Mathematics & Statistics (School of)
https://hdl.handle.net/10023/28
Wed, 14 Aug 2024 22:48:16 GMT2024-08-14T22:48:16ZMathematics & Statistics (School of)https://research-repository.st-andrews.ac.uk:443/bitstream/id/85af3f2b-17df-4c03-9f7d-7db70bcf9950/Mathematics and statistics.gif
https://hdl.handle.net/10023/28
Assouad-type dimensions and the local geometry of fractal sets
https://hdl.handle.net/10023/30361
We study the fine local scaling properties of rough or irregular subsets of a metric space. In particular, we consider the classical Assouad dimension as well as two variants: a scale-refined variant called the Assouad spectrum, and a location-refined variant called the pointwise Assouad dimension.
For the Assouad spectrum, we first give a simple characterization of when a function 𝜑: (0,1) ⟶ [0,𝑑] can be the Assouad spectrum of a general subset of ℝᵈ. Using this, we construct examples exhibiting novel exotic behaviour, answering some questions of Fraser & Yu. We then compute the Assouad spectrum of a certain family planar self-affine sets: the class of Gatzouras–Lalley carpets. Within this family, we establish an explicit formula as the concave conjugate of a certain "column pressure" combined with simple parameter change. This class of sets exhibits novel behaviour in the setting of dynamically invariant sets, such as strict concavity and differentiability on the whole range (0,1).
We then focus on the interrelated concepts of (weak) tangents, Assouad dimension, and a new localized variant which we call the pointwise Assouad dimension. For general attractors of bi-Lipschitz iterated function systems, we establish that the Assouad dimension is given by the Hausdorff dimension of a tangent at some point in the attractor. Under the additional assumption of self-conformality, we moreover prove that this property holds for a subset of full Hausdorff dimension. We then turn our attention again to planar self-affine sets. For Gatzouras–Lalley carpets, we obtain precise information about tangents which, in particular, shows that points with large tangents are very abundant. However, already for Barański carpets, we see that more complex behaviour is possible.
Tue, 03 Dec 2024 00:00:00 GMThttps://hdl.handle.net/10023/303612024-12-03T00:00:00ZRutar, AlexWe study the fine local scaling properties of rough or irregular subsets of a metric space. In particular, we consider the classical Assouad dimension as well as two variants: a scale-refined variant called the Assouad spectrum, and a location-refined variant called the pointwise Assouad dimension.
For the Assouad spectrum, we first give a simple characterization of when a function 𝜑: (0,1) ⟶ [0,𝑑] can be the Assouad spectrum of a general subset of ℝᵈ. Using this, we construct examples exhibiting novel exotic behaviour, answering some questions of Fraser & Yu. We then compute the Assouad spectrum of a certain family planar self-affine sets: the class of Gatzouras–Lalley carpets. Within this family, we establish an explicit formula as the concave conjugate of a certain "column pressure" combined with simple parameter change. This class of sets exhibits novel behaviour in the setting of dynamically invariant sets, such as strict concavity and differentiability on the whole range (0,1).
We then focus on the interrelated concepts of (weak) tangents, Assouad dimension, and a new localized variant which we call the pointwise Assouad dimension. For general attractors of bi-Lipschitz iterated function systems, we establish that the Assouad dimension is given by the Hausdorff dimension of a tangent at some point in the attractor. Under the additional assumption of self-conformality, we moreover prove that this property holds for a subset of full Hausdorff dimension. We then turn our attention again to planar self-affine sets. For Gatzouras–Lalley carpets, we obtain precise information about tangents which, in particular, shows that points with large tangents are very abundant. However, already for Barański carpets, we see that more complex behaviour is possible.Separability properties of semigroups and algebras
https://hdl.handle.net/10023/29944
Separability properties can be seen as generalisations of residual finiteness. In this thesis we investigate four such properties: monogenic subalgebra separability, weak subalgebra separability, strong subalgebra separability and complete separability.
In Chapter 1 we outline the necessary preliminary definitions and results. We define separability properties in terms of universal algebra, in order to be able to study these properties in a range of different settings. We also provide a topological interpretation of these properties. The chapter concludes with the necessary preliminary information to be able to study these properties in semigroups.
In Chapter 2 we investigate the separability properties of free objects in different semigroup varieties. This builds upon work by Hall which shows that the free group is weakly subgroup separable. The varieties considered are groups, semigroups, completely simple semigroups, Clifford semigroups and completely regular semigroups. We also define a new variety, known as α-groups, to aid in our investigation of the free completely simple semigroup.
We begin Chapter 3 by investigating which separability properties are inherited by the Schützenberger groups of a semigroup. We use the theory developed to classify precisely when a finitely generated commutative semi-group has each of four separability properties considered. We conclude the chapter by studying when separability properties of Schützenbeger groups pass to semigroups with finitely many ℋ-classes.
In the final chapter, we consider the preservation of separability properties under various semigroup-theoretic constructions. The constructions considered are the 0-direct union, the direct product, the free product, as well as an investigation into large subsemigroups. We classify precisely when a finite semigroup preserves both monogeinc subsemigroup separability and strong subsemigroup separability in the direct product. We conclude the work by indicating some directions that future research may take.
Tue, 30 Nov 2021 00:00:00 GMThttps://hdl.handle.net/10023/299442021-11-30T00:00:00ZO'Reilly, Gerard AidanSeparability properties can be seen as generalisations of residual finiteness. In this thesis we investigate four such properties: monogenic subalgebra separability, weak subalgebra separability, strong subalgebra separability and complete separability.
In Chapter 1 we outline the necessary preliminary definitions and results. We define separability properties in terms of universal algebra, in order to be able to study these properties in a range of different settings. We also provide a topological interpretation of these properties. The chapter concludes with the necessary preliminary information to be able to study these properties in semigroups.
In Chapter 2 we investigate the separability properties of free objects in different semigroup varieties. This builds upon work by Hall which shows that the free group is weakly subgroup separable. The varieties considered are groups, semigroups, completely simple semigroups, Clifford semigroups and completely regular semigroups. We also define a new variety, known as α-groups, to aid in our investigation of the free completely simple semigroup.
We begin Chapter 3 by investigating which separability properties are inherited by the Schützenberger groups of a semigroup. We use the theory developed to classify precisely when a finitely generated commutative semi-group has each of four separability properties considered. We conclude the chapter by studying when separability properties of Schützenbeger groups pass to semigroups with finitely many ℋ-classes.
In the final chapter, we consider the preservation of separability properties under various semigroup-theoretic constructions. The constructions considered are the 0-direct union, the direct product, the free product, as well as an investigation into large subsemigroups. We classify precisely when a finite semigroup preserves both monogeinc subsemigroup separability and strong subsemigroup separability in the direct product. We conclude the work by indicating some directions that future research may take.Probabilistic generation of almost simple groups and their maximal subgroups
https://hdl.handle.net/10023/29938
See PDF [attached]
Tue, 14 Jun 2022 00:00:00 GMThttps://hdl.handle.net/10023/299382022-06-14T00:00:00ZMordcovich, Adan MarkSee PDF [attached]Automatic evaluation of geopolitical risk
https://hdl.handle.net/10023/29910
This thesis aims to construct programs for automatically evaluating geopolitical risks. This project will use machine learning, specifically sentiment analysis, topic modeling and NER, to build new computer programs to evaluate and assess not just scheduled and predictable geopolitical events, but also unpredictable events. The gap in current literature this project intends to fill is the ability to respond to these risk events in real time. Thus, this project’s objective is to build programs that can digest the vast quantities of data generated by Twitter / X, the data source chosen for this project, focusing on keywords indicating a potential geopolitical event or crisis.
With the use of Twitter / X, I was able to find that information appeared quicker through tweets than traditional news sources, thus I was able to identify emerging geopolitical topics, in some cases, hours or days before they became discussed in the mainstream media. I also achieved success with building a geopolitical risk index program with sentiment analysis to relate the index to the trends in the financial markets surrounding the start of the Ukraine War in 2022 at the daily level. Using Granger Causality, I found that the geopolitical risk index I created from the emotions gleaned from the sentiment analysis of the relevant tweets collected, contained predictive information of the movement of various financial assets over time. In addition, with the NER program, I was able to visualize the different geopolitical risks on a world map. While I managed to create a program that created a geopolitical risk index at the real time level, unfortunately, there was little relationship between the real time risk index and the change in financial markets. In combination, the output of these various programs allowed for the automatic evaluation of geopolitical risk.
Tue, 03 Dec 2024 00:00:00 GMThttps://hdl.handle.net/10023/299102024-12-03T00:00:00ZBurns, John CorcoranThis thesis aims to construct programs for automatically evaluating geopolitical risks. This project will use machine learning, specifically sentiment analysis, topic modeling and NER, to build new computer programs to evaluate and assess not just scheduled and predictable geopolitical events, but also unpredictable events. The gap in current literature this project intends to fill is the ability to respond to these risk events in real time. Thus, this project’s objective is to build programs that can digest the vast quantities of data generated by Twitter / X, the data source chosen for this project, focusing on keywords indicating a potential geopolitical event or crisis.
With the use of Twitter / X, I was able to find that information appeared quicker through tweets than traditional news sources, thus I was able to identify emerging geopolitical topics, in some cases, hours or days before they became discussed in the mainstream media. I also achieved success with building a geopolitical risk index program with sentiment analysis to relate the index to the trends in the financial markets surrounding the start of the Ukraine War in 2022 at the daily level. Using Granger Causality, I found that the geopolitical risk index I created from the emotions gleaned from the sentiment analysis of the relevant tweets collected, contained predictive information of the movement of various financial assets over time. In addition, with the NER program, I was able to visualize the different geopolitical risks on a world map. While I managed to create a program that created a geopolitical risk index at the real time level, unfortunately, there was little relationship between the real time risk index and the change in financial markets. In combination, the output of these various programs allowed for the automatic evaluation of geopolitical risk.Statistical underpinning of mutational signature analyses of cancer sequencing data
https://hdl.handle.net/10023/29876
Cancer is a disease driven and characterised by mutations in the DNA. Thanks to massively parallel sequencing technologies, it is now possible to obtain the sequence of a cancer genome. The advent of modern sequencing technologies has allowed researchers to study the mutations involved in tumour development. More recently, attention has been drawn to the `passenger' mutations that are not involved in tumour development but bear fingerprints of the mutational processes that have been operative over a patient's lifetime.
Those fingerprints, termed mutational signatures, appear consistently across cancer genomes that have been exposed to the underlying mutational processes. Computational analyses have identified over a hundred such signatures, and it is now possible to estimate the relative prevalence of mutational signatures in a cancer genome. Both types of analyses are perhaps unique in the medical literature, in that no confidence intervals or other representations of uncertainty are demanded when reporting the results.
In this thesis, we address the problem of quantifying uncertainty around the reported mutational signatures and their relative prevalence in individual tumours. First, in Chapter 2, we review the available computational methods for mutational signature analyses, assessing the potential of existing approaches to characterise uncertainty. Then, in Chapter 3, we annotate ten statistical challenges. The remainder of the thesis is built on the aim of addressing some of those challenges.
To estimate the relative prevalence of mutational signatures in individual tumours, a method that quantifies the uncertainty around the estimated solution is lacking. Moreover, those analyses assume that the true values for the signatures are `known' as they are propagated from previous analyses. In Chapter 4, we suggest a setting where the signatures are `partially known'. We propose a novel approach for this problem, in a Bayesian setting, providing credible intervals around the estimated solution, propagating prior uncertainty regarding `partially known' signatures, and updating prior beliefs about them.
Estimation of mutational signatures is often performed in a matrix factorisation setting that is not fully probabilistic. While an alternative fully probabilistic approach is available, a post-processing method is needed to characterise the uncertainty around the reported solution. In Chapter 5, we introduce a novel post-processing approach to quantify uncertainty around the mutational signatures estimated in a cohort of cancer patients, along with software that allows investigators to use the proposed method and visualise results.
Tue, 11 Jun 2024 00:00:00 GMThttps://hdl.handle.net/10023/298762024-06-11T00:00:00ZVelasco Pardo, VíctorCancer is a disease driven and characterised by mutations in the DNA. Thanks to massively parallel sequencing technologies, it is now possible to obtain the sequence of a cancer genome. The advent of modern sequencing technologies has allowed researchers to study the mutations involved in tumour development. More recently, attention has been drawn to the `passenger' mutations that are not involved in tumour development but bear fingerprints of the mutational processes that have been operative over a patient's lifetime.
Those fingerprints, termed mutational signatures, appear consistently across cancer genomes that have been exposed to the underlying mutational processes. Computational analyses have identified over a hundred such signatures, and it is now possible to estimate the relative prevalence of mutational signatures in a cancer genome. Both types of analyses are perhaps unique in the medical literature, in that no confidence intervals or other representations of uncertainty are demanded when reporting the results.
In this thesis, we address the problem of quantifying uncertainty around the reported mutational signatures and their relative prevalence in individual tumours. First, in Chapter 2, we review the available computational methods for mutational signature analyses, assessing the potential of existing approaches to characterise uncertainty. Then, in Chapter 3, we annotate ten statistical challenges. The remainder of the thesis is built on the aim of addressing some of those challenges.
To estimate the relative prevalence of mutational signatures in individual tumours, a method that quantifies the uncertainty around the estimated solution is lacking. Moreover, those analyses assume that the true values for the signatures are `known' as they are propagated from previous analyses. In Chapter 4, we suggest a setting where the signatures are `partially known'. We propose a novel approach for this problem, in a Bayesian setting, providing credible intervals around the estimated solution, propagating prior uncertainty regarding `partially known' signatures, and updating prior beliefs about them.
Estimation of mutational signatures is often performed in a matrix factorisation setting that is not fully probabilistic. While an alternative fully probabilistic approach is available, a post-processing method is needed to characterise the uncertainty around the reported solution. In Chapter 5, we introduce a novel post-processing approach to quantify uncertainty around the mutational signatures estimated in a cohort of cancer patients, along with software that allows investigators to use the proposed method and visualise results.Advancements in methods for estimating the abundance of marine megafauna using novel sampling techniques
https://hdl.handle.net/10023/29837
Population size (‘abundance’) is a key parameter for wildlife conservation. As technology advances, new sampling techniques become available for which established estimation methods are extended and new methods developed. We explore, extend, and test new methods for estimating abundance of large marine megafauna that utilise novel sampling techniques: passive acoustic monitoring (PAM), modern genetic analyses, and baited video systems.
The PAM study is motivated by acoustic recordings of bowhead whales (Balaena mysticetus) from multiple sensors in the Beaufort Sea. A high proportion of detections on only one sensor (‘singletons’) are false positives, resulting from the automated detection algorithm. We extend acoustic spatial capture recapture methods to enable singletons to be discarded without biasing the abundance estimate. In our model, detection probability is dependent on received sound level, but we also propose an
extension where detection probability is a function of signal-to-noise ratio, enabling the use of detections at lower received levels.
In the genetic study, we test the recently-developed close-kin mark-recapture (CKMR) method in the situation where the ages of sampled animals are estimated with error. Our work is motivated by a case study of grey reef sharks (Carcharhinus amblyrhynchos) at Palmyra Atoll, Pacific Ocean. CKMR relies on knowing animal age, and for many species this is inferred from measured length. We use simulation to test the effects of misspecifying the age-length relationship (‘growth curve’) and the amount
of length measurement error on abundance estimates. A misspecified growth curve produces substantial bias while misspecifying measurement error does not.
Lastly, we explore the potential of extending random encounter modelling (REM), which is used to estimate population size from remote wildlife cameras, for the situation where baited cameras are used as is common in shark studies. We believe that potential
exists for REM with baited cameras if bait plume intensity is considered.
Tue, 11 Jun 2024 00:00:00 GMThttps://hdl.handle.net/10023/298372024-06-11T00:00:00ZPetersma, Felix ThomasPopulation size (‘abundance’) is a key parameter for wildlife conservation. As technology advances, new sampling techniques become available for which established estimation methods are extended and new methods developed. We explore, extend, and test new methods for estimating abundance of large marine megafauna that utilise novel sampling techniques: passive acoustic monitoring (PAM), modern genetic analyses, and baited video systems.
The PAM study is motivated by acoustic recordings of bowhead whales (Balaena mysticetus) from multiple sensors in the Beaufort Sea. A high proportion of detections on only one sensor (‘singletons’) are false positives, resulting from the automated detection algorithm. We extend acoustic spatial capture recapture methods to enable singletons to be discarded without biasing the abundance estimate. In our model, detection probability is dependent on received sound level, but we also propose an
extension where detection probability is a function of signal-to-noise ratio, enabling the use of detections at lower received levels.
In the genetic study, we test the recently-developed close-kin mark-recapture (CKMR) method in the situation where the ages of sampled animals are estimated with error. Our work is motivated by a case study of grey reef sharks (Carcharhinus amblyrhynchos) at Palmyra Atoll, Pacific Ocean. CKMR relies on knowing animal age, and for many species this is inferred from measured length. We use simulation to test the effects of misspecifying the age-length relationship (‘growth curve’) and the amount
of length measurement error on abundance estimates. A misspecified growth curve produces substantial bias while misspecifying measurement error does not.
Lastly, we explore the potential of extending random encounter modelling (REM), which is used to estimate population size from remote wildlife cameras, for the situation where baited cameras are used as is common in shark studies. We believe that potential
exists for REM with baited cameras if bait plume intensity is considered.Groups defined by language theoretic classes
https://hdl.handle.net/10023/29773
In this thesis we shall study classes of groups defined by formal languages. Our first main topic is the class of groups defined by having an ET0L co-word problem; i.e., the class of co-ET0L groups. We show this class is closed under taking direct products and standard restricted wreath products with virtually free top groups. We also show the class is closed under passing to finite index overgroups and finitely generated subgroups. Further, we show that this class contains the free product Zn * Zm as well as Zn * G for any virtually free group G.
The second topic that we consider is a new class of groups that we introduce called epiregular groups. We show that this class contains all automatic groups and the Baumslag-Solitar group BS(1,2). Further we show that the class of epiregular groups is closed under taking graph products, and passing to finite index overgroups.
Tue, 11 Jun 2024 00:00:00 GMThttps://hdl.handle.net/10023/297732024-06-11T00:00:00ZAl Kohli, Raad Sameer Al SheikhIn this thesis we shall study classes of groups defined by formal languages. Our first main topic is the class of groups defined by having an ET0L co-word problem; i.e., the class of co-ET0L groups. We show this class is closed under taking direct products and standard restricted wreath products with virtually free top groups. We also show the class is closed under passing to finite index overgroups and finitely generated subgroups. Further, we show that this class contains the free product Zn * Zm as well as Zn * G for any virtually free group G.
The second topic that we consider is a new class of groups that we introduce called epiregular groups. We show that this class contains all automatic groups and the Baumslag-Solitar group BS(1,2). Further we show that the class of epiregular groups is closed under taking graph products, and passing to finite index overgroups.Efficient methods for fitting nonlinear non-Gaussian state space models of wildlife population dynamics
https://hdl.handle.net/10023/29716
State-space models (SSMs) are a popular and flexible framework for modelling time series due to their ability to separate changes in the underlying state of a system from the noisy observations made on these states. This thesis explores methods for estimating states and model parameters in non-linear and non-Gaussian Bayesian SSMs. We focus on models of wildlife population dynamics, in particular a case study of the UK grey seal population.
Calculation of the likelihood is fundamental to Bayesian analysis, but direct calculation is typically intractable for non-linear non-Gaussian SSMs. We use a class of simulation-based methods, Sequential Monte Carlo (SMC), which build on repeated importance sampling of simulated states to deliver an unbiased estimate of the likelihood. We find that variance of the estimated likelihood can be high and explore techniques for variance reduction.
For parameter inference, we use particle marginal Metropolis-Hastings (PMMH), which embeds the SMC likelihood within a Markov chain Monte Carlo (MCMC) algorithm. Careful balance is needed between computational effort expended on the SMC step and the number of MCMC samples.
A much faster alternative is the Kalman filter, designed for linear and Gaussian SSMs. We applied the Kalman filter to an approximation of the seal model. The posterior distribution obtained was often close to the true posterior, while reducing computation time by a factor of 1790.
We show the seal model suffers from identifiability issues which cannot be resolved by increasing the accuracy of the observations or allowing more flexibility in the underlying biological process with random effects. However, estimation of underlying states (i.e., population sizes) is unaffected by these issues.
A reduction in PMMH computation time can be achieved by exploiting the structure of the state model: separately estimating likelihood components in each of the 4 seal regions led to a 5-fold increase in speed.
Tue, 11 Jun 2024 00:00:00 GMThttps://hdl.handle.net/10023/297162024-06-11T00:00:00ZEmpacher, FannyState-space models (SSMs) are a popular and flexible framework for modelling time series due to their ability to separate changes in the underlying state of a system from the noisy observations made on these states. This thesis explores methods for estimating states and model parameters in non-linear and non-Gaussian Bayesian SSMs. We focus on models of wildlife population dynamics, in particular a case study of the UK grey seal population.
Calculation of the likelihood is fundamental to Bayesian analysis, but direct calculation is typically intractable for non-linear non-Gaussian SSMs. We use a class of simulation-based methods, Sequential Monte Carlo (SMC), which build on repeated importance sampling of simulated states to deliver an unbiased estimate of the likelihood. We find that variance of the estimated likelihood can be high and explore techniques for variance reduction.
For parameter inference, we use particle marginal Metropolis-Hastings (PMMH), which embeds the SMC likelihood within a Markov chain Monte Carlo (MCMC) algorithm. Careful balance is needed between computational effort expended on the SMC step and the number of MCMC samples.
A much faster alternative is the Kalman filter, designed for linear and Gaussian SSMs. We applied the Kalman filter to an approximation of the seal model. The posterior distribution obtained was often close to the true posterior, while reducing computation time by a factor of 1790.
We show the seal model suffers from identifiability issues which cannot be resolved by increasing the accuracy of the observations or allowing more flexibility in the underlying biological process with random effects. However, estimation of underlying states (i.e., population sizes) is unaffected by these issues.
A reduction in PMMH computation time can be achieved by exploiting the structure of the state model: separately estimating likelihood components in each of the 4 seal regions led to a 5-fold increase in speed.Rearrangement groups of connected spaces
https://hdl.handle.net/10023/29477
We develop a combinatorial framework that assists in finding natural infinite “geometric” presentations for a large subclass of rearrangement groups of fractals – defined by Belk and Forrest, namely rearrangement groups acting on F-type topological spaces. In this framework, for a given fractal set with its group of “rearrangements”, the group generators have a natural one-to-one correspondence with the standard basis of the fractal set, and the relations are all conjugacy relations.
We use this framework to produce a presentation for Richard Thompson’s group F. This presentation has been mentioned before by Dehornoy, but a combinatorial method to find the length of an element in terms of the generating set of this presentation has been hitherto unknown. We provide algorithms that express an element of F in terms of our generating set and reduce a word representing the identity in F to the trivial word.
We conjecture that this framework can be used to find infinite presentations for all groups in the subclass of rearrangement groups acting on F-type topological spaces.
Tue, 28 Jul 2020 00:00:00 GMThttps://hdl.handle.net/10023/294772020-07-28T00:00:00ZKhalid, NayabWe develop a combinatorial framework that assists in finding natural infinite “geometric” presentations for a large subclass of rearrangement groups of fractals – defined by Belk and Forrest, namely rearrangement groups acting on F-type topological spaces. In this framework, for a given fractal set with its group of “rearrangements”, the group generators have a natural one-to-one correspondence with the standard basis of the fractal set, and the relations are all conjugacy relations.
We use this framework to produce a presentation for Richard Thompson’s group F. This presentation has been mentioned before by Dehornoy, but a combinatorial method to find the length of an element in terms of the generating set of this presentation has been hitherto unknown. We provide algorithms that express an element of F in terms of our generating set and reduce a word representing the identity in F to the trivial word.
We conjecture that this framework can be used to find infinite presentations for all groups in the subclass of rearrangement groups acting on F-type topological spaces.Internal interannual variability in the extra-tropical stratosphere
https://hdl.handle.net/10023/29463
We investigate persistent low-frequency variability of the stratospheric winter polar vortex in a rotating spherical shallow-water model under the action of topographic wave-forcing and radiative cooling to a simple time-varying equilibrium state representative of the seasonal cycle in solar heating. A range of modes of variability is obtained, dependent on wave forcing amplitude and characterized by the distribution of quiescent and disturbed winters, defined as winters in which the vortex is either close to radiative equilibrium, with low planetary wave amplitude, or else strongly disturbed from equilibrium by the wave forcing. At low forcing amplitude the vortex is typically quiescent every year, while at higher amplitude it is typically disturbed; in both cases there is little year-to-year variation of the vortex state. For a range of intermediate forcing amplitudes, however, the vortex transitions between quiescent and disturbed states from one winter to the next with a persistent and well-defined pattern of variability. To investigate the extent to which the low-frequency variability found here may be explained in terms of a low-latitude ywheel mechanism, we conduct additional experiments varying a linear drag on the zonal mean ow in the tropics and find that sufficiently strong drag can completely suppress the variability. The robustness of the variability is demonstrated by further experiments using a modified radiative equilibrium profile, associated with a tropical westerly flow: similar variability is obtained but the modified profile is less effective at constraining the tropical ow from a persistent easterly acceleration.
Tue, 30 Nov 2021 00:00:00 GMThttps://hdl.handle.net/10023/294632021-11-30T00:00:00ZHatfield, Luke AnthonyWe investigate persistent low-frequency variability of the stratospheric winter polar vortex in a rotating spherical shallow-water model under the action of topographic wave-forcing and radiative cooling to a simple time-varying equilibrium state representative of the seasonal cycle in solar heating. A range of modes of variability is obtained, dependent on wave forcing amplitude and characterized by the distribution of quiescent and disturbed winters, defined as winters in which the vortex is either close to radiative equilibrium, with low planetary wave amplitude, or else strongly disturbed from equilibrium by the wave forcing. At low forcing amplitude the vortex is typically quiescent every year, while at higher amplitude it is typically disturbed; in both cases there is little year-to-year variation of the vortex state. For a range of intermediate forcing amplitudes, however, the vortex transitions between quiescent and disturbed states from one winter to the next with a persistent and well-defined pattern of variability. To investigate the extent to which the low-frequency variability found here may be explained in terms of a low-latitude ywheel mechanism, we conduct additional experiments varying a linear drag on the zonal mean ow in the tropics and find that sufficiently strong drag can completely suppress the variability. The robustness of the variability is demonstrated by further experiments using a modified radiative equilibrium profile, associated with a tropical westerly flow: similar variability is obtained but the modified profile is less effective at constraining the tropical ow from a persistent easterly acceleration.Expanding the use of spatial models in statistical ecology
https://hdl.handle.net/10023/29306
This thesis is focused on expanding the use of spatial modelling approaches for applications in ecology. Spatial ecology is about understanding the processes that give rise to spatial patterns in ecological data. In addition to developing a purely scientific understanding, insights into these processes are essential for the effective monitoring and conservation management of ecological systems.
However, for many ecological problems, the detectability of animals is imperfect, requiring the use of complex observation models that can account for this. In this thesis we focus on two such models: distance sampling and spatial capture-recapture (SCR). For both these models we incorporate spatially structured random effects to provide a non-parametric method for describing spatial variation in species’ abundance, and to address the problem of spatial auto-correlation.
These complex models require the use of computationally efficient random effect structures and inference methods. In particular, we use a sparse stochastic partial differential equation (SPDE) approach as well as low rank penalised smoothing splines. We also draw links between these two approaches in order to illuminate the technically challenging results underpinning the SPDE approach. For inference in distance sampling models, we use a novel approach to achieve a one-stage model fit based on iterated model fitting using approximate Bayesian methods. For inference in SCR models, we use Laplace approximate maximum likelihood methods. We present models that have the necessary complexity to jointly model complex ecological and observation processes, as well as providing efficient methods to fit the models in practice. We conclude by discussing related avenues for future research that are motivated by applied problems in the field of spatial ecology.
Tue, 14 Jun 2022 00:00:00 GMThttps://hdl.handle.net/10023/293062022-06-14T00:00:00ZSeaton, Andrew ErnestThis thesis is focused on expanding the use of spatial modelling approaches for applications in ecology. Spatial ecology is about understanding the processes that give rise to spatial patterns in ecological data. In addition to developing a purely scientific understanding, insights into these processes are essential for the effective monitoring and conservation management of ecological systems.
However, for many ecological problems, the detectability of animals is imperfect, requiring the use of complex observation models that can account for this. In this thesis we focus on two such models: distance sampling and spatial capture-recapture (SCR). For both these models we incorporate spatially structured random effects to provide a non-parametric method for describing spatial variation in species’ abundance, and to address the problem of spatial auto-correlation.
These complex models require the use of computationally efficient random effect structures and inference methods. In particular, we use a sparse stochastic partial differential equation (SPDE) approach as well as low rank penalised smoothing splines. We also draw links between these two approaches in order to illuminate the technically challenging results underpinning the SPDE approach. For inference in distance sampling models, we use a novel approach to achieve a one-stage model fit based on iterated model fitting using approximate Bayesian methods. For inference in SCR models, we use Laplace approximate maximum likelihood methods. We present models that have the necessary complexity to jointly model complex ecological and observation processes, as well as providing efficient methods to fit the models in practice. We conclude by discussing related avenues for future research that are motivated by applied problems in the field of spatial ecology.Modern computational methods for finitely presented monoids
https://hdl.handle.net/10023/29290
In this thesis we are mainly interested in the development of practical algorithms for semigroups and monoids defined by finite presentations. Although in general nearly every problem about finitely presented semigroups is undecidable, many finitely presented semigroups and monoids of interest are more tractable. Semigroup and monoid presentations have been widely studied in the literature more or less since the inception of the field of semigroup theory. The aim of computational semigroup theory, of which
this thesis forms a part, is to develop algorithms and software tools for computing with semigroups, and on the applications of these tools to research problems.
In this thesis we develop the concept of words graphs, which form the basis for the work presented in the first half of the thesis. We describe an algorithm that computes one-sided congruences of finitely presented semigroups. This is the semigroup theoretic
analogue of an algorithm described by Sims for computing subgroups of small index in finitely presented groups. Furthermore, we focus on the Todd-Coxeter Algorithm, one of the most widely studied algorithms in computational semigroup theory. We describe
a more general version of the Todd-Coxeter Algorithm than the versions available in the literature for computing congruences of finitely presented semigroups.
The remaining part of this thesis is focused on a class of finitely presented monoids, called small overlap monoids. These are, in some sense, the generic finitely presented monoids. They have decidable word problem that can be solved in linear time. We
present the results related to the word problem and the combinatorial theory for small overlap monoids developed by Kambites. In addition, we discuss methods appearing in the literature for normal forms in small overlap monoids and we present a new practical
algorithm for computing normal forms.
Tue, 11 Jun 2024 00:00:00 GMThttps://hdl.handle.net/10023/292902024-06-11T00:00:00ZTsalakou, MariaIn this thesis we are mainly interested in the development of practical algorithms for semigroups and monoids defined by finite presentations. Although in general nearly every problem about finitely presented semigroups is undecidable, many finitely presented semigroups and monoids of interest are more tractable. Semigroup and monoid presentations have been widely studied in the literature more or less since the inception of the field of semigroup theory. The aim of computational semigroup theory, of which
this thesis forms a part, is to develop algorithms and software tools for computing with semigroups, and on the applications of these tools to research problems.
In this thesis we develop the concept of words graphs, which form the basis for the work presented in the first half of the thesis. We describe an algorithm that computes one-sided congruences of finitely presented semigroups. This is the semigroup theoretic
analogue of an algorithm described by Sims for computing subgroups of small index in finitely presented groups. Furthermore, we focus on the Todd-Coxeter Algorithm, one of the most widely studied algorithms in computational semigroup theory. We describe
a more general version of the Todd-Coxeter Algorithm than the versions available in the literature for computing congruences of finitely presented semigroups.
The remaining part of this thesis is focused on a class of finitely presented monoids, called small overlap monoids. These are, in some sense, the generic finitely presented monoids. They have decidable word problem that can be solved in linear time. We
present the results related to the word problem and the combinatorial theory for small overlap monoids developed by Kambites. In addition, we discuss methods appearing in the literature for normal forms in small overlap monoids and we present a new practical
algorithm for computing normal forms.Combinatorial algorithms in semigroups
https://hdl.handle.net/10023/29260
Computational semigroup theory is concerned with developing and implementing algorithms for determining properties of semigroups. The problems in computational semigroup theory can often be divided into sub-problems in computational group theory and combinatorics. In this thesis, we demonstrate how to split a number of problems into group-theoretical and combinatorial sub-problems, and provide algorithms for solving the combinatorial sub-problems. The algorithms presented in this thesis have been implemented in C++ and GAP, and benchmarks are provided to show them practical.
In Chapter 1, we introduce the necessary background in semigroup theory.
In Chapter 2, we introduce a new algorithm for non-exhaustively determining the structure of a semigroup defined by generators, and show that it applies to certain families of matrices over semirings as well as a number of standard and well-known families to which previous algorithms apply.
In Chapter 3 we describe and compute minimal generating sets for several naturally occurring monoids of boolean matrices, in particular the full boolean matrix monoid, Hall monoid, reflexive boolean matrix monoid, and upper and lower triangular boolean matrix monoids. These results extend the dimensions for which the ranks of these monoids are known. We also determine the rank of the 2 × 2 matrices over the max-plus and min-plus semirings with and without threshold, as well as the n × n matrices over Z/kZ relative to their group of units.
Chapter 4 contains new algorithms for determining the translations and bitranslations of arbitrary finite semigroups. We also provide specialised algorithms for computing translations and bitranslations of semigroups defined by finite presentations, completely 0-simple semigroups, congruence-free semigroups, and completely-simple semigroups.
Finally, Chapter 4 contains some further questions raised by the work in this thesis.
Tue, 14 Jun 2022 00:00:00 GMThttps://hdl.handle.net/10023/292602022-06-14T00:00:00ZSmith, Finlay LaughlanComputational semigroup theory is concerned with developing and implementing algorithms for determining properties of semigroups. The problems in computational semigroup theory can often be divided into sub-problems in computational group theory and combinatorics. In this thesis, we demonstrate how to split a number of problems into group-theoretical and combinatorial sub-problems, and provide algorithms for solving the combinatorial sub-problems. The algorithms presented in this thesis have been implemented in C++ and GAP, and benchmarks are provided to show them practical.
In Chapter 1, we introduce the necessary background in semigroup theory.
In Chapter 2, we introduce a new algorithm for non-exhaustively determining the structure of a semigroup defined by generators, and show that it applies to certain families of matrices over semirings as well as a number of standard and well-known families to which previous algorithms apply.
In Chapter 3 we describe and compute minimal generating sets for several naturally occurring monoids of boolean matrices, in particular the full boolean matrix monoid, Hall monoid, reflexive boolean matrix monoid, and upper and lower triangular boolean matrix monoids. These results extend the dimensions for which the ranks of these monoids are known. We also determine the rank of the 2 × 2 matrices over the max-plus and min-plus semirings with and without threshold, as well as the n × n matrices over Z/kZ relative to their group of units.
Chapter 4 contains new algorithms for determining the translations and bitranslations of arbitrary finite semigroups. We also provide specialised algorithms for computing translations and bitranslations of semigroups defined by finite presentations, completely 0-simple semigroups, congruence-free semigroups, and completely-simple semigroups.
Finally, Chapter 4 contains some further questions raised by the work in this thesis.Construction and comparison of semi-Latin rectangles
https://hdl.handle.net/10023/29182
This work is concerned with semi-Latin rectangles (SLRs). These designs are row-column designs with nice combinatorial properties; and were introduced in Bailey and Monod (2001). They generalize the Latin squares (LSs) and semi-Latin squares (SLSs) and are useful for many experimental situations in diverse sectors, ranging from agriculture to the industry. We classify these designs as balanced semi-Latin rectangles (BSLRs) and non- balanced semi-Latin rectangles (NBSLRs) and develop some constructions, via algorithms, for good SLRs, that is, SLRs with good statistical properties for each classification using some combinatorial approaches. BSLRs do not always exist, but when they exist, they are optimal among other SLRs in their class over a range of criteria. When a BSLR does not exist, good designs can be sought among RGSLRs, particularly for large number of blocks, if they exist. Hence for the NBSLRs we concentrate on regular-graph semi-Latin rectangles (RGSLRs). For each classification, constructions are given for designs with block size two and for those with block size larger than two; and for block size two, we consider situations when the number of treatments is odd and also when it is even. The construction involving RGSLRs with block size two having an odd number of treatments is generalized to accommodate more columns and a table showing starters in some cyclic groups of small odd orders, 5 to 15 is given to facilitate the construction. Some direct constructions, for different situations, have been developed for RGSLRs whose number of treatments is even and whose block size is two less the number of treatments. These are backed up with some examples, which when compared with designs of the same size obtained via complementation, they are found to be identical under one of the methods but isomorphic under the other method. Finally, for each of BSLRs and RGSLRs, we have given a table containing sets of parameters, which can combine to give a design alongside their construction and also where the design (or its construction, as the case may be) can be found in the thesis.
Tue, 29 Jun 2021 00:00:00 GMThttps://hdl.handle.net/10023/291822021-06-29T00:00:00ZUto, Nseobong PeterThis work is concerned with semi-Latin rectangles (SLRs). These designs are row-column designs with nice combinatorial properties; and were introduced in Bailey and Monod (2001). They generalize the Latin squares (LSs) and semi-Latin squares (SLSs) and are useful for many experimental situations in diverse sectors, ranging from agriculture to the industry. We classify these designs as balanced semi-Latin rectangles (BSLRs) and non- balanced semi-Latin rectangles (NBSLRs) and develop some constructions, via algorithms, for good SLRs, that is, SLRs with good statistical properties for each classification using some combinatorial approaches. BSLRs do not always exist, but when they exist, they are optimal among other SLRs in their class over a range of criteria. When a BSLR does not exist, good designs can be sought among RGSLRs, particularly for large number of blocks, if they exist. Hence for the NBSLRs we concentrate on regular-graph semi-Latin rectangles (RGSLRs). For each classification, constructions are given for designs with block size two and for those with block size larger than two; and for block size two, we consider situations when the number of treatments is odd and also when it is even. The construction involving RGSLRs with block size two having an odd number of treatments is generalized to accommodate more columns and a table showing starters in some cyclic groups of small odd orders, 5 to 15 is given to facilitate the construction. Some direct constructions, for different situations, have been developed for RGSLRs whose number of treatments is even and whose block size is two less the number of treatments. These are backed up with some examples, which when compared with designs of the same size obtained via complementation, they are found to be identical under one of the methods but isomorphic under the other method. Finally, for each of BSLRs and RGSLRs, we have given a table containing sets of parameters, which can combine to give a design alongside their construction and also where the design (or its construction, as the case may be) can be found in the thesis.Finiteness conditions on semigroups relating to their actions and one-sided congruences
https://hdl.handle.net/10023/28874
The purpose of this thesis is threefold: firstly, to develop a systematic theory of presentations
of monoid acts; secondly, to study finiteness conditions on semigroups
relating to finite generation of one-sided congruences; and thirdly, to establish
connections between each of these finiteness conditions, restricted to the class of
monoids, with finite presentability of acts.
We find general presentations for various monoid act constructions/components,
leading to a number of finite presentability results. In particular, we consider
subacts, Rees quotients, unions of subacts, direct products and wreath products.
A semigroup 𝑆 is called right noetherian if every right congruence on 𝑆 is finitely
generated. We present some fundamental properties of right noetherian semigroups,
discuss how semigroups relate to their substructures with regard to the
property of being right noetherian, and investigate whether this property is preserved
under various semigroup constructions.
Finally, we introduce and study the condition that every right congruence of finite index on a semigroup is finitely generated. We call semigroups satisfying this condition f-noetherian. It turns out that every finitely generated semigroup is
f-noetherian. We investigate, for various semigroup classes, whether the property of being f-noetherian coincides with finite generation.
Tue, 01 Dec 2020 00:00:00 GMThttps://hdl.handle.net/10023/288742020-12-01T00:00:00ZMiller, CraigThe purpose of this thesis is threefold: firstly, to develop a systematic theory of presentations
of monoid acts; secondly, to study finiteness conditions on semigroups
relating to finite generation of one-sided congruences; and thirdly, to establish
connections between each of these finiteness conditions, restricted to the class of
monoids, with finite presentability of acts.
We find general presentations for various monoid act constructions/components,
leading to a number of finite presentability results. In particular, we consider
subacts, Rees quotients, unions of subacts, direct products and wreath products.
A semigroup 𝑆 is called right noetherian if every right congruence on 𝑆 is finitely
generated. We present some fundamental properties of right noetherian semigroups,
discuss how semigroups relate to their substructures with regard to the
property of being right noetherian, and investigate whether this property is preserved
under various semigroup constructions.
Finally, we introduce and study the condition that every right congruence of finite index on a semigroup is finitely generated. We call semigroups satisfying this condition f-noetherian. It turns out that every finitely generated semigroup is
f-noetherian. We investigate, for various semigroup classes, whether the property of being f-noetherian coincides with finite generation.On constructing topology from algebra
https://hdl.handle.net/10023/28769
In this thesis we explore natural procedures through which topological structure can be constructed from specific semigroups. We will do this in two ways: 1) we equip the semigroup object itself with a topological structure, and 2) we find a topological space for the semigroup to act on continuously. We discuss various minimum/maximum topologies which one can define on an arbitrary semigroup (given some topological restrictions). We give explicit descriptions of each these topologies for the monoids of binary relations, partial transformations, transformations, and partial bijections on the set N. Using similar methods we determine whether or not each of these semigroups admits a unique Polish semigroup topology. We also do this for the following semigroups: the monoid of all injective functions on N, the monoid of continuous transformations of the Hilbert cube [0, 1]N, the monoid of continuous transformations of the Cantor space 2N, and the monoid of endomorphisms of the countably infinite atomless boolean algebra. With the exception of the continuous transformation monoid of the Hilbert cube, we also show that all of the above semigroups admit a second countable semigroup topology such that every semigroup homomorphism from the semigroup to a second countable topological semigroup is continuous. In a recent paper, Bleak, Cameron, Maissel, Navas, and Olukoya use a theorem of Rubin to describe the automorphism groups of the Higman-Thompson groups Gₙ,ᵣ via their canonical Rubin action on the Cantor space. In particular they embed these automorphism groups into the rational group R of transducers introduced by Grigorchuk, Nekrashevich, and Sushchanskii. We generalise these transducers to be more suitable to higher dimensional Cantor spaces and give a similar description of the automorphism groups of the Brin-Thompson groups 𝑑Vₙ (although we do not give an embedding into R). Using our description, we show that the outer automorphism group Out(𝑑V₂) of 𝑑V₂ is isomorphic to the wreath product of Out(1V₂) with the symmetric group on 𝑑 points.
Tue, 14 Jun 2022 00:00:00 GMThttps://hdl.handle.net/10023/287692022-06-14T00:00:00ZElliott, LukeIn this thesis we explore natural procedures through which topological structure can be constructed from specific semigroups. We will do this in two ways: 1) we equip the semigroup object itself with a topological structure, and 2) we find a topological space for the semigroup to act on continuously. We discuss various minimum/maximum topologies which one can define on an arbitrary semigroup (given some topological restrictions). We give explicit descriptions of each these topologies for the monoids of binary relations, partial transformations, transformations, and partial bijections on the set N. Using similar methods we determine whether or not each of these semigroups admits a unique Polish semigroup topology. We also do this for the following semigroups: the monoid of all injective functions on N, the monoid of continuous transformations of the Hilbert cube [0, 1]N, the monoid of continuous transformations of the Cantor space 2N, and the monoid of endomorphisms of the countably infinite atomless boolean algebra. With the exception of the continuous transformation monoid of the Hilbert cube, we also show that all of the above semigroups admit a second countable semigroup topology such that every semigroup homomorphism from the semigroup to a second countable topological semigroup is continuous. In a recent paper, Bleak, Cameron, Maissel, Navas, and Olukoya use a theorem of Rubin to describe the automorphism groups of the Higman-Thompson groups Gₙ,ᵣ via their canonical Rubin action on the Cantor space. In particular they embed these automorphism groups into the rational group R of transducers introduced by Grigorchuk, Nekrashevich, and Sushchanskii. We generalise these transducers to be more suitable to higher dimensional Cantor spaces and give a similar description of the automorphism groups of the Brin-Thompson groups 𝑑Vₙ (although we do not give an embedding into R). Using our description, we show that the outer automorphism group Out(𝑑V₂) of 𝑑V₂ is isomorphic to the wreath product of Out(1V₂) with the symmetric group on 𝑑 points.Interpolating between Hausdorff and box dimension
https://hdl.handle.net/10023/28591
Hausdorff and box dimension are two familiar notions of fractal dimension. Box dimension can be larger than Hausdorff dimension, because in the definition of box dimension, all sets in the cover have the same diameter, but for Hausdorff dimension there is no such restriction. This thesis focuses on a family of dimensions parameterised by θ ∈ (0,1), called the intermediate dimensions, which are defined by requiring that diam(U) ⩽ (diam(V))ᶿ for all sets U, V in the cover.
We begin by generalising the intermediate dimensions to allow for greater refinement in how the relative sizes of the covering sets are restricted. These new dimensions can recover the interpolation between Hausdorff and box dimension for compact sets whose intermediate dimensions do not tend to the Hausdorff dimension as θ → 0. We also use a Moran set construction to prove a necessary and sufficient condition, in terms of Dini derivatives, for a given function to be realised as the intermediate dimensions of a set.
We proceed to prove that the intermediate dimensions of limit sets of infinite conformal iterated function systems are given by the maximum of the Hausdorff dimension of the limit set and the intermediate dimensions of the set of fixed points of the contractions. This applies to sets defined using continued fraction expansions, and has applications to dimensions of projections, fractional Brownian images, and general Hölder images.
Finally, we determine a formula for the intermediate dimensions of all self-affine Bedford–McMullen carpets. The functions display features not witnessed in previous examples, such as having countably many phase transitions. We deduce that two carpets have equal intermediate dimensions if and only if the multifractal spectra of the corresponding uniform Bernoulli measures coincide. This shows that if two carpets are bi-Lipschitz equivalent then the multifractal spectra are equal.
Tue, 28 Nov 2023 00:00:00 GMThttps://hdl.handle.net/10023/285912023-11-28T00:00:00ZBanaji, AmlanHausdorff and box dimension are two familiar notions of fractal dimension. Box dimension can be larger than Hausdorff dimension, because in the definition of box dimension, all sets in the cover have the same diameter, but for Hausdorff dimension there is no such restriction. This thesis focuses on a family of dimensions parameterised by θ ∈ (0,1), called the intermediate dimensions, which are defined by requiring that diam(U) ⩽ (diam(V))ᶿ for all sets U, V in the cover.
We begin by generalising the intermediate dimensions to allow for greater refinement in how the relative sizes of the covering sets are restricted. These new dimensions can recover the interpolation between Hausdorff and box dimension for compact sets whose intermediate dimensions do not tend to the Hausdorff dimension as θ → 0. We also use a Moran set construction to prove a necessary and sufficient condition, in terms of Dini derivatives, for a given function to be realised as the intermediate dimensions of a set.
We proceed to prove that the intermediate dimensions of limit sets of infinite conformal iterated function systems are given by the maximum of the Hausdorff dimension of the limit set and the intermediate dimensions of the set of fixed points of the contractions. This applies to sets defined using continued fraction expansions, and has applications to dimensions of projections, fractional Brownian images, and general Hölder images.
Finally, we determine a formula for the intermediate dimensions of all self-affine Bedford–McMullen carpets. The functions display features not witnessed in previous examples, such as having countably many phase transitions. We deduce that two carpets have equal intermediate dimensions if and only if the multifractal spectra of the corresponding uniform Bernoulli measures coincide. This shows that if two carpets are bi-Lipschitz equivalent then the multifractal spectra are equal.Partial differential equation modelling in cancer and development
https://hdl.handle.net/10023/28432
This thesis explores various partial differential equation (PDE) models of the spatiotemporal and evolutionary dynamics of cell populations in different problems in cancer and development. In particular, these models are used to investigate: (i) the emergence of intratumour phenotypic heterogeneity and the development of chemotherapeutic resistance in vascularised tumours; (ii) the formation of endothelial progenitor cell clusters during the early stages of vasculogenesis; (iii) mechanical pattern formation under different linear viscoelasticity assumptions for the extracellular matrix. The mathematical models proposed for these problems are formulated as systems of nonlinear and nonlocal PDEs, which provide a mean-field representation of the underlying cellular dynamics and pose a series of interesting analytical and numerical challenges. These are tackled by means of formal asymptotic methods, linear stability analyses and appropriate numerical schemes preventing the emergence of spurious oscillations. Numerical simulations, relying on parameter values drawn from the extant literature, complement the analytical results and are employed for in silico investigations qualitatively testing the model assumptions against empirical observations. The obtained results help us shed light on the hidden mechanisms responsible for the emergence of the studied phenomena in biology
and medicine, suggesting promising research perspectives.
Tue, 14 Jun 2022 00:00:00 GMThttps://hdl.handle.net/10023/284322022-06-14T00:00:00ZVilla, ChiaraThis thesis explores various partial differential equation (PDE) models of the spatiotemporal and evolutionary dynamics of cell populations in different problems in cancer and development. In particular, these models are used to investigate: (i) the emergence of intratumour phenotypic heterogeneity and the development of chemotherapeutic resistance in vascularised tumours; (ii) the formation of endothelial progenitor cell clusters during the early stages of vasculogenesis; (iii) mechanical pattern formation under different linear viscoelasticity assumptions for the extracellular matrix. The mathematical models proposed for these problems are formulated as systems of nonlinear and nonlocal PDEs, which provide a mean-field representation of the underlying cellular dynamics and pose a series of interesting analytical and numerical challenges. These are tackled by means of formal asymptotic methods, linear stability analyses and appropriate numerical schemes preventing the emergence of spurious oscillations. Numerical simulations, relying on parameter values drawn from the extant literature, complement the analytical results and are employed for in silico investigations qualitatively testing the model assumptions against empirical observations. The obtained results help us shed light on the hidden mechanisms responsible for the emergence of the studied phenomena in biology
and medicine, suggesting promising research perspectives.Counting processes for spatial capture-recapture
https://hdl.handle.net/10023/28389
Estimates of animal density, the number of individuals per unit area, are critically important for understanding ecological processes affecting wildlife management. Increasingly, modern technology like camera traps and acoustic recording units are being used to monitor wildlife populations. These are relatively inexpensive and can reliably record animal detections over a long period of time. When animals have a chance to be recorded on multiple detectors, spatial capture-recapture (SCR) can be used to estimate animal density. However, these models generally require that animal identity is known which is often not the case. For camera trap studies, the animals may not have unique pelage to distinguish them reliably, or image quality may be poor. In an acoustic recording, there may not be any individually identifying features in the individual’s call to distinguish it from others of the same species. The motivating problem of this thesis is how to deal with uncertain animal identity in SCR.
We review current methods in SCR for both known and unknown identity, ways to make Bayesian inference for SCR models using Nimble in R, and how SCR can be written as a Dirichlet process mixture model when identities are unknown. This leads us to reformulate the conventional SCR model as a marked Poisson process, such that the counting process for detections through time no longer depends on identity, but the observed mark distributions do. When identity is latent, the observed marks are distributed over a mixture of N latent animal characteristics (e.g. activity centre, sex), where N is the number of animals at risk of detection. This becomes a generalization of the unmarked SCR model of Chandler and Royle (2013) and allows us to easily add additionally observed covariates to help estimate animal identity. We show through simulation how well the method works and apply it to a camera trap survey of fisher (Pekania pennanti) and an acoustic survey of the Cape Peninsula moss frog (Arthroleptella lightfooti), each with different types of information used to inform identities.
A fundamental assumption of SCR is that detections of an individual occurs independently. This implies that a detection at one detector has no impact on the probability that an animal is seen at any other detectors immediately after. Assumptions of independence allows us to model SCR detections as a spatiotemporal point process, often a Poisson process. As a result, the time of detection becomes uninformative about animal identity. We offer a solution to this by thinking about SCR in terms of a new detection function that depends on a realistic animal movement model. To do this, we relax the independence assumption by building a spatiotemporal dependent point process with a new detection function that depends on where and when the animal was last observed. As a result, we can explicitly model the existing correlation in the detections as well as provide a new method for spatiotemporal clustering of latent identity SCR problems.
Tue, 28 Nov 2023 00:00:00 GMThttps://hdl.handle.net/10023/283892023-11-28T00:00:00Zvan Dam-Bates, PaulEstimates of animal density, the number of individuals per unit area, are critically important for understanding ecological processes affecting wildlife management. Increasingly, modern technology like camera traps and acoustic recording units are being used to monitor wildlife populations. These are relatively inexpensive and can reliably record animal detections over a long period of time. When animals have a chance to be recorded on multiple detectors, spatial capture-recapture (SCR) can be used to estimate animal density. However, these models generally require that animal identity is known which is often not the case. For camera trap studies, the animals may not have unique pelage to distinguish them reliably, or image quality may be poor. In an acoustic recording, there may not be any individually identifying features in the individual’s call to distinguish it from others of the same species. The motivating problem of this thesis is how to deal with uncertain animal identity in SCR.
We review current methods in SCR for both known and unknown identity, ways to make Bayesian inference for SCR models using Nimble in R, and how SCR can be written as a Dirichlet process mixture model when identities are unknown. This leads us to reformulate the conventional SCR model as a marked Poisson process, such that the counting process for detections through time no longer depends on identity, but the observed mark distributions do. When identity is latent, the observed marks are distributed over a mixture of N latent animal characteristics (e.g. activity centre, sex), where N is the number of animals at risk of detection. This becomes a generalization of the unmarked SCR model of Chandler and Royle (2013) and allows us to easily add additionally observed covariates to help estimate animal identity. We show through simulation how well the method works and apply it to a camera trap survey of fisher (Pekania pennanti) and an acoustic survey of the Cape Peninsula moss frog (Arthroleptella lightfooti), each with different types of information used to inform identities.
A fundamental assumption of SCR is that detections of an individual occurs independently. This implies that a detection at one detector has no impact on the probability that an animal is seen at any other detectors immediately after. Assumptions of independence allows us to model SCR detections as a spatiotemporal point process, often a Poisson process. As a result, the time of detection becomes uninformative about animal identity. We offer a solution to this by thinking about SCR in terms of a new detection function that depends on a realistic animal movement model. To do this, we relax the independence assumption by building a spatiotemporal dependent point process with a new detection function that depends on where and when the animal was last observed. As a result, we can explicitly model the existing correlation in the detections as well as provide a new method for spatiotemporal clustering of latent identity SCR problems.Synthetic observational signatures of coronal heating mechanisms from 3D numerical simulations
https://hdl.handle.net/10023/28232
In this thesis, we investigate the synthetic observables from 3D MHD simulations which explore coronal heating mechanisms. These models include the twisting of magnetic flux tubes, the propagation of transverse oscillations through complex braided magnetic fields and a coronal arcade driven by footpoint motions of different characteristic time scales.
Through the use of forward modelling, the numerical model results are transformed into synthetic emission data. Examining such data will teach us more about, and help us identify, the observable features caused by the dynamics and heating of the coronal plasma. It will also help us build a catalogue of characteristics of energy release in the solar corona. This brings numerical models and observations closer together by allowing us to compare models and observations in a meaningful way.
Most of the observables examined within this thesis are as one would expect with knowledge of the plasma parameters (i.e. the density, temperature and velocity field) but some of this information is not readily available from observations. We expected and observed line broadening in regions of fast outflows as a result of magnetic reconnection, high frequency signals when shorter time scale motions are present at the footpoints of a coronal arcade, and the impact the line-of-sight has on estimated kinetic energies. There are also signatures which are not necessarily obvious until the forward modelling is complete but nonetheless are intuitive after the fact. For example, waves helping identify regions of complex magnetic fields; coronal arcade structures are visible in Doppler velocity signatures as well as intensity images, and signatures of Alfvén and fast waves are present within coronal arcades. There is one feature we encounter which could be misinterpreted. During the model which examines transverse oscillations through a braided magnetic field, Doppler signatures are generated which look like those due to torsional motions. In fact, with prior knowledge of the simulation, these are the result of phase mixing and counter-propagating waves through a complex magnetic field.
Finally we round up by examining if there is a relation (more specifically ratio) between wave amplitudes and non-thermal line widths. In order to estimate wave energies, previous studies have used such a relation where the non-thermal line widths are a factor of the square root of 2 smaller than the root mean squared wave amplitudes; however different factors have been used in other studies. We focus on determining whether one true value does exist by examining the simulations already presented in this thesis.
Tue, 14 Jun 2022 00:00:00 GMThttps://hdl.handle.net/10023/282322022-06-14T00:00:00ZFyfe, Lianne ElizabethIn this thesis, we investigate the synthetic observables from 3D MHD simulations which explore coronal heating mechanisms. These models include the twisting of magnetic flux tubes, the propagation of transverse oscillations through complex braided magnetic fields and a coronal arcade driven by footpoint motions of different characteristic time scales.
Through the use of forward modelling, the numerical model results are transformed into synthetic emission data. Examining such data will teach us more about, and help us identify, the observable features caused by the dynamics and heating of the coronal plasma. It will also help us build a catalogue of characteristics of energy release in the solar corona. This brings numerical models and observations closer together by allowing us to compare models and observations in a meaningful way.
Most of the observables examined within this thesis are as one would expect with knowledge of the plasma parameters (i.e. the density, temperature and velocity field) but some of this information is not readily available from observations. We expected and observed line broadening in regions of fast outflows as a result of magnetic reconnection, high frequency signals when shorter time scale motions are present at the footpoints of a coronal arcade, and the impact the line-of-sight has on estimated kinetic energies. There are also signatures which are not necessarily obvious until the forward modelling is complete but nonetheless are intuitive after the fact. For example, waves helping identify regions of complex magnetic fields; coronal arcade structures are visible in Doppler velocity signatures as well as intensity images, and signatures of Alfvén and fast waves are present within coronal arcades. There is one feature we encounter which could be misinterpreted. During the model which examines transverse oscillations through a braided magnetic field, Doppler signatures are generated which look like those due to torsional motions. In fact, with prior knowledge of the simulation, these are the result of phase mixing and counter-propagating waves through a complex magnetic field.
Finally we round up by examining if there is a relation (more specifically ratio) between wave amplitudes and non-thermal line widths. In order to estimate wave energies, previous studies have used such a relation where the non-thermal line widths are a factor of the square root of 2 smaller than the root mean squared wave amplitudes; however different factors have been used in other studies. We focus on determining whether one true value does exist by examining the simulations already presented in this thesis.Combining spatially adaptive statistical modelling methods and computer vision approaches for the automatic detection of animals from high resolution images
https://hdl.handle.net/10023/27871
This study aimed to automate detecting animals in aerial images and improve detection by combining computer vision techniques with statistical modelling of the surveyed area.
Knowing the number of animals in an area is important for wildlife management and existing methods require trained observers in larger planes or photography from smaller aircraft requiring manual counting. Automating detection would allow large areas to be surveyed more frequently with lower human input.
This thesis shows that animals can be automatically detected from aerial images using the YOLO object detection network. Studies ruled out classical computer vision techniques due to excess false positives. The YOLO method detects 61% of the animals compared to 79% detection by humans, however it also detects 11.6 False Positives Per Image (FPPI).
Modelling the distribution of multiple species required a multinomial model. CReSS based GAMs were extended to the multinomial case and simulation studies were carried out to compare CReSS to other multinomial approaches, showing CReSS was preferred for: high noise, low sample sizes or animal densities close to exclusion zones.
Confidence intervals from the statistical model were concatenated with the YOLO model. This reduced the FPPI from 11.6 to 6.6, showing that combining prior knowledge from a statistical model improves performance of animal detection. Manual checking time per image was reduced by 97%, from 5 minutes to 11 seconds. Using the automated detections to guide manual checks spotted additional animals increasing the recall to 0.81, greater than the recall estimated for human performance of 0.79.
The methods described have reduced the estimated manual checking time for the 40,000 photographs covering the 7,500km2 survey area in Namibia from 9 months to 3 weeks, meaning this method could be used frequently to give timely and reliable results.
Tue, 29 Nov 2022 00:00:00 GMThttps://hdl.handle.net/10023/278712022-11-29T00:00:00ZFell, Christina MaryThis study aimed to automate detecting animals in aerial images and improve detection by combining computer vision techniques with statistical modelling of the surveyed area.
Knowing the number of animals in an area is important for wildlife management and existing methods require trained observers in larger planes or photography from smaller aircraft requiring manual counting. Automating detection would allow large areas to be surveyed more frequently with lower human input.
This thesis shows that animals can be automatically detected from aerial images using the YOLO object detection network. Studies ruled out classical computer vision techniques due to excess false positives. The YOLO method detects 61% of the animals compared to 79% detection by humans, however it also detects 11.6 False Positives Per Image (FPPI).
Modelling the distribution of multiple species required a multinomial model. CReSS based GAMs were extended to the multinomial case and simulation studies were carried out to compare CReSS to other multinomial approaches, showing CReSS was preferred for: high noise, low sample sizes or animal densities close to exclusion zones.
Confidence intervals from the statistical model were concatenated with the YOLO model. This reduced the FPPI from 11.6 to 6.6, showing that combining prior knowledge from a statistical model improves performance of animal detection. Manual checking time per image was reduced by 97%, from 5 minutes to 11 seconds. Using the automated detections to guide manual checks spotted additional animals increasing the recall to 0.81, greater than the recall estimated for human performance of 0.79.
The methods described have reduced the estimated manual checking time for the 40,000 photographs covering the 7,500km2 survey area in Namibia from 9 months to 3 weeks, meaning this method could be used frequently to give timely and reliable results.An investigation of the magnetic field structure of flaring solar active regions using global magnetic field models and automated techniques
https://hdl.handle.net/10023/27806
In this thesis we explore methods for analysing flaring active regions, in particular studying X-class flares. We use global magnetic field models and machine learning techniques to carry out this analysis.
Using both potential field source surface (PFSS) models and magnetohydrostatic (MHS) models, the global magnetic skeletons for dates where X-class flares occurred are created. This allows the investigation of topological features found around flaring active regions. The flares analysed all have observable signatures found in Atmospheric Imaging Assembly data (onboard the Solar Dynamics Observatory), in the form of solar flare ribbons which can be mapped by eye to the footpoints of the separatrix structures located in the active regions.
Additionally, we consider techniques for identifying and locating the solar flare ribbons observed. The first technique utilises a convolutional neural network trained using images of M and C-class flares to allow the detection and classification of the types of flare ribbons observed. This includes two-ribbon, compact and limb flares, as well as quiet sun images. After training the network and identifying the flare ribbons in the data, we present an edge detection method which identifies the edges of the flare ribbons, making it easier to compare with the topological features previously found in the global field models. To find the best edge, two methods are presented which correct saturated pixels in the flare ribbon data. Afterwards the corrected images are passed into the edge detector which returns the ribbon edges, which are subsequently compared to the topological features previously found by calculating the Hausdorff and modified Hausdorff distances.
Overall these methods could be put into an automated pipeline which would identify solar flare ribbons in the observations using a CNN, then subsequently creating 3D magnetic field models to investigate the topology around the flare. With the final step taking both the observational and modelled data to be processed by the edge detection method and subsequently outputting a metric which identifies whether they are related. Note however this pipeline was not created in this thesis.
Tue, 14 Jun 2022 00:00:00 GMThttps://hdl.handle.net/10023/278062022-06-14T00:00:00ZLove, TeriIn this thesis we explore methods for analysing flaring active regions, in particular studying X-class flares. We use global magnetic field models and machine learning techniques to carry out this analysis.
Using both potential field source surface (PFSS) models and magnetohydrostatic (MHS) models, the global magnetic skeletons for dates where X-class flares occurred are created. This allows the investigation of topological features found around flaring active regions. The flares analysed all have observable signatures found in Atmospheric Imaging Assembly data (onboard the Solar Dynamics Observatory), in the form of solar flare ribbons which can be mapped by eye to the footpoints of the separatrix structures located in the active regions.
Additionally, we consider techniques for identifying and locating the solar flare ribbons observed. The first technique utilises a convolutional neural network trained using images of M and C-class flares to allow the detection and classification of the types of flare ribbons observed. This includes two-ribbon, compact and limb flares, as well as quiet sun images. After training the network and identifying the flare ribbons in the data, we present an edge detection method which identifies the edges of the flare ribbons, making it easier to compare with the topological features previously found in the global field models. To find the best edge, two methods are presented which correct saturated pixels in the flare ribbon data. Afterwards the corrected images are passed into the edge detector which returns the ribbon edges, which are subsequently compared to the topological features previously found by calculating the Hausdorff and modified Hausdorff distances.
Overall these methods could be put into an automated pipeline which would identify solar flare ribbons in the observations using a CNN, then subsequently creating 3D magnetic field models to investigate the topology around the flare. With the final step taking both the observational and modelled data to be processed by the edge detection method and subsequently outputting a metric which identifies whether they are related. Note however this pipeline was not created in this thesis.Mathematical models of atmospheric convection : a Lagrangian perspective
https://hdl.handle.net/10023/27696
This thesis presents extensions and enhancements to an existing Lagrangian model for atmospheric convection. The Moist Parcel-In-Cell (MPIC) method, developed by Dritschel et al. (2018), is a novel approach which avoids some shortcomings of
conventional large-eddy simulation models, particularly concerning the representation of sub-grid turbulence. While the method is in a relatively early stage, we provide case studies to show the model’s potential.
The first case study simulates the ascent of a rising thermal, subject to constant vertical wind shear. Using MPIC, we find that low to intermediate shear appears to promote cloud growth, but high shear tears the thermal apart. The air is
partitioned into cloud-air and dry-air components based on the liquid water content and the evolution of the enstrophy associated with each component suggests that the increasing shear is causing more dry air to become turbulent and influence the growth of the cloud.
Our second study analyses the potential energy in MPIC and its sensitivity to numerical parameters. The results show an abnormal growth in the total energy at early times, which is attributed to a failure to enforce incompressibility in
the method. The energy evolutions appear to converge with resolution and the numerical mixing parameters in the model for early to intermediate times, although the turbulent flow leads to greater discrepancies in the late-stage energy evolution.
Finally, we present a simulation of the shear-free atmospheric boundary layer, modelled through the implementation of surface fluxes of parcel attributes in MPIC. Our results show evidence of the two-layer entrainment zone structure observed in previous studies. Comparisons of the vertical enstrophy distribution and vorticity field further support this. We also compute entrainment rate parameters that suggest that MPIC underestimates entrainment compared to the zero-order model and overestimates entrainment when using the global buoyancy increment across the entire entrainment zone. Nonetheless, the entrainment rate in MPIC compares favourably to results in the literature when using a local buoyancy increment computed at the height of minimum buoyancy flux.
Tue, 13 Jun 2023 00:00:00 GMThttps://hdl.handle.net/10023/276962023-06-13T00:00:00ZWallace, Samuel JosephThis thesis presents extensions and enhancements to an existing Lagrangian model for atmospheric convection. The Moist Parcel-In-Cell (MPIC) method, developed by Dritschel et al. (2018), is a novel approach which avoids some shortcomings of
conventional large-eddy simulation models, particularly concerning the representation of sub-grid turbulence. While the method is in a relatively early stage, we provide case studies to show the model’s potential.
The first case study simulates the ascent of a rising thermal, subject to constant vertical wind shear. Using MPIC, we find that low to intermediate shear appears to promote cloud growth, but high shear tears the thermal apart. The air is
partitioned into cloud-air and dry-air components based on the liquid water content and the evolution of the enstrophy associated with each component suggests that the increasing shear is causing more dry air to become turbulent and influence the growth of the cloud.
Our second study analyses the potential energy in MPIC and its sensitivity to numerical parameters. The results show an abnormal growth in the total energy at early times, which is attributed to a failure to enforce incompressibility in
the method. The energy evolutions appear to converge with resolution and the numerical mixing parameters in the model for early to intermediate times, although the turbulent flow leads to greater discrepancies in the late-stage energy evolution.
Finally, we present a simulation of the shear-free atmospheric boundary layer, modelled through the implementation of surface fluxes of parcel attributes in MPIC. Our results show evidence of the two-layer entrainment zone structure observed in previous studies. Comparisons of the vertical enstrophy distribution and vorticity field further support this. We also compute entrainment rate parameters that suggest that MPIC underestimates entrainment compared to the zero-order model and overestimates entrainment when using the global buoyancy increment across the entire entrainment zone. Nonetheless, the entrainment rate in MPIC compares favourably to results in the literature when using a local buoyancy increment computed at the height of minimum buoyancy flux.Solving decision problems in finitely presented groups via generalized small cancellation theory
https://hdl.handle.net/10023/27549
This thesis studies two decision problems for finitely presented groups. Using a standard RAM model of computation, in which the basic arithmetical operations on integers are assumed to take constant time, in Part I we develop an algorithm IsConjugate, which on input a (finite) presentation defining a hyperbolic group 𝐺, correctly decides whether 𝑤₁ ϵ 𝑋* and 𝑤₂ ϵ 𝑋* are conjugate in 𝐺, and if so, then for each 𝑖 ϵ {1,2}, returns a cyclically reduced word 𝑟ᵢ that is conjugate in 𝐺 to 𝑤ᵢ, and an 𝑥 ϵ 𝑋* such that r₂= G 𝑥^{-1} r_1 x (hence if 𝑤₁ and 𝑤₂ are already cyclically reduced, then it returns an 𝑥 ϵ 𝑋* such that 𝑤₂=_G x^{-1} w_1 x). Moreover, IsConjugate can be constructed in polynomial-time in the input presentation < 𝑋 | 𝑅 >, and IsConjugate runs in time O((|𝑤₁| + |𝑤₂| min{|𝑤₁|, |𝑤₂|}).
IsConjugate has been implemented in the MAGMA software, and our experiments show that the run times agree with the worst-case time complexities. Thus, IsConjugate is the most efficient general practically implementable conjugacy problem solver for hyperbolic groups.
It is undecidable in general whether a given finitely presented group is hyperbolic. In Part II of this thesis, we present a polynomial-time procedure VerifyHypVertex which on input a finite presentation for a group G, returns true only if G is hyperbolic. VerifyHypVertex generalizes the methods from [34], and in particular succeeds on all presentations on which the implementation from [34] succeeds, and many additional presentations as well. The algorithms have been implemented in MAGMA, and the experiments show that they return a positive answer on many examples on which other comparable publicly available methods fail, such as KBMAG.
Tue, 13 Jun 2023 00:00:00 GMThttps://hdl.handle.net/10023/275492023-06-13T00:00:00ZJurina, SimonThis thesis studies two decision problems for finitely presented groups. Using a standard RAM model of computation, in which the basic arithmetical operations on integers are assumed to take constant time, in Part I we develop an algorithm IsConjugate, which on input a (finite) presentation defining a hyperbolic group 𝐺, correctly decides whether 𝑤₁ ϵ 𝑋* and 𝑤₂ ϵ 𝑋* are conjugate in 𝐺, and if so, then for each 𝑖 ϵ {1,2}, returns a cyclically reduced word 𝑟ᵢ that is conjugate in 𝐺 to 𝑤ᵢ, and an 𝑥 ϵ 𝑋* such that r₂= G 𝑥^{-1} r_1 x (hence if 𝑤₁ and 𝑤₂ are already cyclically reduced, then it returns an 𝑥 ϵ 𝑋* such that 𝑤₂=_G x^{-1} w_1 x). Moreover, IsConjugate can be constructed in polynomial-time in the input presentation < 𝑋 | 𝑅 >, and IsConjugate runs in time O((|𝑤₁| + |𝑤₂| min{|𝑤₁|, |𝑤₂|}).
IsConjugate has been implemented in the MAGMA software, and our experiments show that the run times agree with the worst-case time complexities. Thus, IsConjugate is the most efficient general practically implementable conjugacy problem solver for hyperbolic groups.
It is undecidable in general whether a given finitely presented group is hyperbolic. In Part II of this thesis, we present a polynomial-time procedure VerifyHypVertex which on input a finite presentation for a group G, returns true only if G is hyperbolic. VerifyHypVertex generalizes the methods from [34], and in particular succeeds on all presentations on which the implementation from [34] succeeds, and many additional presentations as well. The algorithms have been implemented in MAGMA, and the experiments show that they return a positive answer on many examples on which other comparable publicly available methods fail, such as KBMAG.Solar cycle variation of photospheric and chromospheric magnetic and ultraviolet emission features observed by the Solar Dynamics Observatory
https://hdl.handle.net/10023/27370
The solar magnetic field exhibits cyclic behaviour over a period of 22 years, continually reprocessing the poloidal, dipolar magnetic field into toroidal, quadrupolar magnetic field and vice versa. The cyclic behaviour of the solar magnetic field has been revealed through long-term measurements of the photospheric magnetic field strength. In addition, the long-term behaviour of phenomena associated with the magnetic field, such as sunspots and coronal loops, can be observed through ultraviolet emission measurements. We use statistical tools to analyse magnetic field strength and ultraviolet emission intensity data from the Solar Dynamics Observatory to determine the variation of photospheric and chromospheric magnetic and ultraviolet emission features over a full solar cycle. The nature of the observed distributions of these features at different times during the solar cycle may contribute towards a better understanding of the magnetic field generation mechanisms. This contribution may be in the form of theoretical interpretation of the physical processes which lead to such distributions or by providing physical constraints to numerical models of the solar dynamo, turbulent convection, and flux emergence. We consider a number of statistical models including a single power law, a truncated Weibull-lognormal, and a smooth double power law, amongst others. We investigate the plausibility and goodness-of-fit of such models for four full cycle datasets; magnetic features observed by the Helioseismic and Magnetic Imager, and emission features observed by the 304\AA, 1600\AA, and 1700\AA\ channels of the Atmospheric Imaging Assembly. We determine that a double power law performs well over the full solar cycle in all four cases and discuss the potential implications of a double power law distribution for solar magnetic field generation. We propose that the double power law is a suitable fit as the flexibility of two separate power law regimes accurately reflects the physical conditions which produce the observed magnetic field and ultraviolet emission features.
Tue, 29 Nov 2022 00:00:00 GMThttps://hdl.handle.net/10023/273702022-11-29T00:00:00ZNoble, Callan NicholasThe solar magnetic field exhibits cyclic behaviour over a period of 22 years, continually reprocessing the poloidal, dipolar magnetic field into toroidal, quadrupolar magnetic field and vice versa. The cyclic behaviour of the solar magnetic field has been revealed through long-term measurements of the photospheric magnetic field strength. In addition, the long-term behaviour of phenomena associated with the magnetic field, such as sunspots and coronal loops, can be observed through ultraviolet emission measurements. We use statistical tools to analyse magnetic field strength and ultraviolet emission intensity data from the Solar Dynamics Observatory to determine the variation of photospheric and chromospheric magnetic and ultraviolet emission features over a full solar cycle. The nature of the observed distributions of these features at different times during the solar cycle may contribute towards a better understanding of the magnetic field generation mechanisms. This contribution may be in the form of theoretical interpretation of the physical processes which lead to such distributions or by providing physical constraints to numerical models of the solar dynamo, turbulent convection, and flux emergence. We consider a number of statistical models including a single power law, a truncated Weibull-lognormal, and a smooth double power law, amongst others. We investigate the plausibility and goodness-of-fit of such models for four full cycle datasets; magnetic features observed by the Helioseismic and Magnetic Imager, and emission features observed by the 304\AA, 1600\AA, and 1700\AA\ channels of the Atmospheric Imaging Assembly. We determine that a double power law performs well over the full solar cycle in all four cases and discuss the potential implications of a double power law distribution for solar magnetic field generation. We propose that the double power law is a suitable fit as the flexibility of two separate power law regimes accurately reflects the physical conditions which produce the observed magnetic field and ultraviolet emission features.Title redacted
https://hdl.handle.net/10023/27008
Abstract redacted
Tue, 28 Nov 2023 00:00:00 GMThttps://hdl.handle.net/10023/270082023-11-28T00:00:00ZFranchini, Filippo Luciano CharlyAbstract redactedLimit sets, Julia sets and Sullivan’s dictionary : a dimension theoretic analysis
https://hdl.handle.net/10023/26908
This thesis includes work from four papers that were written during the author’s time as a PhD student
with Jonathan Fraser, namely [40, 41, 42, 43]. Chapter 1 introduces the two main settings that will
be studied throughout this thesis along with several tools that will be used. This will include various
notions of dimensions of sets and measures, the setting of hyperbolic geometry and limit sets, and
the setting of rational maps and Julia sets. Chapter 2 will state and prove results in the hyperbolic
geometry setting, where we calculate the Assouad and lower spectra for limit sets of geometrically finite
Kleinian groups along with their associated Patterson-Sullivan measure. The broad approach takes
some ideas from [35] where the Assouad and lower dimensions were calculated, but many of the ideas
require adjustment or replacement due to the Assouad and lower spectra requiring finer control. An
important tool made use of is the notion of a ‘global measure formula’ in order to obtain estimates on
efficient covers. Chapter 3 involves adapting this approach to calculate the Assouad type dimensions
of Julia sets of parabolic rational maps and their associated h-conformal measures, where h denotes
the Hausdorff dimension of the Julia set. Chapter 4 is then dedicated to a discussion of the results in
Chapters 2 and 3 in the context of Sullivan’s dictionary, a framework which draws many connections
between the settings of hyperbolic geometry and rational maps. We draw several interesting parallels
between the two settings, along with some notable differences, using our results on the Assouad type
dimensions which are not witnessed by other notions of dimension. In Chapter 5, we obtain results for
counting horoballs of certain sizes, and then discuss some applications of these results to Diophantine
approximation and the calculation of dimensions of conformal measures. We finish in Chapter 6 with
discussion about further questions which stem from our research.
Tue, 13 Jun 2023 00:00:00 GMThttps://hdl.handle.net/10023/269082023-06-13T00:00:00ZStuart, LiamThis thesis includes work from four papers that were written during the author’s time as a PhD student
with Jonathan Fraser, namely [40, 41, 42, 43]. Chapter 1 introduces the two main settings that will
be studied throughout this thesis along with several tools that will be used. This will include various
notions of dimensions of sets and measures, the setting of hyperbolic geometry and limit sets, and
the setting of rational maps and Julia sets. Chapter 2 will state and prove results in the hyperbolic
geometry setting, where we calculate the Assouad and lower spectra for limit sets of geometrically finite
Kleinian groups along with their associated Patterson-Sullivan measure. The broad approach takes
some ideas from [35] where the Assouad and lower dimensions were calculated, but many of the ideas
require adjustment or replacement due to the Assouad and lower spectra requiring finer control. An
important tool made use of is the notion of a ‘global measure formula’ in order to obtain estimates on
efficient covers. Chapter 3 involves adapting this approach to calculate the Assouad type dimensions
of Julia sets of parabolic rational maps and their associated h-conformal measures, where h denotes
the Hausdorff dimension of the Julia set. Chapter 4 is then dedicated to a discussion of the results in
Chapters 2 and 3 in the context of Sullivan’s dictionary, a framework which draws many connections
between the settings of hyperbolic geometry and rational maps. We draw several interesting parallels
between the two settings, along with some notable differences, using our results on the Assouad type
dimensions which are not witnessed by other notions of dimension. In Chapter 5, we obtain results for
counting horoballs of certain sizes, and then discuss some applications of these results to Diophantine
approximation and the calculation of dimensions of conformal measures. We finish in Chapter 6 with
discussion about further questions which stem from our research.Diameters of graphs related to groups and base sizes of primitive groups
https://hdl.handle.net/10023/26895
In this thesis, we study three problems. First, we determine new bounds for base sizes b(G,Ω) of primitive subspace actions of finite almost simple classical groups G. Such base sizes are useful statistics in computational group theory. We show that if the underlying set Ω consists of k-dimensional subspaces of the natural module V = F_q^n for G, then b(G,Ω) ≥ ⌈n/k⌉ + c, where c ∈ {-2,-1,0,1} depends on n, q, k and the type of G. If instead Ω consists of pairs {X,Y} of subspaces of V with k:=dim(X) < dim(Y), and G is generated by PGL(n,q) and the graph automorphism of PSL(n,q), then b(G,Ω) ≤ max{⌈n/k⌉,4}.
The second part of the thesis concerns the intersection graph Δ_G of a finite simple group G. This graph has vertices the nontrivial proper subgroups of G, and its edges are the pairs of subgroups that intersect nontrivially. We prove that Δ_G has diameter at most 5, and that a diameter of 5 is achieved only by the graphs of the baby monster group and certain unitary groups of odd prime dimension. This answers a question posed by Shen.
Finally, we study the non-commuting, non-generating graph Ξ(G) of a group G, where G/Z(G) is either finite or non-simple. This graph is closely related to the hierarchy of graphs introduced by Cameron. The graph's vertices are the non-central elements of G, and its edges are the pairs {x,y} such that ⟨x, y⟩ ≠ G and xy ≠ yx. We show that if Ξ(G) has an edge, then either the graph is connected with diameter at most 5; the graph has exactly two connected components, each of diameter 2; or the graph consists of isolated vertices and a component of diameter at most 4. In this last case, either the nontrivial component has diameter 2, or G/Z(G) is a non-simple insoluble primitive group with every proper quotient cyclic.
Tue, 29 Nov 2022 00:00:00 GMThttps://hdl.handle.net/10023/268952022-11-29T00:00:00ZFreedman, Saul DanielIn this thesis, we study three problems. First, we determine new bounds for base sizes b(G,Ω) of primitive subspace actions of finite almost simple classical groups G. Such base sizes are useful statistics in computational group theory. We show that if the underlying set Ω consists of k-dimensional subspaces of the natural module V = F_q^n for G, then b(G,Ω) ≥ ⌈n/k⌉ + c, where c ∈ {-2,-1,0,1} depends on n, q, k and the type of G. If instead Ω consists of pairs {X,Y} of subspaces of V with k:=dim(X) < dim(Y), and G is generated by PGL(n,q) and the graph automorphism of PSL(n,q), then b(G,Ω) ≤ max{⌈n/k⌉,4}.
The second part of the thesis concerns the intersection graph Δ_G of a finite simple group G. This graph has vertices the nontrivial proper subgroups of G, and its edges are the pairs of subgroups that intersect nontrivially. We prove that Δ_G has diameter at most 5, and that a diameter of 5 is achieved only by the graphs of the baby monster group and certain unitary groups of odd prime dimension. This answers a question posed by Shen.
Finally, we study the non-commuting, non-generating graph Ξ(G) of a group G, where G/Z(G) is either finite or non-simple. This graph is closely related to the hierarchy of graphs introduced by Cameron. The graph's vertices are the non-central elements of G, and its edges are the pairs {x,y} such that ⟨x, y⟩ ≠ G and xy ≠ yx. We show that if Ξ(G) has an edge, then either the graph is connected with diameter at most 5; the graph has exactly two connected components, each of diameter 2; or the graph consists of isolated vertices and a component of diameter at most 4. In this last case, either the nontrivial component has diameter 2, or G/Z(G) is a non-simple insoluble primitive group with every proper quotient cyclic.Synchronising and separating permutation groups through graphs
https://hdl.handle.net/10023/26470
About 15 years ago, Araújo, Arnold and Steinberg introduced the notion of synchronisation to the theory of finite permutation groups. Synchronisation property is closely related to another property which is called separation, but they are not the same. Interestingly, the study of the two properties for finite groups involves many combinatorial problems. In this thesis, we tried to extend the current knowledge about synchronising and separating groups and suggest some questions. The introduction and the background are represented in Chapter 1 and Chapter 2, respectively. The main work is divided into three chapters.
In Chapter 3, we started by extending the notions of synchronisation and separation to association schemes. Then, we considered two important families of almost simple permutation groups. Firstly, the group 𝐺 induced by the action of the symmetric group Sym(𝑛) on the set Ω of 𝑘-element subsets of an 𝑛-set, say {1, ..., 𝑛} (we call this the first group). Secondly, the group 𝐺 induced by the action of the symmetric group Sym(𝑛) on the set Ω of uniform 𝑙-partitions of an 𝑛-set, {1, ..., 𝑛}, into subsets of size 𝑘 where 𝑛 = 𝑘𝑙 (we call this the second group).
For first group, when 𝑘 = 2, 3, 4 and 5, we showed that for large enough 𝑛 the group is non-separating (resp. non-synchronizing) if and only if there is a Steiner system S(𝑡, 𝑘, 𝑛) (resp. large set) for some 𝑡 < 𝑘. In general, we stated a conjecture that is if true would be a crucial extension of the remarkable result by Peter Keevash that considers the existence of Steiner systems. For the second group, we gave similar results to the first group when 𝐾 = 2, 3, 4, 5, 6 and 𝑙 = 2. We stated conjecture for 𝑘 > 6 and 𝑙 = 2. Also, we showed that the group is non-synchronising when 𝑙 > 2.
In Chapter 4, the synchronisation property of a ne distance transitive permutation groups is considered. We showed that the separation and the synchronising properties are equivalent for a ne groups. We determined when some groups are synchronising, for example, automorphism groups of Hamming graphs, halved graphs, folded halved graphs, bilinear form graphs, some alternating form graphs and cosets graphs of some Golay codes. In addition, we stated a conjecture for distance regular graphs which connects this chapter and the previous one.
In Chapter 5, we started by defining the diagonal factorisation of finite groups and proved some related basic results. Then, we showed that the diagonal group 𝐷(𝑇, 2) is non-separating if and only if 𝑇 admits a diagonal factorisation. Also, we showed that the group 𝐷(𝑇, 2) is non-separating when 𝑇 = 𝐴ₙ. We proved that the diagonal group 𝐷(𝑇, 𝑑) for 𝑑 ≥ 3, is non-synchronising. In the last section, we showed the equivalence between the separation and the synchronisation properties for groups of diagonal types.
Tue, 14 Jun 2022 00:00:00 GMThttps://hdl.handle.net/10023/264702022-06-14T00:00:00ZAljohani, Mohammed HamoudAbout 15 years ago, Araújo, Arnold and Steinberg introduced the notion of synchronisation to the theory of finite permutation groups. Synchronisation property is closely related to another property which is called separation, but they are not the same. Interestingly, the study of the two properties for finite groups involves many combinatorial problems. In this thesis, we tried to extend the current knowledge about synchronising and separating groups and suggest some questions. The introduction and the background are represented in Chapter 1 and Chapter 2, respectively. The main work is divided into three chapters.
In Chapter 3, we started by extending the notions of synchronisation and separation to association schemes. Then, we considered two important families of almost simple permutation groups. Firstly, the group 𝐺 induced by the action of the symmetric group Sym(𝑛) on the set Ω of 𝑘-element subsets of an 𝑛-set, say {1, ..., 𝑛} (we call this the first group). Secondly, the group 𝐺 induced by the action of the symmetric group Sym(𝑛) on the set Ω of uniform 𝑙-partitions of an 𝑛-set, {1, ..., 𝑛}, into subsets of size 𝑘 where 𝑛 = 𝑘𝑙 (we call this the second group).
For first group, when 𝑘 = 2, 3, 4 and 5, we showed that for large enough 𝑛 the group is non-separating (resp. non-synchronizing) if and only if there is a Steiner system S(𝑡, 𝑘, 𝑛) (resp. large set) for some 𝑡 < 𝑘. In general, we stated a conjecture that is if true would be a crucial extension of the remarkable result by Peter Keevash that considers the existence of Steiner systems. For the second group, we gave similar results to the first group when 𝐾 = 2, 3, 4, 5, 6 and 𝑙 = 2. We stated conjecture for 𝑘 > 6 and 𝑙 = 2. Also, we showed that the group is non-synchronising when 𝑙 > 2.
In Chapter 4, the synchronisation property of a ne distance transitive permutation groups is considered. We showed that the separation and the synchronising properties are equivalent for a ne groups. We determined when some groups are synchronising, for example, automorphism groups of Hamming graphs, halved graphs, folded halved graphs, bilinear form graphs, some alternating form graphs and cosets graphs of some Golay codes. In addition, we stated a conjecture for distance regular graphs which connects this chapter and the previous one.
In Chapter 5, we started by defining the diagonal factorisation of finite groups and proved some related basic results. Then, we showed that the diagonal group 𝐷(𝑇, 2) is non-separating if and only if 𝑇 admits a diagonal factorisation. Also, we showed that the group 𝐷(𝑇, 2) is non-separating when 𝑇 = 𝐴ₙ. We proved that the diagonal group 𝐷(𝑇, 𝑑) for 𝑑 ≥ 3, is non-synchronising. In the last section, we showed the equivalence between the separation and the synchronisation properties for groups of diagonal types.Advances on priors for the Dirichlet process mixture model with Gaussian kernels
https://hdl.handle.net/10023/26299
Abstract redacted
Tue, 29 Nov 2022 00:00:00 GMThttps://hdl.handle.net/10023/262992022-11-29T00:00:00ZJing, WeiAbstract redactedApplications of likelihood-free parameter inference methods on numerical models of cancer invasion
https://hdl.handle.net/10023/25952
In this thesis, we present two different methods to estimate parameters within a Partial Differential Equation (PDE) model of cancer invasion and an Individual-based Model (IBM) derived from it. The PDE model was fitted to synthetic spatial 1D data generated from the model, and the IBM was fitted to authentic spatial 2D data derived from the invasion patterns observed in in vitro and ex vivo organotypic assays. The first estimation method is a likelihood-free approach related to Approximate Bayesian Computation (ABC). The second is a two-stage gradient matching method based on smoothing the data with a Generalized Additive Model (GAM) and matching gradients from the GAM to those from the model. Both methods performed well on spatial 1D synthetic data when the synthetic data was generated assuming no measurement errors. To increase realism we tested both methods with simulated measurement errors, and found that the ability to estimate some model parameters deteriorated rapidly as measurement error increased for the gradient matching method. The ABC method was more robust to the introduction of measurement errors.
For spatial 2D authentic data, we only applied the ABC method. The simulated patterns produced at the end of the estimation procedure were quantitatively close to the observed ones. In addition, most of the final parameter samples obtained in the spatial 2D inference passed a set of ABC posterior diagnostics, implying they are valid posteriors from the perspective of Bayesian inference. The ABC-derived calibration methods developed in this thesis are not limited to models of cancer invasion alone, and can potentially be applied in a wide variety of application areas where the system under examination can be described using PDE models.
Wed, 27 Jul 2022 00:00:00 GMThttps://hdl.handle.net/10023/259522022-07-27T00:00:00ZXiao, YunchenIn this thesis, we present two different methods to estimate parameters within a Partial Differential Equation (PDE) model of cancer invasion and an Individual-based Model (IBM) derived from it. The PDE model was fitted to synthetic spatial 1D data generated from the model, and the IBM was fitted to authentic spatial 2D data derived from the invasion patterns observed in in vitro and ex vivo organotypic assays. The first estimation method is a likelihood-free approach related to Approximate Bayesian Computation (ABC). The second is a two-stage gradient matching method based on smoothing the data with a Generalized Additive Model (GAM) and matching gradients from the GAM to those from the model. Both methods performed well on spatial 1D synthetic data when the synthetic data was generated assuming no measurement errors. To increase realism we tested both methods with simulated measurement errors, and found that the ability to estimate some model parameters deteriorated rapidly as measurement error increased for the gradient matching method. The ABC method was more robust to the introduction of measurement errors.
For spatial 2D authentic data, we only applied the ABC method. The simulated patterns produced at the end of the estimation procedure were quantitatively close to the observed ones. In addition, most of the final parameter samples obtained in the spatial 2D inference passed a set of ABC posterior diagnostics, implying they are valid posteriors from the perspective of Bayesian inference. The ABC-derived calibration methods developed in this thesis are not limited to models of cancer invasion alone, and can potentially be applied in a wide variety of application areas where the system under examination can be described using PDE models.Base size and generating graphs of primitive permutation groups
https://hdl.handle.net/10023/25826
In this thesis we consider base size and properties of the generating graph for finite groups.
Let Ω = {1,...,n}, let Sₙ = Sym({1,...,n}) and let G ≤ Sₙ. A base for G is a sequence Λ = (ω₁, . . . , ωₖ) of points in Ω such that the pointwise stabilizer, G_{ω₁,...,ωₖ} , is the identity. The base size of G, denoted by b(G, Ω) or b(G), is the length of the shortest base. We say that Λ is an irredundant base if
G > G_{ω₁} > G_{ω₁,ω₂} > ··· > G_{ω₁,ω₂,...,ωₖ} = 1.
If no irredundant base is longer than Λ, then we say that Λ is a maximal irredundant base for G and denote its length by I(G). A group is called large base if it is either a product action or almost simple group, and its socle is one or more copies of the alternating group Aᵣ acting on k-sets.
Let G be a primitive subgroup of Sₙ that is not large base. We prove that any irredundant base for G has size at most 5log₂n. This bound is best possible up to a small multiplicative constant and is the first logarithmic bound on the size of an irredundant base for such groups. We show that for any constant c, there are infinitely many primitive groups with maximal irredundant base size at least c times the minimal base size. As a corollary of the first result, the relational complexity of G, denoted RC(G) (see Definition 2.2.10), is at most 5log₂n + 1. In addition the maximal size of a minimal base and the height, denoted B(G) and H(G) (see Definitions 2.2.1 and 2.2.5), are both at most 5log₂n. Furthermore, we deduce that a base for G of size at most 5log₂n can be computed in polynomial time.
The generating graph Γ(G) of a finite group G has vertex set the non-identity elements of G, with two elements connected exactly when they generate G. A coclique in a graph is an empty induced subgraph, so a coclique in Γ(G) is a subset of G such that no pair of elements generate G. A coclique is maximal if it is contained in no larger coclique. It is easy to see that the non-identity elements of a maximal subgroup of G form a coclique in Γ(G), but this coclique need not be maximal.
Let G = Sₙ or Aₙ. We first determine when the intransitive maximal subgroups of G are maximal cocliques in Γ(G), and when they are not we find the unique maximal coclique in which they are contained. We then show that for sufficiently large n, the imprimitive maximal subgroups of G are all maximal cocliques in Γ(G).
In addition, using the result on intransitive maximal subgroups we prove that a conjecture of Cameron, Lucchini, and Roney-Dougal holds for G under certain restrictions on n. Namely we prove that two elements of G have identical sets of neighbours in Γ(G) if and only if they belong to exactly the same maximal subgroups. Finally under another set of restrictions on n we then determine precisely which maximal subgroups are maximal cocliques in Γ(G).
Tue, 14 Jun 2022 00:00:00 GMThttps://hdl.handle.net/10023/258262022-06-14T00:00:00ZKelsey, VeronicaIn this thesis we consider base size and properties of the generating graph for finite groups.
Let Ω = {1,...,n}, let Sₙ = Sym({1,...,n}) and let G ≤ Sₙ. A base for G is a sequence Λ = (ω₁, . . . , ωₖ) of points in Ω such that the pointwise stabilizer, G_{ω₁,...,ωₖ} , is the identity. The base size of G, denoted by b(G, Ω) or b(G), is the length of the shortest base. We say that Λ is an irredundant base if
G > G_{ω₁} > G_{ω₁,ω₂} > ··· > G_{ω₁,ω₂,...,ωₖ} = 1.
If no irredundant base is longer than Λ, then we say that Λ is a maximal irredundant base for G and denote its length by I(G). A group is called large base if it is either a product action or almost simple group, and its socle is one or more copies of the alternating group Aᵣ acting on k-sets.
Let G be a primitive subgroup of Sₙ that is not large base. We prove that any irredundant base for G has size at most 5log₂n. This bound is best possible up to a small multiplicative constant and is the first logarithmic bound on the size of an irredundant base for such groups. We show that for any constant c, there are infinitely many primitive groups with maximal irredundant base size at least c times the minimal base size. As a corollary of the first result, the relational complexity of G, denoted RC(G) (see Definition 2.2.10), is at most 5log₂n + 1. In addition the maximal size of a minimal base and the height, denoted B(G) and H(G) (see Definitions 2.2.1 and 2.2.5), are both at most 5log₂n. Furthermore, we deduce that a base for G of size at most 5log₂n can be computed in polynomial time.
The generating graph Γ(G) of a finite group G has vertex set the non-identity elements of G, with two elements connected exactly when they generate G. A coclique in a graph is an empty induced subgraph, so a coclique in Γ(G) is a subset of G such that no pair of elements generate G. A coclique is maximal if it is contained in no larger coclique. It is easy to see that the non-identity elements of a maximal subgroup of G form a coclique in Γ(G), but this coclique need not be maximal.
Let G = Sₙ or Aₙ. We first determine when the intransitive maximal subgroups of G are maximal cocliques in Γ(G), and when they are not we find the unique maximal coclique in which they are contained. We then show that for sufficiently large n, the imprimitive maximal subgroups of G are all maximal cocliques in Γ(G).
In addition, using the result on intransitive maximal subgroups we prove that a conjecture of Cameron, Lucchini, and Roney-Dougal holds for G under certain restrictions on n. Namely we prove that two elements of G have identical sets of neighbours in Γ(G) if and only if they belong to exactly the same maximal subgroups. Finally under another set of restrictions on n we then determine precisely which maximal subgroups are maximal cocliques in Γ(G).Numerical experiments on reconnection in magnetic field conﬁgurations containing null points and separators
https://hdl.handle.net/10023/25676
Magnetic ﬁelds containing null points offer favourable conditions for reconnection.
In this thesis, numerical experiments attempt to gain insight into the reconnection
process at these topological features.
Null point reconnection is studied under the assumption that the initial state is
a quasi-equilibrium. A non-equilibrium ﬁeld with a current density component
parallel to the fan plane is relaxed ideally to obtain the initial condition, forming
fan current layers centred on the null point. Rapid reconnection occurs at the
beginning of the resistive experiment, before there is a transition to impulsive
behaviour. The orientation of the initial current density relative to the strongest
magnetic ﬁeld in the fan plane determines the current layer dimensions and also
inﬂuences the reconnection rate.
A single separator magnetic ﬁeld is derived from the 2D Corrugated Sheet Pinch
by the addition of a perturbation magnetic ﬁeld. The evolution of the system is
investigated numerically, using the 3D ﬁeld with the pressure proﬁle from the 2D
case. The current is transferred from the original current sheet to the separatrix
surfaces and null point bifurcations occur.
A potential double separator ﬁeld is used as the basis for a study of reconnection at
multiply-connected null points. The potential ﬁeld is perturbed by the addition
of ﬂux rings and ideal relaxation results in a quasi-equilibrium with separator
current layers. In the resistive experiment, reconnection occurs at the centre of
the separators and its effects are localised.
In each of the experiments containing a single null point, reconnection occurs at
current layers in the vicinity of the null. When there are multiple nulls connected
by separators, the reconnection often takes place away from the nulls. However,
the dynamics of separator reconnection appear to be inﬂuenced by the choice of
initial conditions.
Tue, 26 Oct 2021 00:00:00 GMThttps://hdl.handle.net/10023/256762021-10-26T00:00:00ZChambers, Daniel ThomasMagnetic ﬁelds containing null points offer favourable conditions for reconnection.
In this thesis, numerical experiments attempt to gain insight into the reconnection
process at these topological features.
Null point reconnection is studied under the assumption that the initial state is
a quasi-equilibrium. A non-equilibrium ﬁeld with a current density component
parallel to the fan plane is relaxed ideally to obtain the initial condition, forming
fan current layers centred on the null point. Rapid reconnection occurs at the
beginning of the resistive experiment, before there is a transition to impulsive
behaviour. The orientation of the initial current density relative to the strongest
magnetic ﬁeld in the fan plane determines the current layer dimensions and also
inﬂuences the reconnection rate.
A single separator magnetic ﬁeld is derived from the 2D Corrugated Sheet Pinch
by the addition of a perturbation magnetic ﬁeld. The evolution of the system is
investigated numerically, using the 3D ﬁeld with the pressure proﬁle from the 2D
case. The current is transferred from the original current sheet to the separatrix
surfaces and null point bifurcations occur.
A potential double separator ﬁeld is used as the basis for a study of reconnection at
multiply-connected null points. The potential ﬁeld is perturbed by the addition
of ﬂux rings and ideal relaxation results in a quasi-equilibrium with separator
current layers. In the resistive experiment, reconnection occurs at the centre of
the separators and its effects are localised.
In each of the experiments containing a single null point, reconnection occurs at
current layers in the vicinity of the null. When there are multiple nulls connected
by separators, the reconnection often takes place away from the nulls. However,
the dynamics of separator reconnection appear to be inﬂuenced by the choice of
initial conditions.On singular pencils of matrices
https://hdl.handle.net/10023/24945
"This thesis is a study of Singular Matrix Pencils under various aspects. In part (I) a new derivation of the Canonical Form of matrix pencils is given. This suggests investigation of the transformations of a pencil into itself (part (II)). Finally, part (III) deals with the canonical form of singular pencils of special types, namely those whose members are induced (or invariant) matrices." -- Preface
Wed, 01 Jan 1936 00:00:00 GMThttps://hdl.handle.net/10023/249451936-01-01T00:00:00ZLedermann, Walter"This thesis is a study of Singular Matrix Pencils under various aspects. In part (I) a new derivation of the Canonical Form of matrix pencils is given. This suggests investigation of the transformations of a pencil into itself (part (II)). Finally, part (III) deals with the canonical form of singular pencils of special types, namely those whose members are induced (or invariant) matrices." -- PrefaceThe invariant theory of linear complexes associated with a quaternary quadric
https://hdl.handle.net/10023/24930
"The invariants and covariants of a quaternary system of a linear complex and a quadric have been discussed by Weitzenbock, but he excludes the mixed concomitants. In the present paper the concomitants (which will include mixed forms) of a quadric and two linear complexes are discussed. The Reduced Prepared System is given in §3, the Complete System in §8 and some invariants of the covariant forms in §9. The concomitants of a quadric and 𝑛 linear complexes have next been considered and the corresponding reduced system of typical forms, is given at the end of the paper." -- From the Introduction.
Tue, 01 Jan 1929 00:00:00 GMThttps://hdl.handle.net/10023/249301929-01-01T00:00:00ZDasGupta, Pramathanath"The invariants and covariants of a quaternary system of a linear complex and a quadric have been discussed by Weitzenbock, but he excludes the mixed concomitants. In the present paper the concomitants (which will include mixed forms) of a quadric and two linear complexes are discussed. The Reduced Prepared System is given in §3, the Complete System in §8 and some invariants of the covariant forms in §9. The concomitants of a quadric and 𝑛 linear complexes have next been considered and the corresponding reduced system of typical forms, is given at the end of the paper." -- From the Introduction.Studies in form-theory : 1. Mixed determinants - 2. The pedal correspondence
https://hdl.handle.net/10023/24896
Tue, 01 Jan 1924 00:00:00 GMThttps://hdl.handle.net/10023/248961924-01-01T00:00:00ZVaidyanathaswamy, R.Contributions to the theory of apolarity
https://hdl.handle.net/10023/23956
Tue, 01 Jan 1924 00:00:00 GMThttps://hdl.handle.net/10023/239561924-01-01T00:00:00ZVaidyanathaswamy, R.Multifractal measures : from self-affine to nonlinear
https://hdl.handle.net/10023/23786
This thesis is based on three papers the author wrote during his time as a PhD student
[28, 17, 33].
In Chapter 2 we study 𝐿[sup]𝑞-spectra of planar self-affine measures generated by diagonal
matrices. We introduce a new technique for constructing and understanding examples
based on combinatorial estimates for the exponential growth of certain split binomial
sums. Using this approach we find counterexamples to a statement of Falconer and Miao
from 2007 and a conjecture of Miao from 2008 concerning a closed form expression for
the generalised dimensions of generic self-affine measures.
We also answer a question of Fraser from 2016 in the negative by proving that a certain natural closed form expression does not generally give the 𝐿[sup]𝑞-spectrum. As a further
application we provide examples of self-affine measures whose 𝐿[sup]𝑞-spectra exhibit new
types of phase transitions. Finally, we provide new non-trivial closed form bounds for
the 𝐿[sup]𝑞-spectra, which in certain cases yield sharp results.
In Chapter 3 we study 𝐿[sup]𝑞-spectra of measures in the plane generated by certain nonlinear maps. In particular we study attractors of iterated function systems consisting
of maps whose components are 𝐶[sup](1+α) and for which the Jacobian is a lower triangular
matrix at every point subject to a natural domination condition on the entries. We
calculate the 𝐿[sup]𝑞-spectrum of Bernoulli measures supported on such sets using an appropriately defined analogue of the singular value function and an appropriate pressure function.
In Chapter 4 we study a more general class of invariant measures supported on the attractors introduced in Chapter 3. These are pushforward quasi-Bernoulli measures, a class which includes the well-known class of Gibbs measures for Hölder continuous potentials. We show these measures are exact dimensional and that their exact dimensions satisfy a Ledrappier-Young formula.
Wed, 01 Dec 2021 00:00:00 GMThttps://hdl.handle.net/10023/237862021-12-01T00:00:00ZLee, Lawrence DavidThis thesis is based on three papers the author wrote during his time as a PhD student
[28, 17, 33].
In Chapter 2 we study 𝐿[sup]𝑞-spectra of planar self-affine measures generated by diagonal
matrices. We introduce a new technique for constructing and understanding examples
based on combinatorial estimates for the exponential growth of certain split binomial
sums. Using this approach we find counterexamples to a statement of Falconer and Miao
from 2007 and a conjecture of Miao from 2008 concerning a closed form expression for
the generalised dimensions of generic self-affine measures.
We also answer a question of Fraser from 2016 in the negative by proving that a certain natural closed form expression does not generally give the 𝐿[sup]𝑞-spectrum. As a further
application we provide examples of self-affine measures whose 𝐿[sup]𝑞-spectra exhibit new
types of phase transitions. Finally, we provide new non-trivial closed form bounds for
the 𝐿[sup]𝑞-spectra, which in certain cases yield sharp results.
In Chapter 3 we study 𝐿[sup]𝑞-spectra of measures in the plane generated by certain nonlinear maps. In particular we study attractors of iterated function systems consisting
of maps whose components are 𝐶[sup](1+α) and for which the Jacobian is a lower triangular
matrix at every point subject to a natural domination condition on the entries. We
calculate the 𝐿[sup]𝑞-spectrum of Bernoulli measures supported on such sets using an appropriately defined analogue of the singular value function and an appropriate pressure function.
In Chapter 4 we study a more general class of invariant measures supported on the attractors introduced in Chapter 3. These are pushforward quasi-Bernoulli measures, a class which includes the well-known class of Gibbs measures for Hölder continuous potentials. We show these measures are exact dimensional and that their exact dimensions satisfy a Ledrappier-Young formula.Enumerating 0-simple semigroups
https://hdl.handle.net/10023/23558
Computational semigroup theory involves the study and implementation of algorithms to compute with semigroups. Efficiency is of central concern and often follows from the insight of semigroup theoretic results. In turn, computational methods allow for analysis of semigroups which can provide intuition leading to theoretical breakthroughs. More efficient algorithms allow for more cases to be computed and increases the potential for insight. In this way, research into computational semigroup theory and abstract semigroup theory forms a feedback loop with each benefiting the other.
In this thesis the primary focus will be on counting isomorphism classes of finite 0-simple semigroups. These semigroups are in some sense the building blocks of finite semigroups due to their correspondence with the Greens 𝒟-classes of a semigroup. The theory of Rees 0-matrix semigroups links these semigroups to matrices with entries from 0-groups. Special consideration will be given to the enumeration of certain sub-cases, most prominently the case of congruence free semigroups. The author has implemented these enumeration techniques and applied them to count isomorphism classes of 0-simple semigroups and congruence free semigroups by order. Included in this thesis are tables of the number of 0-simple semigroups of orders less than or equal to 130, up to isomorphism. Also included are tables of the numbers of congruence free semigroups, up to isomorphism, with m Green’s ℒ-classes and n Green’s ℛ-classes for all mn less than or equal to 100, as well as for various other values of m,n. Furthermore a database of finite 0-simple semigroups has been created and we detail how this was done. The implementation of these enumeration methods and the database are publicly available as GAP code. In order to achieve these results pertaining to finite 0-simple semigroups we invoke the theory of group actions and prove novel combinatorial results. Most notably, we have deduced formulae for enumerating the number of binary matrices with distinct rows and columns up to row and column permutations.
There are also two sections dedicated to covers of E-unitary inverse semigroups, and presentations of factorisable orthodox monoids, respectively. In the first, we explore the concept of a minimal E-unitary inverse cover, up to isomorphism, by defining various sensible orderings. We provide examples of Clifford semigroups showing that, in general, these orderings do not have a unique minimal element. Finally, we pose conjectures about the existence of unique minimal E-unitary inverse covers of Clifford semigroups, when considered up to an equivalence weaker than isomorphism. In the latter section, we generalise the theory of presentations of factorisable inverse monoids to the more general setting of factorisable orthodox monoids. These topics were explored early in the authors doctoral studies but ultimately in less depth than the research on 0-simple semigroups.
Tue, 29 Jun 2021 00:00:00 GMThttps://hdl.handle.net/10023/235582021-06-29T00:00:00ZRussell, ChristopherComputational semigroup theory involves the study and implementation of algorithms to compute with semigroups. Efficiency is of central concern and often follows from the insight of semigroup theoretic results. In turn, computational methods allow for analysis of semigroups which can provide intuition leading to theoretical breakthroughs. More efficient algorithms allow for more cases to be computed and increases the potential for insight. In this way, research into computational semigroup theory and abstract semigroup theory forms a feedback loop with each benefiting the other.
In this thesis the primary focus will be on counting isomorphism classes of finite 0-simple semigroups. These semigroups are in some sense the building blocks of finite semigroups due to their correspondence with the Greens 𝒟-classes of a semigroup. The theory of Rees 0-matrix semigroups links these semigroups to matrices with entries from 0-groups. Special consideration will be given to the enumeration of certain sub-cases, most prominently the case of congruence free semigroups. The author has implemented these enumeration techniques and applied them to count isomorphism classes of 0-simple semigroups and congruence free semigroups by order. Included in this thesis are tables of the number of 0-simple semigroups of orders less than or equal to 130, up to isomorphism. Also included are tables of the numbers of congruence free semigroups, up to isomorphism, with m Green’s ℒ-classes and n Green’s ℛ-classes for all mn less than or equal to 100, as well as for various other values of m,n. Furthermore a database of finite 0-simple semigroups has been created and we detail how this was done. The implementation of these enumeration methods and the database are publicly available as GAP code. In order to achieve these results pertaining to finite 0-simple semigroups we invoke the theory of group actions and prove novel combinatorial results. Most notably, we have deduced formulae for enumerating the number of binary matrices with distinct rows and columns up to row and column permutations.
There are also two sections dedicated to covers of E-unitary inverse semigroups, and presentations of factorisable orthodox monoids, respectively. In the first, we explore the concept of a minimal E-unitary inverse cover, up to isomorphism, by defining various sensible orderings. We provide examples of Clifford semigroups showing that, in general, these orderings do not have a unique minimal element. Finally, we pose conjectures about the existence of unique minimal E-unitary inverse covers of Clifford semigroups, when considered up to an equivalence weaker than isomorphism. In the latter section, we generalise the theory of presentations of factorisable inverse monoids to the more general setting of factorisable orthodox monoids. These topics were explored early in the authors doctoral studies but ultimately in less depth than the research on 0-simple semigroups.Improved methods for estimating spatial and temporal trends from point transect survey data
https://hdl.handle.net/10023/23506
This thesis is about methods for improving estimates of abundance and trends from distance sampling surveys. My particular focus is on point transect surveys of endemic Hawaiian songbirds. When critical assumptions are met, design-based distance sampling provides unbiased abundance estimates; however, for rare endangered Hawaiian forest birds, the estimates can have high variance, hindering their use in assessing conservation efforts.
One approach to improve precision is to use spatial models instead of design-based methods. I fitted density surface models (DSMs), accounting for spatial and temporal correlation, using a two-stage approach that separated modelling of detection probability from modelling spatio-temporal patterns in density using generalized additive models (GAMs). Precision was improved and maps depicted spatio-temporal patterns in densities.
I compared the model that I fitted for a single year to two alternative approaches: spatial point-process model based on a log-Gaussian Cox process with a Matérn covariance (LGCP) and a soap-film smoother. The GAM-based DSMs and LGCP approaches produced better precision than the design-based method but varied in how they captured pattern in the data. I also implemented a GAM that used a smoother which took into account the study area boundary (a soap-film smoother) and found this produced better extrapolations into parts of the study area not surveyed.
Including biological realism is another approach to improve modelling of population change over time is to link design-based abundance estimates to an underlying population dynamics model, using a state-space modelling framework. This constrains population changes to be biologically realistic, as I demonstrate with a set of models that make different assumptions about the demographic parameters driving population changes.
Overall, I demonstrate that spatial, spatio-temporal and population dynamics modelling procedures reduced the variance in density estimates in single- and multi-year abundance data compared to design-based methods, thus better informing management and conservation decisions.
Tue, 29 Jun 2021 00:00:00 GMThttps://hdl.handle.net/10023/235062021-06-29T00:00:00ZCamp, Richard JosephThis thesis is about methods for improving estimates of abundance and trends from distance sampling surveys. My particular focus is on point transect surveys of endemic Hawaiian songbirds. When critical assumptions are met, design-based distance sampling provides unbiased abundance estimates; however, for rare endangered Hawaiian forest birds, the estimates can have high variance, hindering their use in assessing conservation efforts.
One approach to improve precision is to use spatial models instead of design-based methods. I fitted density surface models (DSMs), accounting for spatial and temporal correlation, using a two-stage approach that separated modelling of detection probability from modelling spatio-temporal patterns in density using generalized additive models (GAMs). Precision was improved and maps depicted spatio-temporal patterns in densities.
I compared the model that I fitted for a single year to two alternative approaches: spatial point-process model based on a log-Gaussian Cox process with a Matérn covariance (LGCP) and a soap-film smoother. The GAM-based DSMs and LGCP approaches produced better precision than the design-based method but varied in how they captured pattern in the data. I also implemented a GAM that used a smoother which took into account the study area boundary (a soap-film smoother) and found this produced better extrapolations into parts of the study area not surveyed.
Including biological realism is another approach to improve modelling of population change over time is to link design-based abundance estimates to an underlying population dynamics model, using a state-space modelling framework. This constrains population changes to be biologically realistic, as I demonstrate with a set of models that make different assumptions about the demographic parameters driving population changes.
Overall, I demonstrate that spatial, spatio-temporal and population dynamics modelling procedures reduced the variance in density estimates in single- and multi-year abundance data compared to design-based methods, thus better informing management and conservation decisions.Magnetohydrodynamic waves in the solar corona : a mathematical investigation of the role of resonant absorption and phase mixing in coronal heating
https://hdl.handle.net/10023/23436
Background: The Sun is a massive and highly dynamic ball of plasma, and oscillations in kinetic and magnetic energy are commonplace throughout its atmosphere. Since the plasma conducts electricity, we model the fluid using magnetohydrodynamics (MHD) instead of hydrodynamics which is used for non-ionised fluids. We study two MHD wave phenomena, namely, phase mixing and resonant absorption. These are both phenomena that occur exclusively in MHD fluids and do not occur in hydrodynamic fluids. We study their implications for the coronal heating problem and coronal seismology. The solar surface is significantly denser than the atmosphere, and we model it as a solid wall. In other words, we impose line-tied boundary conditions at the solar surface where the velocity is set equal to zero.
Aims:
1) The first research chapter introduces some of the key properties of footpoint driven Alfvén waves (a type of MHD wave) which are relevant for the rest of this thesis.
2) The third chapter calculates an upper bound for the heat that linear phase-mixed Alfvén waves can produce at observed frequencies and amplitudes to assess its viability as a coronal heating mechanism.
3) The fourth chapter tests if line-tied boundary conditions still apply in a resonant absorption experiment where the transverse length-scales can be very short.
Methods:
We take an analytic and theoretical approach to solving each problem and then check the results numerically.
Results:
1) We show that the growth of energy in closed loops for a sinusoidal footpoint driver is highly dependent on the driver frequency. If a resonance is excited, then the energy grows quadratically with time, and for a broadband driver, the energy grows linearly on average. If the loop is partially closed (i.e. only a fraction of the wave amplitude reflects at the boundary), the energy will converge towards a steady-state in which the energy of the loop remains constant with time.
2) We calculate an upper bound for the heat produced by phase-mixed Alfvén waves and find that it is, on average, too small to play a significant role in coronal heating.
3) We show that if the length-scales perpendicular or parallel to the boundary is sufficiently short, imposing line-tied boundary conditions may no longer be valid. However, researchers may wish to continue to use them in their models for their simplicity and ability to significantly reduce computation time if they understand and are aware of their limitations.
Tue, 29 Jun 2021 00:00:00 GMThttps://hdl.handle.net/10023/234362021-06-29T00:00:00ZProkopyszyn, AlexanderBackground: The Sun is a massive and highly dynamic ball of plasma, and oscillations in kinetic and magnetic energy are commonplace throughout its atmosphere. Since the plasma conducts electricity, we model the fluid using magnetohydrodynamics (MHD) instead of hydrodynamics which is used for non-ionised fluids. We study two MHD wave phenomena, namely, phase mixing and resonant absorption. These are both phenomena that occur exclusively in MHD fluids and do not occur in hydrodynamic fluids. We study their implications for the coronal heating problem and coronal seismology. The solar surface is significantly denser than the atmosphere, and we model it as a solid wall. In other words, we impose line-tied boundary conditions at the solar surface where the velocity is set equal to zero.
Aims:
1) The first research chapter introduces some of the key properties of footpoint driven Alfvén waves (a type of MHD wave) which are relevant for the rest of this thesis.
2) The third chapter calculates an upper bound for the heat that linear phase-mixed Alfvén waves can produce at observed frequencies and amplitudes to assess its viability as a coronal heating mechanism.
3) The fourth chapter tests if line-tied boundary conditions still apply in a resonant absorption experiment where the transverse length-scales can be very short.
Methods:
We take an analytic and theoretical approach to solving each problem and then check the results numerically.
Results:
1) We show that the growth of energy in closed loops for a sinusoidal footpoint driver is highly dependent on the driver frequency. If a resonance is excited, then the energy grows quadratically with time, and for a broadband driver, the energy grows linearly on average. If the loop is partially closed (i.e. only a fraction of the wave amplitude reflects at the boundary), the energy will converge towards a steady-state in which the energy of the loop remains constant with time.
2) We calculate an upper bound for the heat produced by phase-mixed Alfvén waves and find that it is, on average, too small to play a significant role in coronal heating.
3) We show that if the length-scales perpendicular or parallel to the boundary is sufficiently short, imposing line-tied boundary conditions may no longer be valid. However, researchers may wish to continue to use them in their models for their simplicity and ability to significantly reduce computation time if they understand and are aware of their limitations.Coincidence and disparity of fractal dimensions
https://hdl.handle.net/10023/23381
We investigate the dimension and structure of four fractal families: inhomogeneous attractors, fractal projections, fractional Brownian images, and elliptical polynomial spirals. For each family, particular attention is given to the relationships between different notions of dimension. This may take the form of determining conditions for them to coincide, or, in the case they differ, calculating the spectrum of dimensions interpolating between them. Material for this thesis is drawn from the papers [6,7,8,9,10].
First, we develop the dimension theory of inhomogeneous attractors for non-linear and affine iterated function systems. In both cases, we find natural quantities that bound the upper box-counting dimension from above and identify sufficient conditions for these bounds to be obtained. Our work improves and unifies previous theorems on inhomogeneous self-affine carpets, while providing inhomogeneous analogues of Falconer's seminal results on homogeneous self-affine sets.
Second, we prove that the intermediate dimensions of the orthogonal projection of a Borel set 𝐸 ⸦ ℝⁿ onto a linear subspace 𝑉 are almost surely independent of the choice of subspace. Similar methods identify the almost sure value of the dimension of Borel sets under index-α fractional Brownian motion. Various applications are given, including a surprising result that relates the box dimension of the Hölder images of a set to the Hausdorff dimension of the preimages.
Finally, we investigate fractal aspects of elliptical polynomial spirals; that is, planar spirals with differing polynomial rates of decay in the two axis directions. We give a full dimensional analysis, computing explicitly their intermediate, box-counting and Assouad-type dimensions. Relying on this, we bound the Hölder regularity of maps that deform one spiral into another, generalising the `winding problem’ of when spirals are bi-Lipschitz equivalent to a line segment. A novel feature is the use of fractional Brownian motion and dimension profiles to bound the Hölder exponents.
Tue, 29 Jun 2021 00:00:00 GMThttps://hdl.handle.net/10023/233812021-06-29T00:00:00ZBurrell, Stuart AndrewWe investigate the dimension and structure of four fractal families: inhomogeneous attractors, fractal projections, fractional Brownian images, and elliptical polynomial spirals. For each family, particular attention is given to the relationships between different notions of dimension. This may take the form of determining conditions for them to coincide, or, in the case they differ, calculating the spectrum of dimensions interpolating between them. Material for this thesis is drawn from the papers [6,7,8,9,10].
First, we develop the dimension theory of inhomogeneous attractors for non-linear and affine iterated function systems. In both cases, we find natural quantities that bound the upper box-counting dimension from above and identify sufficient conditions for these bounds to be obtained. Our work improves and unifies previous theorems on inhomogeneous self-affine carpets, while providing inhomogeneous analogues of Falconer's seminal results on homogeneous self-affine sets.
Second, we prove that the intermediate dimensions of the orthogonal projection of a Borel set 𝐸 ⸦ ℝⁿ onto a linear subspace 𝑉 are almost surely independent of the choice of subspace. Similar methods identify the almost sure value of the dimension of Borel sets under index-α fractional Brownian motion. Various applications are given, including a surprising result that relates the box dimension of the Hölder images of a set to the Hausdorff dimension of the preimages.
Finally, we investigate fractal aspects of elliptical polynomial spirals; that is, planar spirals with differing polynomial rates of decay in the two axis directions. We give a full dimensional analysis, computing explicitly their intermediate, box-counting and Assouad-type dimensions. Relying on this, we bound the Hölder regularity of maps that deform one spiral into another, generalising the `winding problem’ of when spirals are bi-Lipschitz equivalent to a line segment. A novel feature is the use of fractional Brownian motion and dimension profiles to bound the Hölder exponents.Computer-assisted proofs and the F[super](a,b,c) conjecture
https://hdl.handle.net/10023/22019
This thesis studies finitely presented groups and the process known as coset enumeration, which finds the index of a finitely generated subgroup in a finitely presented group, provided this index is finite. The Todd-Coxeter algorithm for coset enumeration is described, as well as its modified version, additionally finding a presentation for the subgroup. Coset enumeration is suitable for computer implementation, and GAP and ACE, two programs containing such functions using different strategies, are outlined. Proof Extraction After Coset Enumeration (PEACE) is a computer pro¬ gram that allows one to show a group element is in the subgroup. Descriptions are provided of modifications to PEACE, giving this program the extra functionality of creating subgroup presentations with the Modified ToddCoxeter algorithm. Using different strategies during the enumeration to determine varied subgroup presentations is also discussed. Additionally, a program converting the output of the original PEACE program, showing an element's membership of the subgroup, into a lemma-based step by step proof is implemented and described.
'The Fᵃᵇᶜ conjecture' was proposed by Campbell, Coxeter and Robertson in 1977 to classify the groups
Fᵃ,ᵇ,ᶜ =〈r,s|r²,rsᵃrsᵇrsᶜ〉
By considering the homomorphic image Hᵃᵇᶜ=〈r,s|r²,rsᵃrsᵇrsᶜ,s²⁽ᵃᵇᶜ⁾〉The lemma-based proof generating program is used as an aid in considering the groups Fᵃ,ᵇ,ᶜ and the corresponding conjecture. Lastly, a proof showing this conjecture to be true is provided.
Sun, 01 Jan 2006 00:00:00 GMThttps://hdl.handle.net/10023/220192006-01-01T00:00:00ZSutherland, Dale C.This thesis studies finitely presented groups and the process known as coset enumeration, which finds the index of a finitely generated subgroup in a finitely presented group, provided this index is finite. The Todd-Coxeter algorithm for coset enumeration is described, as well as its modified version, additionally finding a presentation for the subgroup. Coset enumeration is suitable for computer implementation, and GAP and ACE, two programs containing such functions using different strategies, are outlined. Proof Extraction After Coset Enumeration (PEACE) is a computer pro¬ gram that allows one to show a group element is in the subgroup. Descriptions are provided of modifications to PEACE, giving this program the extra functionality of creating subgroup presentations with the Modified ToddCoxeter algorithm. Using different strategies during the enumeration to determine varied subgroup presentations is also discussed. Additionally, a program converting the output of the original PEACE program, showing an element's membership of the subgroup, into a lemma-based step by step proof is implemented and described.
'The Fᵃᵇᶜ conjecture' was proposed by Campbell, Coxeter and Robertson in 1977 to classify the groups
Fᵃ,ᵇ,ᶜ =〈r,s|r²,rsᵃrsᵇrsᶜ〉
By considering the homomorphic image Hᵃᵇᶜ=〈r,s|r²,rsᵃrsᵇrsᶜ,s²⁽ᵃᵇᶜ⁾〉The lemma-based proof generating program is used as an aid in considering the groups Fᵃ,ᵇ,ᶜ and the corresponding conjecture. Lastly, a proof showing this conjecture to be true is provided.Transition region blinkers
https://hdl.handle.net/10023/22021
Blinkers are small intensity enhancements seen in the transition region of the solar atmosphere. They are important because they give us a unique insight into the transition region which until recently, has not been studied in much detail. An automated method of identifying blinkers is presented and used to identify blinkers in the quiet Sun and active regions from SOHO/CDS O V data. The general properties of the blinkers are discussed. They have typical areas of 3 x 10⁷ km² and lifetimes of 16 minutes. Their typical global frequency and intensity enhancement factors in the quiet Sun are 7s ⁻¹ and 1.8, respectively, whereas these values increase in active regions to 13s⁻¹and 2.4. Blinkers are best seen in the O V (629Å) transition-region line, but they also have strong signatures in O IV (554Å), and the chromospheric line, He I (584Å). The strongest O V blinkers can also be identified in O III (599Å). No significant signatures are found in the coronal lines Mg IX (368 Å) and Mg X (624 Å) for quiet Sun blinkers, but some increases can be seen in active-region blinkers. The ratios of the oxygen lines in blinkers were found to be flat confirming the result that blinkers are not temperature events, but are either density enhancements or increases in filling factor. Blinkers appear to occur preferentially over regions of enhanced chromospheric, transition region or coronal emission such as network boundaries. The plasma velocities of the O V blinkers and the chromosphere below have been studied. The Doppler and non-thermal velocities found are preferentially more red-shifted and greater than the normal chromospheric and transition region plasma, respectively. The ranges of these enhanced velocities, however, are no larger than the typical spread of Doppler and non-thermal velocities in these regions. Analysis of the magnetic field below blinkers shows that blinkers preferentially occur above regions of large or strong magnetic fragments with 55% of quiet Sun and 50% of active-region blinkers occurring in regions where one polarity dominates. Active-region blinkers are found above both active-region (plage) magnetic fields, as well as above the umbra and penumbra of sunspots. There appears to be no correlation between the strength of these single polarity magnetic fields or the ratio of mixed magnetic fields beneath blinkers and blinker characteristics. Furthermore, following a comparison of explosive events and blinkers, only one case is found where the two phenomena are coincident. Initial probability analysis suggests that the hypothesis that explosive events occur independently of blinkers cannot be ruled out.
Wed, 01 Jan 2003 00:00:00 GMThttps://hdl.handle.net/10023/220212003-01-01T00:00:00ZBewsher, DanielleBlinkers are small intensity enhancements seen in the transition region of the solar atmosphere. They are important because they give us a unique insight into the transition region which until recently, has not been studied in much detail. An automated method of identifying blinkers is presented and used to identify blinkers in the quiet Sun and active regions from SOHO/CDS O V data. The general properties of the blinkers are discussed. They have typical areas of 3 x 10⁷ km² and lifetimes of 16 minutes. Their typical global frequency and intensity enhancement factors in the quiet Sun are 7s ⁻¹ and 1.8, respectively, whereas these values increase in active regions to 13s⁻¹and 2.4. Blinkers are best seen in the O V (629Å) transition-region line, but they also have strong signatures in O IV (554Å), and the chromospheric line, He I (584Å). The strongest O V blinkers can also be identified in O III (599Å). No significant signatures are found in the coronal lines Mg IX (368 Å) and Mg X (624 Å) for quiet Sun blinkers, but some increases can be seen in active-region blinkers. The ratios of the oxygen lines in blinkers were found to be flat confirming the result that blinkers are not temperature events, but are either density enhancements or increases in filling factor. Blinkers appear to occur preferentially over regions of enhanced chromospheric, transition region or coronal emission such as network boundaries. The plasma velocities of the O V blinkers and the chromosphere below have been studied. The Doppler and non-thermal velocities found are preferentially more red-shifted and greater than the normal chromospheric and transition region plasma, respectively. The ranges of these enhanced velocities, however, are no larger than the typical spread of Doppler and non-thermal velocities in these regions. Analysis of the magnetic field below blinkers shows that blinkers preferentially occur above regions of large or strong magnetic fragments with 55% of quiet Sun and 50% of active-region blinkers occurring in regions where one polarity dominates. Active-region blinkers are found above both active-region (plage) magnetic fields, as well as above the umbra and penumbra of sunspots. There appears to be no correlation between the strength of these single polarity magnetic fields or the ratio of mixed magnetic fields beneath blinkers and blinker characteristics. Furthermore, following a comparison of explosive events and blinkers, only one case is found where the two phenomena are coincident. Initial probability analysis suggests that the hypothesis that explosive events occur independently of blinkers cannot be ruled out.Two-component relativistic wave equations for spin 1/2 particles
https://hdl.handle.net/10023/21991
Tue, 01 Jan 1974 00:00:00 GMThttps://hdl.handle.net/10023/219911974-01-01T00:00:00ZKoutroulos, Christos GeorgeGeneration and presentations of semigroup constructions : Bruck-Reilly extensions and P-semigroups
https://hdl.handle.net/10023/21953
In this thesis we study problems regarding finite presentability of Bruck-Reilly extensions, finite generation of the underlying monoids, and finite generation of P-unitary inverse semigroups.
The first main question we consider is: Let M be a monoid and θ and endomorphism of M. If the Bruck-Reilly extension BR(M, θ) is finitely presented is the monoid M necessarily finitely generated? We answer this question for the following classes of monoids: semilattices; Clifford monoids; zero monoids; free monoids; completely (0-)simple semigroups; and semidirect products of semilattices by groups. This allows us to obtain necessary and sufficient conditions for the Bruck-Reilly extensions of these classes of monoids to be finitely presented.
We also show that, like the free inverse monoid, a Bruck-Reilly extension (of an inverse monoid) is not necessarily finitely presented as a monoid when it happens to be finitely presented as an inverse monoid.
We then consider the question: When are P-semigroups, or P-unitary inverse semigroups, finitely generated? We give necessary and sufficient conditions for a P-semigroup P(G, X, Y) to be finitely generated in the case when X\Y is finite, and consider several particular cases when X\Y is infinite.
Sun, 01 Jan 2006 00:00:00 GMThttps://hdl.handle.net/10023/219532006-01-01T00:00:00ZCarvalho, Catarina A. S.In this thesis we study problems regarding finite presentability of Bruck-Reilly extensions, finite generation of the underlying monoids, and finite generation of P-unitary inverse semigroups.
The first main question we consider is: Let M be a monoid and θ and endomorphism of M. If the Bruck-Reilly extension BR(M, θ) is finitely presented is the monoid M necessarily finitely generated? We answer this question for the following classes of monoids: semilattices; Clifford monoids; zero monoids; free monoids; completely (0-)simple semigroups; and semidirect products of semilattices by groups. This allows us to obtain necessary and sufficient conditions for the Bruck-Reilly extensions of these classes of monoids to be finitely presented.
We also show that, like the free inverse monoid, a Bruck-Reilly extension (of an inverse monoid) is not necessarily finitely presented as a monoid when it happens to be finitely presented as an inverse monoid.
We then consider the question: When are P-semigroups, or P-unitary inverse semigroups, finitely generated? We give necessary and sufficient conditions for a P-semigroup P(G, X, Y) to be finitely generated in the case when X\Y is finite, and consider several particular cases when X\Y is infinite.Multi-dimensional modelling of physiologically and temporally structured populations
https://hdl.handle.net/10023/21943
Physiologically-structured population counts are sometimes the only available source of information about a population. Because such data are often sparse and noisy, they are difficult to model. Moreover the parameters of interest may be underlying demographic rates rather than population abundance. In this thesis, the possibility of using smoothing splines, demographic processes and environmental variables to improve estimates of birth and death rates from physiologically-structured population counts is explored.
A smooth physiologically-structured population model is proposed which makes appropriate use of demographic processes and allows explicit space-time dependence in estimated quantities (birth and death rates). A model fitting process is also defined. The model is tested using simulated data and is applied to Dover sole (Solea solea) data from the Bristol Channel. It can be applied when the life-history stages are defined by age. The method presented avoids mis-specification bias in birth and death rate estimates and improves their precision. It allows study of the relationship between vital rate estimates and environmental variables and, when coded, is straightforward to apply.
The more general inverse problem of obtaining birth, death and growth rates from stage abundances is also examined, when the physiological trait distinguishing stages need not be age. It is proven mathematically that unique solutions do not exist unless the trait is age, though limits for the growth and hence birth and death rates do exist when the growth rate is independent of the physiological trait. Simulations are used to demonstrate that plausible estimates of the 'true' birth, death and growth rates, cannot be identified in practice.
To overcome this difficulty, stage-age experiment data may be incorporated into physiologically-structured population models. A new method is discussed for doing this in a statistically justified manner.
Thu, 01 Jan 2004 00:00:00 GMThttps://hdl.handle.net/10023/219432004-01-01T00:00:00ZDixon, Camilla ElizabethPhysiologically-structured population counts are sometimes the only available source of information about a population. Because such data are often sparse and noisy, they are difficult to model. Moreover the parameters of interest may be underlying demographic rates rather than population abundance. In this thesis, the possibility of using smoothing splines, demographic processes and environmental variables to improve estimates of birth and death rates from physiologically-structured population counts is explored.
A smooth physiologically-structured population model is proposed which makes appropriate use of demographic processes and allows explicit space-time dependence in estimated quantities (birth and death rates). A model fitting process is also defined. The model is tested using simulated data and is applied to Dover sole (Solea solea) data from the Bristol Channel. It can be applied when the life-history stages are defined by age. The method presented avoids mis-specification bias in birth and death rate estimates and improves their precision. It allows study of the relationship between vital rate estimates and environmental variables and, when coded, is straightforward to apply.
The more general inverse problem of obtaining birth, death and growth rates from stage abundances is also examined, when the physiological trait distinguishing stages need not be age. It is proven mathematically that unique solutions do not exist unless the trait is age, though limits for the growth and hence birth and death rates do exist when the growth rate is independent of the physiological trait. Simulations are used to demonstrate that plausible estimates of the 'true' birth, death and growth rates, cannot be identified in practice.
To overcome this difficulty, stage-age experiment data may be incorporated into physiologically-structured population models. A new method is discussed for doing this in a statistically justified manner.Quasi-Newton methods for unconstrained function minimization and the solution of systems of nonlinear equations
https://hdl.handle.net/10023/21940
This thesis is concerned with the unconstrained minimization of a function of n variables, and, to a lesser extent, with the numerical solution of systems of nonlinear equations.
The first chapter contains an account of the fundamental ideas and theorems which are related to the subject of this thesis, and also gives a brief description of some methods which historically precede quasi-Newton methods, such as the method of steepest descent, Newton's method, the conjugate direction methods, the contraction mapping method, and the parameter variation method.
Newton's method, among the aforementioned methods, is considered the most effective one. It is rapidly convergent, and is capable of handling a variety of problems efficiently. But from a computational point of view, Newton's method is expensive. The second chapter of this thesis demonstrates how quasi-Newton methods are considered as an improvement of Newton's method by being able to circumvent the difficulties which face Newton's method. Also a general procedure for deriving quasi-Newton algorithms is described.
All methods generate a sequence of estimates which tend to the solution of the problem. In general all the methods which precede quasi-Newton methods employ information at the present stage, but quasi-Newton methods employ information at the present stage, and at the stage immediately previous to the present. In chapters 3 and 4 we will discuss methods which employ information from previous stages. Such methods are unified in one general scheme called "supermemory descent methods". Numerical experience with members of this class of methods is reported and compared with quasi-Newton methods.
Tue, 01 Jan 1974 00:00:00 GMThttps://hdl.handle.net/10023/219401974-01-01T00:00:00ZViazminsky, Caesar P.This thesis is concerned with the unconstrained minimization of a function of n variables, and, to a lesser extent, with the numerical solution of systems of nonlinear equations.
The first chapter contains an account of the fundamental ideas and theorems which are related to the subject of this thesis, and also gives a brief description of some methods which historically precede quasi-Newton methods, such as the method of steepest descent, Newton's method, the conjugate direction methods, the contraction mapping method, and the parameter variation method.
Newton's method, among the aforementioned methods, is considered the most effective one. It is rapidly convergent, and is capable of handling a variety of problems efficiently. But from a computational point of view, Newton's method is expensive. The second chapter of this thesis demonstrates how quasi-Newton methods are considered as an improvement of Newton's method by being able to circumvent the difficulties which face Newton's method. Also a general procedure for deriving quasi-Newton algorithms is described.
All methods generate a sequence of estimates which tend to the solution of the problem. In general all the methods which precede quasi-Newton methods employ information at the present stage, but quasi-Newton methods employ information at the present stage, and at the stage immediately previous to the present. In chapters 3 and 4 we will discuss methods which employ information from previous stages. Such methods are unified in one general scheme called "supermemory descent methods". Numerical experience with members of this class of methods is reported and compared with quasi-Newton methods.Multifractal analysis and modelling of rainfall
https://hdl.handle.net/10023/21926
In this thesis we study novel aspects of the multifractal properties of rainfall. Our aim is to use multifractal methods to improve the representation of rainfall distributions in climate simulations, in particular by disaggregating spatial rainfall using random cascades. For this we utilise recent mathematical ideas from multifractal analysis, develop computational methods based on these ideas and apply them to the central hydrological question of rainfall representation in climate simulations. First we present the background to fractal and multifractal theory, with an introduction to the fine, coarse and Legendre transform multifractal spectrum. We consider algorithms for computing the auxiliary function and the Legendre multifractal spectrum, which we test 011 some measures with well-known multifractal properties. We review the motivation for applying multifractal analysis, in particular random cascade measures, to hydrological problems. To provide a non-isotropic test example, we introduce and analyse a self-affine measure supported by a variant of the Sierpinski triangle. As a main application, we incorporate a random cascade disaggregation of spatial rain¬ fall into the hydrological component of the UK Meteorological Office Surface Exchange Scheme and compare the resulting water balance variables with those gained from simulations using more conventional rainfall distributions. We show that a disaggregation using random cascades gives closer values to the reference simulation than the other approaches. The multifractal properties of random cascades depend on the parameters used in their generation process. We present some simple schemes for estimating parameters that give random cascades with specific multifractal features. These schemes are applied to standard examples and spatial rainfall data. We question the assumption that random cascades are always appropriate for modelling observed multifractals. In the final chapter we discuss the relationship of the multifractal functions of a plane measure and those of slices of the measure by a line. Based on recent mathematical ideas about the multifractal properties of slices we formulate the 'slice hypothesis'. We investigate the use of the slice hypothesis to estimate multifractal properties of spatial rainfall fields from data from slices and from temporal data at a fixed site.
Sat, 01 Jan 2000 00:00:00 GMThttps://hdl.handle.net/10023/219262000-01-01T00:00:00ZLammering, BirgerIn this thesis we study novel aspects of the multifractal properties of rainfall. Our aim is to use multifractal methods to improve the representation of rainfall distributions in climate simulations, in particular by disaggregating spatial rainfall using random cascades. For this we utilise recent mathematical ideas from multifractal analysis, develop computational methods based on these ideas and apply them to the central hydrological question of rainfall representation in climate simulations. First we present the background to fractal and multifractal theory, with an introduction to the fine, coarse and Legendre transform multifractal spectrum. We consider algorithms for computing the auxiliary function and the Legendre multifractal spectrum, which we test 011 some measures with well-known multifractal properties. We review the motivation for applying multifractal analysis, in particular random cascade measures, to hydrological problems. To provide a non-isotropic test example, we introduce and analyse a self-affine measure supported by a variant of the Sierpinski triangle. As a main application, we incorporate a random cascade disaggregation of spatial rain¬ fall into the hydrological component of the UK Meteorological Office Surface Exchange Scheme and compare the resulting water balance variables with those gained from simulations using more conventional rainfall distributions. We show that a disaggregation using random cascades gives closer values to the reference simulation than the other approaches. The multifractal properties of random cascades depend on the parameters used in their generation process. We present some simple schemes for estimating parameters that give random cascades with specific multifractal features. These schemes are applied to standard examples and spatial rainfall data. We question the assumption that random cascades are always appropriate for modelling observed multifractals. In the final chapter we discuss the relationship of the multifractal functions of a plane measure and those of slices of the measure by a line. Based on recent mathematical ideas about the multifractal properties of slices we formulate the 'slice hypothesis'. We investigate the use of the slice hypothesis to estimate multifractal properties of spatial rainfall fields from data from slices and from temporal data at a fixed site.Techniques for computing exact Hausdorff measure with application to a Sierpinski sponge in R³
https://hdl.handle.net/10023/21914
Sun, 01 Jan 2006 00:00:00 GMThttps://hdl.handle.net/10023/219142006-01-01T00:00:00ZRidge, BarryA statistical analysis of tactical movement patterns in Association Football
https://hdl.handle.net/10023/21904
The main purpose of this study was an attempt to reveal more information about types of attacking patterns in association football. Seven types of attacking formation were identified from a total of eighteen league matches. These were analysed using a number of statistical and visual techniques. For set plays, twenty-four matches were analysed using the same statistical and visual techniques as were used for the ordinary attacks. There are, of course, two types of pattern for corner-kicks and throw-ins; one from the right-hand side and the other from the left-hand side of the pitch. For free-kicks, however, there were no obvious patterns. In further analysis, the result showed that there were significant relationships for ordinary attacks, thus: Types of pattern with final actions (significance level = 0.001); final actions with the number of long passes (significance level = 0.02); types of pattern with the number of short passes and dribbling sections (significance level = 0.001 for both). For set plays, it was found that only for cornerkicks were the number of short passes with the final actions statistically significant (significance level = 0.01).
Also, the result indicated that the most successful attacking pattern formations in providing shooting and scoring opportunities were 1 and 6. This means that the most successful moves are those which proceed along the length of either wing. Although pattern 2 (an attack initiated close to the centre spot of the pitch, towards the left side line briefly along the wing, and then into the penalty area by a number of passes, and terminated by shooting) succeeded in providing shooting and scoring opportunities, it also resulted in more corner-kicks being awarded. Furthermore, attacks culminating in final action 7 (off-side) have a very high average number of long passes involved within the attack pattern. Generally, the more complex the attacking pattern, the less likely it was to result in potential scoring opportunitie, e.g. pattern 7 (an attack initiated from the middle of the pitch, about halfway between the centre and the left side line, diagonally towards the left side line with a number of short passes, followed by a pass into the penalty area and terminated by shooting). It was found that corner-kicks that included a number of short passes were more successful in providing scoring opportunities, than those that consisted of a single cross into the goal area.
Wed, 01 Jan 1986 00:00:00 GMThttps://hdl.handle.net/10023/219041986-01-01T00:00:00ZAli, Araz H.The main purpose of this study was an attempt to reveal more information about types of attacking patterns in association football. Seven types of attacking formation were identified from a total of eighteen league matches. These were analysed using a number of statistical and visual techniques. For set plays, twenty-four matches were analysed using the same statistical and visual techniques as were used for the ordinary attacks. There are, of course, two types of pattern for corner-kicks and throw-ins; one from the right-hand side and the other from the left-hand side of the pitch. For free-kicks, however, there were no obvious patterns. In further analysis, the result showed that there were significant relationships for ordinary attacks, thus: Types of pattern with final actions (significance level = 0.001); final actions with the number of long passes (significance level = 0.02); types of pattern with the number of short passes and dribbling sections (significance level = 0.001 for both). For set plays, it was found that only for cornerkicks were the number of short passes with the final actions statistically significant (significance level = 0.01).
Also, the result indicated that the most successful attacking pattern formations in providing shooting and scoring opportunities were 1 and 6. This means that the most successful moves are those which proceed along the length of either wing. Although pattern 2 (an attack initiated close to the centre spot of the pitch, towards the left side line briefly along the wing, and then into the penalty area by a number of passes, and terminated by shooting) succeeded in providing shooting and scoring opportunities, it also resulted in more corner-kicks being awarded. Furthermore, attacks culminating in final action 7 (off-side) have a very high average number of long passes involved within the attack pattern. Generally, the more complex the attacking pattern, the less likely it was to result in potential scoring opportunitie, e.g. pattern 7 (an attack initiated from the middle of the pitch, about halfway between the centre and the left side line, diagonally towards the left side line with a number of short passes, followed by a pass into the penalty area and terminated by shooting). It was found that corner-kicks that included a number of short passes were more successful in providing scoring opportunities, than those that consisted of a single cross into the goal area.The numerical solution of finite difference equations with applications to problems in fluid dynamics
https://hdl.handle.net/10023/21871
Sun, 01 Jan 1956 00:00:00 GMThttps://hdl.handle.net/10023/218711956-01-01T00:00:00ZMitchell, Andrew RonaldReduction of the principal series representation of Lorentz groups on an Abelian subgroup
https://hdl.handle.net/10023/21839
Let G be a locally compact group and G'CG a subgroup. Suppose that we are given an irreducible unitary representation Γ of G (which is infinite dimensional and is specified by the so-called principal series representation) and we wish to find out how this representation reduces on the subgroup G'.
In the special case where the representation Γ of G is an induced representation, then one can first apply Mackey's subgroup theorem. This gives a representation Γ’of G* defined on a Hilbert space 𝓗. However it often happens that Γ is not an irreducible representation of G , nor even a direct sum of irreducible representations, but is a "direct integral" of irreducible representations.
In this work, a unitary transformation will be introduced that maps 𝓗’ into another Hilbert space 𝓗’^ which is a direct integral of the Hilbert spaces of the irreducible representation that appear in the reduction of the principal series representation Γ on G’. The group G under consideration is taken to be SL(2,C) which is the universal covering of Lorentz Group , 1 B and G an Abelian subgroup (1,0 β 1)which is called the horospherical subgroup of SL(2,C) and we seek the reduction as mentioned above.
Tue, 01 Jan 1980 00:00:00 GMThttps://hdl.handle.net/10023/218391980-01-01T00:00:00ZAssar, Ali RezaLet G be a locally compact group and G'CG a subgroup. Suppose that we are given an irreducible unitary representation Γ of G (which is infinite dimensional and is specified by the so-called principal series representation) and we wish to find out how this representation reduces on the subgroup G'.
In the special case where the representation Γ of G is an induced representation, then one can first apply Mackey's subgroup theorem. This gives a representation Γ’of G* defined on a Hilbert space 𝓗. However it often happens that Γ is not an irreducible representation of G , nor even a direct sum of irreducible representations, but is a "direct integral" of irreducible representations.
In this work, a unitary transformation will be introduced that maps 𝓗’ into another Hilbert space 𝓗’^ which is a direct integral of the Hilbert spaces of the irreducible representation that appear in the reduction of the principal series representation Γ on G’. The group G under consideration is taken to be SL(2,C) which is the universal covering of Lorentz Group , 1 B and G an Abelian subgroup (1,0 β 1)which is called the horospherical subgroup of SL(2,C) and we seek the reduction as mentioned above.Discriminant analysis with respect to paired observations
https://hdl.handle.net/10023/21814
The thesis is divided into two sections. In PART I the theory of discrimant analysis is presented in a standard manner. Particular attention is paid to those aspects of the subject which also have relevance to-the allocation of pairs. PART II mainly concerns the author's work on the theory of the assignment of paired observations. It is concluded by an example which illustrates the advantages of allocation by the discriminator developed.
Tue, 01 Jan 1974 00:00:00 GMThttps://hdl.handle.net/10023/218141974-01-01T00:00:00ZMurphy, Alan RobertThe thesis is divided into two sections. In PART I the theory of discrimant analysis is presented in a standard manner. Particular attention is paid to those aspects of the subject which also have relevance to-the allocation of pairs. PART II mainly concerns the author's work on the theory of the assignment of paired observations. It is concluded by an example which illustrates the advantages of allocation by the discriminator developed.An implementation of algorithms to define and manipulate finite automata and regular expressions
https://hdl.handle.net/10023/21800
The main purpose of the work described in this chapter is to enable the user to define and modify deterministic and non-deterministic finite automata. The first step is to allow the user to define states and transitions for both deterministic and non-deterministic finite state automata and to subsequently modify them. It is possible at this stage to systematically rename final and other states. Functions are provided to convert non-deterministic finite automata to deterministic finite automata and also to trim and minimize automata. The minimization process produces a minimum state automaton. We will abbreviate the expressions deterministic finite automata and non-deterministic finite automata to DFA and NBFA respectively.
Tue, 01 Jan 1985 00:00:00 GMThttps://hdl.handle.net/10023/218001985-01-01T00:00:00ZMaroof, Adnan Abdul KadirThe main purpose of the work described in this chapter is to enable the user to define and modify deterministic and non-deterministic finite automata. The first step is to allow the user to define states and transitions for both deterministic and non-deterministic finite state automata and to subsequently modify them. It is possible at this stage to systematically rename final and other states. Functions are provided to convert non-deterministic finite automata to deterministic finite automata and also to trim and minimize automata. The minimization process produces a minimum state automaton. We will abbreviate the expressions deterministic finite automata and non-deterministic finite automata to DFA and NBFA respectively.A syntax directed translator
https://hdl.handle.net/10023/21795
'This Thesis describes a program, based on the mechanism discussed by Metcalfe [AL] , which performs indicated operations automatically to check syntax of various source languages and also translates a source language into a pre-determined target language. A mechanism, an improvement on that of Metcalfe or rather not discussed by Metcalfe, which in certain respects will speed up translation is also described. The program written in the IBM 1620 symbolic programming language simulates a special purpose stored-program computer (Syntax machine [AL] ). A program for this machine (syntax program [AL] ) represents the syntax and semantics of some language to be translated. Since the syntax machine can be programmed, it can translate any number of source language-target language pairs, that is it is a parametrized compiler. Two sets of parameters have to be provided for the compiler in order to carry out a specific translation : a specification of the source language and a specification of the target language. Only one set of parameters is required if only syntax checking is to be performed, that is, the specification of the source language. The source language description will be in an expanded form of constituent (phrase-structure) grammar. The target language description will be in a form which is consistent with and embedded in the source language description. This total specification of the source and target languages is referred to as a "grammar", and the notation in which it is written as a "meta-language". Thus the scope of translation is limited to those language pairs which are completely definable by phrase -structure analysis end synthesis. Such analysis can be referred to as "parsing" or analysis in terms of the syntax for a particular grammar. The synthesis is often called "unparsing"' --Taken from Introduction
Wed, 01 Jan 1969 00:00:00 GMThttps://hdl.handle.net/10023/217951969-01-01T00:00:00ZKadwa, Abdul Khaliq'This Thesis describes a program, based on the mechanism discussed by Metcalfe [AL] , which performs indicated operations automatically to check syntax of various source languages and also translates a source language into a pre-determined target language. A mechanism, an improvement on that of Metcalfe or rather not discussed by Metcalfe, which in certain respects will speed up translation is also described. The program written in the IBM 1620 symbolic programming language simulates a special purpose stored-program computer (Syntax machine [AL] ). A program for this machine (syntax program [AL] ) represents the syntax and semantics of some language to be translated. Since the syntax machine can be programmed, it can translate any number of source language-target language pairs, that is it is a parametrized compiler. Two sets of parameters have to be provided for the compiler in order to carry out a specific translation : a specification of the source language and a specification of the target language. Only one set of parameters is required if only syntax checking is to be performed, that is, the specification of the source language. The source language description will be in an expanded form of constituent (phrase-structure) grammar. The target language description will be in a form which is consistent with and embedded in the source language description. This total specification of the source and target languages is referred to as a "grammar", and the notation in which it is written as a "meta-language". Thus the scope of translation is limited to those language pairs which are completely definable by phrase -structure analysis end synthesis. Such analysis can be referred to as "parsing" or analysis in terms of the syntax for a particular grammar. The synthesis is often called "unparsing"' --Taken from IntroductionHarmonic mean estimates for recapture debugging
https://hdl.handle.net/10023/21793
In this thesis we examine the problem of estimating the number of errors (bugs) in a reliability system using the recapture debugging model suggested by Nayak (1988). The reliability system contains a certain number N of errors. Each causes system failures independently of the others. The times between failures for any bug are assumed to be independent exponential random variables with a parameter X common to all bugs. We assume the system is observed for a fixed length of time. The maximum likelihood estimate of N was considered by Nayak. We derive the profile likelihood interval for N, and consider as a point estimate the harmonic mean of the endpoints. This estimate was used for the Jelinski-Moranda model by Joe and Reid (1985). The exact probability distribution of the harmonic mean estimator is computed. A generalization of the harmonic mean estimate, called the weighted harmonic mean estimate is proposed as a further improvement. A comparison is drawn between this estimator and the maximum likelihood estimator, using their computed distributions for various values of N.
Mon, 01 Jan 1990 00:00:00 GMThttps://hdl.handle.net/10023/217931990-01-01T00:00:00ZAl-Harbi, Abdulghani A. GhonaimIn this thesis we examine the problem of estimating the number of errors (bugs) in a reliability system using the recapture debugging model suggested by Nayak (1988). The reliability system contains a certain number N of errors. Each causes system failures independently of the others. The times between failures for any bug are assumed to be independent exponential random variables with a parameter X common to all bugs. We assume the system is observed for a fixed length of time. The maximum likelihood estimate of N was considered by Nayak. We derive the profile likelihood interval for N, and consider as a point estimate the harmonic mean of the endpoints. This estimate was used for the Jelinski-Moranda model by Joe and Reid (1985). The exact probability distribution of the harmonic mean estimator is computed. A generalization of the harmonic mean estimate, called the weighted harmonic mean estimate is proposed as a further improvement. A comparison is drawn between this estimator and the maximum likelihood estimator, using their computed distributions for various values of N.A model for solar flares and coronal heating based on magnetohydrodynamic avalanches
https://hdl.handle.net/10023/21489
The coronal heating problem addresses how temperatures in the solar atmosphere increase by millions of degrees, moving up from the cool surface, into the upper reaches of the atmosphere. This heating, known to be attributable to the magnetic field, has attracted several possible explanations. Attention here focuses on the viability and onset of magnetohydrodynamic avalanches.
One means of transferring energy from interior convective motions is through photospheric motions, for example granulation and super-granulation. The power transferred by these photospheric motions stores energy in the global coronal magnetic field, and in coronal loops within active regions. The fields become highly braided, such that an ideal kink-mode instability occurs within one strand of a loop. Then, this instability rapidly expands, destabilizing the surrounding magnetic field, and destroying neighbouring threads. Bursty events occur in an intermittent, impulsive series, Parker's so-called nanoflares, above a largely steady background.
This heating is then investigated in order to determine the respective contributions of the physical mechanisms of viscosity and resistivity. Arbitrary distributions of non-uniform heating occur across the domain. The localization and impulsiveness are investigated in respect of field-aligned heating properties.
These heating profiles are tested in a hydrodynamic model of single coronal strands. This heating is found capable of maintaining approximately coronal conditions. Interestingly, three-dimensional MHD simulations and field-aligned, one-dimensional models produce similar behaviours, apart from in velocity, and this may have observable consequences.
Throughout, magnetic reconnection is vital in contributing to coronal heating. Therefore, the onset and locations of reconnection are identified, and compared with several possible indicators. The widely used squashing factor, Q, little agrees with the sites of reconnection in a braided MHD field.
Based on these results, magnetohydrodynamic avalanches can occur and produce sufficient energy to maintain the elevated temperatures of coronal loops.
Tue, 01 Dec 2020 00:00:00 GMThttps://hdl.handle.net/10023/214892020-12-01T00:00:00ZReid, JackThe coronal heating problem addresses how temperatures in the solar atmosphere increase by millions of degrees, moving up from the cool surface, into the upper reaches of the atmosphere. This heating, known to be attributable to the magnetic field, has attracted several possible explanations. Attention here focuses on the viability and onset of magnetohydrodynamic avalanches.
One means of transferring energy from interior convective motions is through photospheric motions, for example granulation and super-granulation. The power transferred by these photospheric motions stores energy in the global coronal magnetic field, and in coronal loops within active regions. The fields become highly braided, such that an ideal kink-mode instability occurs within one strand of a loop. Then, this instability rapidly expands, destabilizing the surrounding magnetic field, and destroying neighbouring threads. Bursty events occur in an intermittent, impulsive series, Parker's so-called nanoflares, above a largely steady background.
This heating is then investigated in order to determine the respective contributions of the physical mechanisms of viscosity and resistivity. Arbitrary distributions of non-uniform heating occur across the domain. The localization and impulsiveness are investigated in respect of field-aligned heating properties.
These heating profiles are tested in a hydrodynamic model of single coronal strands. This heating is found capable of maintaining approximately coronal conditions. Interestingly, three-dimensional MHD simulations and field-aligned, one-dimensional models produce similar behaviours, apart from in velocity, and this may have observable consequences.
Throughout, magnetic reconnection is vital in contributing to coronal heating. Therefore, the onset and locations of reconnection are identified, and compared with several possible indicators. The widely used squashing factor, Q, little agrees with the sites of reconnection in a braided MHD field.
Based on these results, magnetohydrodynamic avalanches can occur and produce sufficient energy to maintain the elevated temperatures of coronal loops.Subdirect products of free semigroups and monoids
https://hdl.handle.net/10023/21333
Subdirect products are special types of subalgebras of direct products. The purpose of this thesis is to initiate a study of combinatorial properties of subdirect products and fiber products of semigroups and monoids, motivated by the previous work on free groups, and some recent advances in general algebra.
In Chapter 1, we outline the necessary preliminary definitions and results, including elements of algebraic semigroup theory, formal language theory, automata theory and universal algebra.
In Chapter 2, we consider the number of subsemigroups and subdirect products of ℕ𝗑ℕ up to isomorphism. We obtain uncountably many such objects, and characterise the finite semigroups 𝘚 for which ℕ𝗑𝘚 has uncountable many subsemigroups and subdirect products up to isomorphism.
In Chapter 3, we consider particular finite generating sets for subdirect products of free semigroups introduced as "sets of letter pairs". We classify and count these sets which generate subdirect and fiber products, and discuss their abundance.
In Chapter 4, we consider finite generation and presentation for fiber products of free semigroups and monoids over finite fibers. We give a characterisation for finite generation of the fiber product of two free monoids over a finite fiber, and show that this implies finite presentation. We show that the fiber product of two free semigroups over a finite fiber is never finitely generated, and obtain necessary conditions on an infinite fiber for finite generation.
In Chapter 5, we consider the problem of finite generation for fiber products of free semigroups and monoids over a free fiber. We construct two-tape automata which we use to determine the language of indecomposable elements of the fiber product, which algorithmically decides when they are finitely generated.
Finally in Chapter 6, we summarise our findings, providing some further questions based on the results of the thesis.
Tue, 01 Dec 2020 00:00:00 GMThttps://hdl.handle.net/10023/213332020-12-01T00:00:00ZClayton, AshleySubdirect products are special types of subalgebras of direct products. The purpose of this thesis is to initiate a study of combinatorial properties of subdirect products and fiber products of semigroups and monoids, motivated by the previous work on free groups, and some recent advances in general algebra.
In Chapter 1, we outline the necessary preliminary definitions and results, including elements of algebraic semigroup theory, formal language theory, automata theory and universal algebra.
In Chapter 2, we consider the number of subsemigroups and subdirect products of ℕ𝗑ℕ up to isomorphism. We obtain uncountably many such objects, and characterise the finite semigroups 𝘚 for which ℕ𝗑𝘚 has uncountable many subsemigroups and subdirect products up to isomorphism.
In Chapter 3, we consider particular finite generating sets for subdirect products of free semigroups introduced as "sets of letter pairs". We classify and count these sets which generate subdirect and fiber products, and discuss their abundance.
In Chapter 4, we consider finite generation and presentation for fiber products of free semigroups and monoids over finite fibers. We give a characterisation for finite generation of the fiber product of two free monoids over a finite fiber, and show that this implies finite presentation. We show that the fiber product of two free semigroups over a finite fiber is never finitely generated, and obtain necessary conditions on an infinite fiber for finite generation.
In Chapter 5, we consider the problem of finite generation for fiber products of free semigroups and monoids over a free fiber. We construct two-tape automata which we use to determine the language of indecomposable elements of the fiber product, which algorithmically decides when they are finitely generated.
Finally in Chapter 6, we summarise our findings, providing some further questions based on the results of the thesis.Numerical modelling of MHD waves in coronal loops
https://hdl.handle.net/10023/21057
Waves in the solar corona have been investigated for many years, as a potential coronal heating mechanism and in the context of coronal seismology, and they play an important role in our understanding of the solar corona. In this thesis, we present the results of numerical simulations of transverse MHD waves in coronal loops. In a first study, we consider an atmospheric model for a coronal loop where the chromosphere is included as a simple mass reservoir and the effects of gravity, thermal conduction and optically thin radiation are taken into account, and we investigate the dissipation of phase-mixed, driven Alfvén waves and the subsequent heating and evaporation from the lower atmosphere. It has been argued that this evaporation can significantly affect the transverse density profile in the boundary of the loop, thereby changing the Alfvén speed gradient and the phase mixing process. We analyse the heating from the phase-mixed Alfvén waves and the evaporation and find that in our setup, with a high-frequency driver, the effect of the evaporation on the phase mixing process is negligible as a significant amount of the wave energy in the corona is lost to the lower atmosphere.
Waves usually originate in the lower parts of the solar atmosphere, where the convective motions beneath the photosphere shuffle the magnetic field around, and they are then transmitted into the corona. However, recent observations have shown that transverse MHD waves can also be generated in-situ in the corona, by the collision of counter-propagating plasma clumps (coronal rain). When falling down, these coronal rain clumps can collide with upflows or other coronal rain clumps, and generate transverse oscillations. In order to investigate this mechanism, we develop a 2D model for the collision of counter-propagating plasma clumps based on detailed observations and statistical analysis of these events and study the generation of transverse MHD waves. We first study the relationship between various physical parameters of the clumps and the resulting oscillations and subsequently apply the model using observed coronal rain properties and investigate the likelihood of collisions and oscillations in coronal loops. In our simulations, we find that the properties of the oscillations are linked to the properties of the counter-propagating clumps, but also that coronal rain collisions and oscillations are rather unlikely in active region loops, due to the relatively large background pressure and magnetic field strength.
Thu, 03 Dec 2020 00:00:00 GMThttps://hdl.handle.net/10023/210572020-12-03T00:00:00ZVan Damme, Hendrik-JanWaves in the solar corona have been investigated for many years, as a potential coronal heating mechanism and in the context of coronal seismology, and they play an important role in our understanding of the solar corona. In this thesis, we present the results of numerical simulations of transverse MHD waves in coronal loops. In a first study, we consider an atmospheric model for a coronal loop where the chromosphere is included as a simple mass reservoir and the effects of gravity, thermal conduction and optically thin radiation are taken into account, and we investigate the dissipation of phase-mixed, driven Alfvén waves and the subsequent heating and evaporation from the lower atmosphere. It has been argued that this evaporation can significantly affect the transverse density profile in the boundary of the loop, thereby changing the Alfvén speed gradient and the phase mixing process. We analyse the heating from the phase-mixed Alfvén waves and the evaporation and find that in our setup, with a high-frequency driver, the effect of the evaporation on the phase mixing process is negligible as a significant amount of the wave energy in the corona is lost to the lower atmosphere.
Waves usually originate in the lower parts of the solar atmosphere, where the convective motions beneath the photosphere shuffle the magnetic field around, and they are then transmitted into the corona. However, recent observations have shown that transverse MHD waves can also be generated in-situ in the corona, by the collision of counter-propagating plasma clumps (coronal rain). When falling down, these coronal rain clumps can collide with upflows or other coronal rain clumps, and generate transverse oscillations. In order to investigate this mechanism, we develop a 2D model for the collision of counter-propagating plasma clumps based on detailed observations and statistical analysis of these events and study the generation of transverse MHD waves. We first study the relationship between various physical parameters of the clumps and the resulting oscillations and subsequently apply the model using observed coronal rain properties and investigate the likelihood of collisions and oscillations in coronal loops. In our simulations, we find that the properties of the oscillations are linked to the properties of the counter-propagating clumps, but also that coronal rain collisions and oscillations are rather unlikely in active region loops, due to the relatively large background pressure and magnetic field strength.Nonlinear mixed effect models for modeling initial viral decay rates in an HIV study
https://hdl.handle.net/10023/20947
The Nonlinear Mixed Effect Viral Dynamic Model can easily handle unbalanced repeated and continuous measures data for individuals and is also popular in many other research areas such as biology and pharmacokinetics. Wu 𝘦𝘵 𝘢𝘭. (2004) described a Nonlinear Mixed Effects Biphasic Model to estimate short-term population and individual viral decay rates in their study. Perelson 𝘦𝘵 𝘢𝘭. (1999) and Ding 𝘦𝘵 𝘢𝘭. (1999) reported that initial viral decay estimated for viral decay models would be good markers of the potency of antiretroviral regimens. The aim of this study was to model viral decay rates, and check the validity of the model for the set of data provided and investigate whether the relationships found with baseline covariates and long-term response are consistent with Wu 𝘦𝘵 𝘢𝘭.’s (2004) findings.
The Nonlinear Mixed Effect Single and Biphasic Viral Dynamic Models were fitted, and their respective initial viral decay rates were derived. In this study, analyses and reports are focused on the first-phase viral decay rates of the models. The study found that the actual treatment groups were more potent than the control group. It was found that actual treatment effect and the number of multi-PI mutations at baseline had impacts on the initial viral decay rates for both models. Besides, baseline HIV-1 RNA levels had an impact on the initial viral decay rates for the biphasic model. There were no significant differences in the initial viral decay rates for different ages, ethnicities, and gender groups.
The study also shows that the initial viral decay rates were somewhat negatively correlated with the baseline HIV-1 RNA levels. A strong correlation between the initial viral decay rates and week 1 virus load reduction from baseline was observed. It was also observed that individuals with the higher initial viral decay rates were more likely to have suppressed virus load at week 24. Also, individuals with higher week 1 virus load reduction, i.e. early viral dynamics, were more likely to have suppressed virus load at week 24. These findings suggest that the antiviral potency or the initial viral decay rates are predictive of long-term viral load response.
Thu, 27 Nov 2008 00:00:00 GMThttps://hdl.handle.net/10023/209472008-11-27T00:00:00ZKang, SujinThe Nonlinear Mixed Effect Viral Dynamic Model can easily handle unbalanced repeated and continuous measures data for individuals and is also popular in many other research areas such as biology and pharmacokinetics. Wu 𝘦𝘵 𝘢𝘭. (2004) described a Nonlinear Mixed Effects Biphasic Model to estimate short-term population and individual viral decay rates in their study. Perelson 𝘦𝘵 𝘢𝘭. (1999) and Ding 𝘦𝘵 𝘢𝘭. (1999) reported that initial viral decay estimated for viral decay models would be good markers of the potency of antiretroviral regimens. The aim of this study was to model viral decay rates, and check the validity of the model for the set of data provided and investigate whether the relationships found with baseline covariates and long-term response are consistent with Wu 𝘦𝘵 𝘢𝘭.’s (2004) findings.
The Nonlinear Mixed Effect Single and Biphasic Viral Dynamic Models were fitted, and their respective initial viral decay rates were derived. In this study, analyses and reports are focused on the first-phase viral decay rates of the models. The study found that the actual treatment groups were more potent than the control group. It was found that actual treatment effect and the number of multi-PI mutations at baseline had impacts on the initial viral decay rates for both models. Besides, baseline HIV-1 RNA levels had an impact on the initial viral decay rates for the biphasic model. There were no significant differences in the initial viral decay rates for different ages, ethnicities, and gender groups.
The study also shows that the initial viral decay rates were somewhat negatively correlated with the baseline HIV-1 RNA levels. A strong correlation between the initial viral decay rates and week 1 virus load reduction from baseline was observed. It was also observed that individuals with the higher initial viral decay rates were more likely to have suppressed virus load at week 24. Also, individuals with higher week 1 virus load reduction, i.e. early viral dynamics, were more likely to have suppressed virus load at week 24. These findings suggest that the antiviral potency or the initial viral decay rates are predictive of long-term viral load response.On the regularity dimensions of measures
https://hdl.handle.net/10023/20218
This body of work is based upon the following three papers that the author wrote during his PhD with Jonathan Fraser and Han Yu: [FH20, HY17, How19].
Chapter 1 starts by introducing many of the common tools and notation that will be used throughout this thesis. This will cover the main notions of dimensions discussed from both the set and the measure perspectives. An emphasis will be placed on their relationships where possible. This will provide a solid base upon which to expand. Many of the standard results in this part can be found in fractal geometry textbooks such as [Fal03, Mat95] if further reading was desired.
The first results discussed in Chapter 2 will cover some of the regularity dimensions’ properties such as general bounds in relation to the Assouad and lower dimensions, local dimensions and the Lq-spectrum. The Assoaud and lower dimensions are known to interact pleasantly with weak tangents and these ideas are discussed in the regularity dimension setting. We then calculate the regularity dimensions for several specific example measures such as self-similar and self-affine measures which provides an opportunity to discuss the sharpness of the previously obtained bounds. This work originates in [FH20] where the upper regularity dimension was studied, with many of the lower regularity dimension results being natural extensions.
In Chapter 3 we continue the study of the upper and lower regularity dimensions with an emphasis on how they can be used to quantify doubling and uniform perfectness of measures. This starts with an explicit relation between the upper regularity dimension and the doubling constants along with a similar link between the lower regularity dimension and the constants of uniform perfectness. We then turn our attention to a technical result which can be made more quantitative thanks to the regularity dimensions. It is interesting to study how properties, such as doubling, change under pushforwards by different types of maps, here we study the regularity dimensions of pushforward measures with respect to quasisymmetric homeomorphisms. We round this chapter out with an interesting application of the lower regularity to Diophantine approximation by noting the equivalence between uniform perfectness and weakly absolutely α-decaying measures. The original material for this part can be found in [How19] with part of the first section integrating a result of [FH20].
Finally, in Chapter 4, we will consider graphs of Brownian motion, and more generally, graphs of Levy processes. This will involve the calculation of the lower and Assouad dimensions for such sets and then the regularity dimensions of measures pushed onto these graphs from the real line. These graphs are the only examples in this thesis for which the Assouad and lower dimensions had not been previously calculated so we delve deeper into the area, studying graphs of functions defined as stochastic integrals as well. This chapter is based on the paper [HY17] for the set theoretic half, with the regularity dimension results coming from [How19].
Wed, 01 Jan 2020 00:00:00 GMThttps://hdl.handle.net/10023/202182020-01-01T00:00:00ZHowroyd, Douglas CharlesThis body of work is based upon the following three papers that the author wrote during his PhD with Jonathan Fraser and Han Yu: [FH20, HY17, How19].
Chapter 1 starts by introducing many of the common tools and notation that will be used throughout this thesis. This will cover the main notions of dimensions discussed from both the set and the measure perspectives. An emphasis will be placed on their relationships where possible. This will provide a solid base upon which to expand. Many of the standard results in this part can be found in fractal geometry textbooks such as [Fal03, Mat95] if further reading was desired.
The first results discussed in Chapter 2 will cover some of the regularity dimensions’ properties such as general bounds in relation to the Assouad and lower dimensions, local dimensions and the Lq-spectrum. The Assoaud and lower dimensions are known to interact pleasantly with weak tangents and these ideas are discussed in the regularity dimension setting. We then calculate the regularity dimensions for several specific example measures such as self-similar and self-affine measures which provides an opportunity to discuss the sharpness of the previously obtained bounds. This work originates in [FH20] where the upper regularity dimension was studied, with many of the lower regularity dimension results being natural extensions.
In Chapter 3 we continue the study of the upper and lower regularity dimensions with an emphasis on how they can be used to quantify doubling and uniform perfectness of measures. This starts with an explicit relation between the upper regularity dimension and the doubling constants along with a similar link between the lower regularity dimension and the constants of uniform perfectness. We then turn our attention to a technical result which can be made more quantitative thanks to the regularity dimensions. It is interesting to study how properties, such as doubling, change under pushforwards by different types of maps, here we study the regularity dimensions of pushforward measures with respect to quasisymmetric homeomorphisms. We round this chapter out with an interesting application of the lower regularity to Diophantine approximation by noting the equivalence between uniform perfectness and weakly absolutely α-decaying measures. The original material for this part can be found in [How19] with part of the first section integrating a result of [FH20].
Finally, in Chapter 4, we will consider graphs of Brownian motion, and more generally, graphs of Levy processes. This will involve the calculation of the lower and Assouad dimensions for such sets and then the regularity dimensions of measures pushed onto these graphs from the real line. These graphs are the only examples in this thesis for which the Assouad and lower dimensions had not been previously calculated so we delve deeper into the area, studying graphs of functions defined as stochastic integrals as well. This chapter is based on the paper [HY17] for the set theoretic half, with the regularity dimension results coming from [How19].Movement ecology and conservation : the case of African vultures
https://hdl.handle.net/10023/20210
The movements of critically endangered vultures, equipped with satellite-based tracking devices in Namibia, were inspected using Generalized Additive Models. Models incorporated spatially adaptive (1D and 2D) smooths via the Spatially Adaptive Local Smoothing Algorithm (SALSA) and Complex REgion Spatial Smoother (CReSS) method. The correlated nature of geo-location data was address via robust standard errors.
The results of this thorough and integrative study of movement ecology have an unprecedented level of detail, far exceeding what is available in the literature. Namely, vultures were seen throughout Namibia and its five neighbouring countries with three individuals visiting locations farther than 1,000 km from where they were initially seen. Large variability was found both within and between birds. Differences were perceived in four daily movement properties, even though temporal differences were only captured for daily distance travelled (monthly) and daily maximum displacement (seasonally). There was noticeable variation in the size of the areas each bird used from month to month, often showing very little spatial overlap. Home ranges varied greatly; one bird expanded its monthly home range as much as nineteen times its smaller size. Contrastingly, core areas remained sometimes constant. Home ranges were three to five times larger than the respective core areas, clearly indicating a non-uniform use of the environment. The extensive study area (2.3 million sq.km) was characterised using habitat features, climate conditions and indices of human presence. Vegetation index, minimum distance to river and minimum distance to road were consistently important in explaining the probability of bird presence. Nonetheless, each vulture used its environment in its own way.
These novel findings support trans-frontier conservation measures, represent crucial support to revise the geographic extent of existing conservation actions and constitute the basis to predict the risk of exposure of vultures to lethal threats or to assess changes under Climate Change scenarios.
Tue, 28 Jul 2020 00:00:00 GMThttps://hdl.handle.net/10023/202102020-07-28T00:00:00ZEstevinho Santos Faustino, CláudiaThe movements of critically endangered vultures, equipped with satellite-based tracking devices in Namibia, were inspected using Generalized Additive Models. Models incorporated spatially adaptive (1D and 2D) smooths via the Spatially Adaptive Local Smoothing Algorithm (SALSA) and Complex REgion Spatial Smoother (CReSS) method. The correlated nature of geo-location data was address via robust standard errors.
The results of this thorough and integrative study of movement ecology have an unprecedented level of detail, far exceeding what is available in the literature. Namely, vultures were seen throughout Namibia and its five neighbouring countries with three individuals visiting locations farther than 1,000 km from where they were initially seen. Large variability was found both within and between birds. Differences were perceived in four daily movement properties, even though temporal differences were only captured for daily distance travelled (monthly) and daily maximum displacement (seasonally). There was noticeable variation in the size of the areas each bird used from month to month, often showing very little spatial overlap. Home ranges varied greatly; one bird expanded its monthly home range as much as nineteen times its smaller size. Contrastingly, core areas remained sometimes constant. Home ranges were three to five times larger than the respective core areas, clearly indicating a non-uniform use of the environment. The extensive study area (2.3 million sq.km) was characterised using habitat features, climate conditions and indices of human presence. Vegetation index, minimum distance to river and minimum distance to road were consistently important in explaining the probability of bird presence. Nonetheless, each vulture used its environment in its own way.
These novel findings support trans-frontier conservation measures, represent crucial support to revise the geographic extent of existing conservation actions and constitute the basis to predict the risk of exposure of vultures to lethal threats or to assess changes under Climate Change scenarios.Electron acceleration in auroral field-aligned currents
https://hdl.handle.net/10023/19520
Field-aligned currents at Earth's high latitudes are principally carried by accelerated electrons. Current
densities, typically ~µAm⁻² at ionospheric altitudes, are sustained by parallel potential drops of ~100 - 1000 V. This Thesis
presents Vlasov models of upward and downward current regions, where electrons are
described via distribution functions. The ion
density profile is fixed, and quasi-neutrality is invoked to solve
numerically for the potential variation.
In both cases, an ambipolar electric field traps ionospheric electrons. For downward currents, an energetic ionospheric electron beam emerges into the magnetosphere where it is accelerated around the B/n
peak at altitudes of 500 - 6000 km to carry the current. The electric field maximises just Earthward of
the
B/n peak. The magnitude and altitude of the potential is found to depend solely on the equilibrium
properties immediately above the B/n peak. An analytic non-linear current-voltage relation, analagous to the linear
Knight relation for upward currents, is derived.
Energetic magnetospheric electrons precipitate into the ionosphere to carry upward currents. The continuous
potential variation is solved for current densities ~1 µAm⁻². Acceleration extends above the B/n
peak for ~1 R[sub]E, and is increasingly concentrated at the peak for higher current densities. The presence
of
mirroring electrons is vital to the system, as they play a major role in satisfying quasi-neutrality, and
support the majority of the parallel electric field.
Ion outflow is a feature of both current
regions, but is stronger and extends to lower altitudes for down¬
ward currents: this is presented as a possible explanation for observed lower-altitude acceleration in downward currents
compared to upward currents.
The effect of downward currents on E region number density is studied using an Alfven wave model
of
magnetosphere-ionosphere interaction, employing a height-integrated Pedersen conductivity. It is found
that
significant E region depletion and current broadening are more common on the nightside than on the
dayside, and occur in ~ 10 - 100 s.
Thu, 30 Nov 2006 00:00:00 GMThttps://hdl.handle.net/10023/195202006-11-30T00:00:00ZCran-McGreehin, Alexandra P.Field-aligned currents at Earth's high latitudes are principally carried by accelerated electrons. Current
densities, typically ~µAm⁻² at ionospheric altitudes, are sustained by parallel potential drops of ~100 - 1000 V. This Thesis
presents Vlasov models of upward and downward current regions, where electrons are
described via distribution functions. The ion
density profile is fixed, and quasi-neutrality is invoked to solve
numerically for the potential variation.
In both cases, an ambipolar electric field traps ionospheric electrons. For downward currents, an energetic ionospheric electron beam emerges into the magnetosphere where it is accelerated around the B/n
peak at altitudes of 500 - 6000 km to carry the current. The electric field maximises just Earthward of
the
B/n peak. The magnitude and altitude of the potential is found to depend solely on the equilibrium
properties immediately above the B/n peak. An analytic non-linear current-voltage relation, analagous to the linear
Knight relation for upward currents, is derived.
Energetic magnetospheric electrons precipitate into the ionosphere to carry upward currents. The continuous
potential variation is solved for current densities ~1 µAm⁻². Acceleration extends above the B/n
peak for ~1 R[sub]E, and is increasingly concentrated at the peak for higher current densities. The presence
of
mirroring electrons is vital to the system, as they play a major role in satisfying quasi-neutrality, and
support the majority of the parallel electric field.
Ion outflow is a feature of both current
regions, but is stronger and extends to lower altitudes for down¬
ward currents: this is presented as a possible explanation for observed lower-altitude acceleration in downward currents
compared to upward currents.
The effect of downward currents on E region number density is studied using an Alfven wave model
of
magnetosphere-ionosphere interaction, employing a height-integrated Pedersen conductivity. It is found
that
significant E region depletion and current broadening are more common on the nightside than on the
dayside, and occur in ~ 10 - 100 s.Modelling solar coronal magnetic field evolution
https://hdl.handle.net/10023/19238
Footpoint motions at the photosphere can inject energy into the magnetic ﬁeld in the solar
corona. This energy is then released in the corona as heat. There are many mathematical
approaches to model the evolution of these magnetic ﬁelds. Magnetohydrodynamics (MHD)
provides the most convenient and practical approach. However, there are many alternative
approximate methods. It is diﬃcult to know when an approximate method is valid and
how well the assumptions need to be satisﬁed for the solutions to be accurate enough.
To illustrate this, a simple experiment is performed. Four approximate methods, including
Reduced MHD (RMHD), are used to model the evolution of a footpoint driven coronal loop
through sequences of equilibria. The predicted evolution from each method is compared
to the solution from full MHD simulations to test the accuracy of each method when
the relevant assumptions are adjusted. After this initial test, the validity of RMHD is
investigated for the particular case of the magnetic ﬁeld evolution involving the development
of the tearing instability. Full MHD simulations are used to argue the applicability of the
assumptions and conditions of RMHD for this evolution. The potential of this setup to
heat the corona is considered by performing full MHD simulations including thermodynamic
processes of optically thin radiation and thermal conduction. These additional processes
are not included in RMHD.
Tue, 03 Dec 2019 00:00:00 GMThttps://hdl.handle.net/10023/192382019-12-03T00:00:00ZGoldstraw, Erin ElizabethFootpoint motions at the photosphere can inject energy into the magnetic ﬁeld in the solar
corona. This energy is then released in the corona as heat. There are many mathematical
approaches to model the evolution of these magnetic ﬁelds. Magnetohydrodynamics (MHD)
provides the most convenient and practical approach. However, there are many alternative
approximate methods. It is diﬃcult to know when an approximate method is valid and
how well the assumptions need to be satisﬁed for the solutions to be accurate enough.
To illustrate this, a simple experiment is performed. Four approximate methods, including
Reduced MHD (RMHD), are used to model the evolution of a footpoint driven coronal loop
through sequences of equilibria. The predicted evolution from each method is compared
to the solution from full MHD simulations to test the accuracy of each method when
the relevant assumptions are adjusted. After this initial test, the validity of RMHD is
investigated for the particular case of the magnetic ﬁeld evolution involving the development
of the tearing instability. Full MHD simulations are used to argue the applicability of the
assumptions and conditions of RMHD for this evolution. The potential of this setup to
heat the corona is considered by performing full MHD simulations including thermodynamic
processes of optically thin radiation and thermal conduction. These additional processes
are not included in RMHD.Mathematical modelling of cancer invasion and metastatic spread
https://hdl.handle.net/10023/19080
Metastatic spread—the dissemination of cancer cells from a primary tumour with subsequent re-colonisation at secondary sites in the body—causes around 90% of cancer-related deaths. Mathematical modelling may provide a complementary approach to help understand the complex mechanisms underlying metastasis. In particular, the spatiotemporal evolution of individual cancer cells during the so-called invasion-metastasis cascade—i.e. during cancer cell invasion, intravasation, vascular travel, extravasation and metastatic growth—is an aspect not yet explored through existing mathematical models. In this thesis, such a spatially explicit hybrid multi-organ metastasis modelling framework is developed. It describes the invasive growth dynamics of individual cancer cells both at a primary site and at potential secondary metastatic sites in the body, as well as their transport from the primary to the secondary sites. Throughout, the interactions between the cancer cells, matrix-degrading enzymes (MDEs) and the extracellular matrix (ECM) are accounted for. Furthermore, the individual-based framework models phenotypic variation by distinguishing between cancer cells of an epithelial-like, a mesenchymal-like and a mixed phenotype. It also describes permanent and transient mutations between these cell phenotypes in the form of epithelial-mesenchymal transition (EMT) and its reverse process mesenchymal-epithelial transition (MET). Both of these mechanisms are implemented at the biologically appropriate locations of the invasion-metastasis cascade. Finally, cancer cell dormancy and death at the metastatic sites are considered to model the frequently observed maladaptation of metastasised cancer cells to their new microenvironments. To investigate the EMT-process further, an additional three-dimensional discrete-continuum model of EMT- and MET-dependent cancer cell invasion is developed. It consists of a hybrid system of partial and stochastic differential equations that describe the evolution of epithelial-like and mesenchymal-like cancer cells, again under the consideration of MDE concentrations and the ECM density. Using inverse parameter estimation and sensitivity analysis, this model is calibrated to an in vitro organotypic assay experiment that examines the invasion of HSC-3 cancer cells.
Tue, 03 Dec 2019 00:00:00 GMThttps://hdl.handle.net/10023/190802019-12-03T00:00:00ZFranssen, Linnea ChristinMetastatic spread—the dissemination of cancer cells from a primary tumour with subsequent re-colonisation at secondary sites in the body—causes around 90% of cancer-related deaths. Mathematical modelling may provide a complementary approach to help understand the complex mechanisms underlying metastasis. In particular, the spatiotemporal evolution of individual cancer cells during the so-called invasion-metastasis cascade—i.e. during cancer cell invasion, intravasation, vascular travel, extravasation and metastatic growth—is an aspect not yet explored through existing mathematical models. In this thesis, such a spatially explicit hybrid multi-organ metastasis modelling framework is developed. It describes the invasive growth dynamics of individual cancer cells both at a primary site and at potential secondary metastatic sites in the body, as well as their transport from the primary to the secondary sites. Throughout, the interactions between the cancer cells, matrix-degrading enzymes (MDEs) and the extracellular matrix (ECM) are accounted for. Furthermore, the individual-based framework models phenotypic variation by distinguishing between cancer cells of an epithelial-like, a mesenchymal-like and a mixed phenotype. It also describes permanent and transient mutations between these cell phenotypes in the form of epithelial-mesenchymal transition (EMT) and its reverse process mesenchymal-epithelial transition (MET). Both of these mechanisms are implemented at the biologically appropriate locations of the invasion-metastasis cascade. Finally, cancer cell dormancy and death at the metastatic sites are considered to model the frequently observed maladaptation of metastasised cancer cells to their new microenvironments. To investigate the EMT-process further, an additional three-dimensional discrete-continuum model of EMT- and MET-dependent cancer cell invasion is developed. It consists of a hybrid system of partial and stochastic differential equations that describe the evolution of epithelial-like and mesenchymal-like cancer cells, again under the consideration of MDE concentrations and the ECM density. Using inverse parameter estimation and sensitivity analysis, this model is calibrated to an in vitro organotypic assay experiment that examines the invasion of HSC-3 cancer cells.Estimating abundance of African great apes
https://hdl.handle.net/10023/18859
All species and subspecies of African great apes are listed by the International Union for the Conservation of Nature as endangered or critically endangered, and populations continue to decline. As human populations and industry expand into great ape habitat, efficient, reliable estimators of great ape abundance are needed to inform conservation status and land-use planning, to assess adverse and beneficial effects of human activities, and to help funding agencies and donors make informed and efficient contributions. Fortunately, technological advances have improved our ability to sample great apes remotely, and new statistical methods for estimating abundance are constantly in development. Following a brief general introduction, this thesis reviews established and emerging approaches to estimating great ape abundance, then describes new methods for estimating animal density from photographic data by distance sampling with camera traps, and for selecting among models of the distance sampling detection function when distance data are overdispersed. Subsequent chapters quantify the effect of violating the assumption of demographic closure when estimating abundance using spatially explicit capture–recapture models for closed populations, and describe the design and implementation of a camera trapping survey of chimpanzees at the landscape scale in Kibale National Park, Uganda. The new methods developed have generated considerable interest, and allow abundances of multiple species, including great apes, to be estimated from data collected during a single photographic survey. Spatially explicit capture–recapture analyses of photographic data from small study areas yielded accurate and precise estimates of chimpanzee abundance, and this combination of methods could be used to enumerate great apes over large areas and in dense forests more reliably and efficiently than previously possible.
Tue, 03 Dec 2019 00:00:00 GMThttps://hdl.handle.net/10023/188592019-12-03T00:00:00ZHowe, Eric J.All species and subspecies of African great apes are listed by the International Union for the Conservation of Nature as endangered or critically endangered, and populations continue to decline. As human populations and industry expand into great ape habitat, efficient, reliable estimators of great ape abundance are needed to inform conservation status and land-use planning, to assess adverse and beneficial effects of human activities, and to help funding agencies and donors make informed and efficient contributions. Fortunately, technological advances have improved our ability to sample great apes remotely, and new statistical methods for estimating abundance are constantly in development. Following a brief general introduction, this thesis reviews established and emerging approaches to estimating great ape abundance, then describes new methods for estimating animal density from photographic data by distance sampling with camera traps, and for selecting among models of the distance sampling detection function when distance data are overdispersed. Subsequent chapters quantify the effect of violating the assumption of demographic closure when estimating abundance using spatially explicit capture–recapture models for closed populations, and describe the design and implementation of a camera trapping survey of chimpanzees at the landscape scale in Kibale National Park, Uganda. The new methods developed have generated considerable interest, and allow abundances of multiple species, including great apes, to be estimated from data collected during a single photographic survey. Spatially explicit capture–recapture analyses of photographic data from small study areas yielded accurate and precise estimates of chimpanzee abundance, and this combination of methods could be used to enumerate great apes over large areas and in dense forests more reliably and efficiently than previously possible.Theoretical models of charged particle acceleration motivated by solar flares
https://hdl.handle.net/10023/18614
In this thesis we examine non-thermal particle behaviour in the presence of
modelled electromagnetic ﬁelds motivated by various aspects of solar ﬂares.
We ﬁrst investigate particle dynamics in magnetic reconnection scenarios,
in particular 2D reconnection in force-free current sheets and 3D separator
reconnection. The electromagnetic ﬁelds are obtained by performing resistive
magnetohydrodynamic (MHD) simulations with a non-zero anomalous resistivity
speciﬁed in regions where the local current density exceeds a speciﬁed
threshold. Test particle orbits and energy spectra are computed in the resulting
electromagnetic ﬁelds using the relativistic guiding centre equations. Motivated
by the enhanced anomalous resistivity, which is several orders of magnitude
greater than the Spitzer resistivity, pitch angle scattering linked to the resistivity
is introduced into guiding centre formalism when the test particle is located in
regions of non-zero resistivity.
In 2D reconnection, pitch angle scattering modiﬁes the particle trajectories,
energy gain and orbit duration. In certain cases, pitch angle scattering allows
test particles to gain more energy than would be possible in the absence of
scattering due to particles traversing the reconnection region multiple times, hence
experiencing a parallel electric ﬁeld component along a greater portion of their
orbits. We observe many of the same phenomena in 3D separator reconnection
simulations, however changes in particle energy spectra are minimal in comparison
to the 2D case.
We also investigate test particle behaviour in an analytical model of a collapsing
magnetic trap with the inclusion of a jet braking region at the loop apex, which
consists of an indentation in the loops caused by an interaction of a reconnection
outﬂow with low lying magnetic ﬁeld. New types of particle orbits that are not
observed in the absence of the braking jet are characterised. The effects of different
trap parameters on particle energisation and orbit behaviour are also examined.
Tue, 25 Jun 2019 00:00:00 GMThttps://hdl.handle.net/10023/186142019-06-25T00:00:00ZBorissov, AlexeiIn this thesis we examine non-thermal particle behaviour in the presence of
modelled electromagnetic ﬁelds motivated by various aspects of solar ﬂares.
We ﬁrst investigate particle dynamics in magnetic reconnection scenarios,
in particular 2D reconnection in force-free current sheets and 3D separator
reconnection. The electromagnetic ﬁelds are obtained by performing resistive
magnetohydrodynamic (MHD) simulations with a non-zero anomalous resistivity
speciﬁed in regions where the local current density exceeds a speciﬁed
threshold. Test particle orbits and energy spectra are computed in the resulting
electromagnetic ﬁelds using the relativistic guiding centre equations. Motivated
by the enhanced anomalous resistivity, which is several orders of magnitude
greater than the Spitzer resistivity, pitch angle scattering linked to the resistivity
is introduced into guiding centre formalism when the test particle is located in
regions of non-zero resistivity.
In 2D reconnection, pitch angle scattering modiﬁes the particle trajectories,
energy gain and orbit duration. In certain cases, pitch angle scattering allows
test particles to gain more energy than would be possible in the absence of
scattering due to particles traversing the reconnection region multiple times, hence
experiencing a parallel electric ﬁeld component along a greater portion of their
orbits. We observe many of the same phenomena in 3D separator reconnection
simulations, however changes in particle energy spectra are minimal in comparison
to the 2D case.
We also investigate test particle behaviour in an analytical model of a collapsing
magnetic trap with the inclusion of a jet braking region at the loop apex, which
consists of an indentation in the loops caused by an interaction of a reconnection
outﬂow with low lying magnetic ﬁeld. New types of particle orbits that are not
observed in the absence of the braking jet are characterised. The effects of different
trap parameters on particle energisation and orbit behaviour are also examined.Orderings on words and permutations
https://hdl.handle.net/10023/18465
Substructure orderings are ubiquitous throughout combinatorics and all of mathematics.
In this thesis we consider various orderings on words, as well as the consecutive
involvement ordering on permutations. Throughout there will be a focus
on deciding certain order-theoretic properties, primarily the properties of being well-quasi-ordered
(WQO) and of being atomic.
In Chapter 1, we establish the background material required for the remainder of
the thesis. This will include concepts from order theory, formal language theory, automata
theory, and the theory of permutations. We also introduce various orderings
on words, and the consecutive involvement ordering on permutations.
In Chapter 2, we consider the prefix, suffix and factor orderings on words. For the
prefix and suffix orderings, we give a characterisation of the regular languages which
are WQO, and of those which are atomic. We then consider the factor ordering and
show that the atomicity is decidable for finitely-based sets. We also give a new proof
that WQO is decidable for finitely-based sets, which is a special case of a result of
Atminas et al.
In Chapters 3 and 4, we consider some general families of orderings on words. In
Chapter 3 we consider orderings on words which are rational, meaning that they can
be generated by transducers. We discuss the class of insertion relations introduced
in a paper by the author, and introduce a generalisation. In Chapter 4, we
consider three other variations of orderings on words. Throughout these chapters we
prove various decidability results.
In Chapter 5, we consider the consecutive involvement on permutations. We generalise
our results for the factor ordering on words to show that WQO and atomicity
are decidable. Through this investigation we answer some questions which have been
asked (and remain open) for the involvement on permutations.
Tue, 03 Dec 2019 00:00:00 GMThttps://hdl.handle.net/10023/184652019-12-03T00:00:00ZMcDevitt, MatthewSubstructure orderings are ubiquitous throughout combinatorics and all of mathematics.
In this thesis we consider various orderings on words, as well as the consecutive
involvement ordering on permutations. Throughout there will be a focus
on deciding certain order-theoretic properties, primarily the properties of being well-quasi-ordered
(WQO) and of being atomic.
In Chapter 1, we establish the background material required for the remainder of
the thesis. This will include concepts from order theory, formal language theory, automata
theory, and the theory of permutations. We also introduce various orderings
on words, and the consecutive involvement ordering on permutations.
In Chapter 2, we consider the prefix, suffix and factor orderings on words. For the
prefix and suffix orderings, we give a characterisation of the regular languages which
are WQO, and of those which are atomic. We then consider the factor ordering and
show that the atomicity is decidable for finitely-based sets. We also give a new proof
that WQO is decidable for finitely-based sets, which is a special case of a result of
Atminas et al.
In Chapters 3 and 4, we consider some general families of orderings on words. In
Chapter 3 we consider orderings on words which are rational, meaning that they can
be generated by transducers. We discuss the class of insertion relations introduced
in a paper by the author, and introduce a generalisation. In Chapter 4, we
consider three other variations of orderings on words. Throughout these chapters we
prove various decidability results.
In Chapter 5, we consider the consecutive involvement on permutations. We generalise
our results for the factor ordering on words to show that WQO and atomicity
are decidable. Through this investigation we answer some questions which have been
asked (and remain open) for the involvement on permutations.Methods in spatially explicit capture-recapture
https://hdl.handle.net/10023/18233
Capture-recapture (CR) methods are a ubiquitous means of estimating animal abundance from wildlife surveys. They rely on the detection and subsequent redetection of individuals over a number of sampling occasions. It is usually necessary for individuals to be recognised upon redetection. Spatially explicit capture-recapture (SECR) methods generalise those of CR by accounting for the locations at which each detection occurs. This allows spatial heterogeneity in detection probabilities to be accounted for: individuals with home-range centres near the detector array are more likely to be detected. They also permit estimation of animal density in addition to abundance.
One particular advantage of SECR methods is that they can be used when individuals are detected via the cues they produce---examples include birdsongs detected by microphones and whale surfacings detected by human observers. In such situations each cue may be detected by multiple detectors at different fixed locations. Redetections are then spatial (rather than temporal) in nature, and density can be estimated from a single survey occasion.
Existing methods, however, cannot generally be appropriately applied to the resulting cue-detection data without making assumptions that rarely hold. Additionally, they usually estimate cue density rather than animal density, which does not usually have the same biological importance. This thesis extends SECR methodology primarily for the appropriate estimation of animal density from cue-based SECR surveys. These extensions include (i) incorporation of auxiliary survey data into SECR estimators, (ii) appropriate point and variance estimators of animal density for a range of scenarios, and (iii) methods to account for both heterogeneity in detectability and cues that are directional in nature.
Moreover, a general class of methods is presented for the estimation of demographic parameters from wildlife surveys on which individuals cannot be recognised. These can variously be applied to CR and---potentially---SECR.
Fri, 24 Jun 2016 00:00:00 GMThttps://hdl.handle.net/10023/182332016-06-24T00:00:00ZStevenson, Ben C.Capture-recapture (CR) methods are a ubiquitous means of estimating animal abundance from wildlife surveys. They rely on the detection and subsequent redetection of individuals over a number of sampling occasions. It is usually necessary for individuals to be recognised upon redetection. Spatially explicit capture-recapture (SECR) methods generalise those of CR by accounting for the locations at which each detection occurs. This allows spatial heterogeneity in detection probabilities to be accounted for: individuals with home-range centres near the detector array are more likely to be detected. They also permit estimation of animal density in addition to abundance.
One particular advantage of SECR methods is that they can be used when individuals are detected via the cues they produce---examples include birdsongs detected by microphones and whale surfacings detected by human observers. In such situations each cue may be detected by multiple detectors at different fixed locations. Redetections are then spatial (rather than temporal) in nature, and density can be estimated from a single survey occasion.
Existing methods, however, cannot generally be appropriately applied to the resulting cue-detection data without making assumptions that rarely hold. Additionally, they usually estimate cue density rather than animal density, which does not usually have the same biological importance. This thesis extends SECR methodology primarily for the appropriate estimation of animal density from cue-based SECR surveys. These extensions include (i) incorporation of auxiliary survey data into SECR estimators, (ii) appropriate point and variance estimators of animal density for a range of scenarios, and (iii) methods to account for both heterogeneity in detectability and cues that are directional in nature.
Moreover, a general class of methods is presented for the estimation of demographic parameters from wildlife surveys on which individuals cannot be recognised. These can variously be applied to CR and---potentially---SECR.The statistical development of integrated multi-state stopover models
https://hdl.handle.net/10023/18206
This thesis focusses on the analysis of ecological capture-recapture data and the
estimation of population parameters of interest. Many of the common models applied
to such data, for example the Cormack-Jolly-Seber model, condition on the first capture of an individual or on the number of individuals encountered. A consequence
of this conditioning is that it is not possible to estimate the total abundance
directly. Stopover models remove the conditioning on first capture and instead
explicitly model the arrival of individuals into the population. This permits the
estimation of abundance through the likelihood along with other parameters such
as capture and retention probabilities.
We develop an integrated stopover model capable of analysing multiple years of
data within a single likelihood and allowing parameters to be shared across years.
We consider special cases of this model, writing the likelihood using sufficient statistics
as well as utilising the hidden Markov model framework to allow for efficient
evaluation of the likelihood. We further extend this model to an integrated multistate-stopover model which incorporates any available discrete state information.
The new stopover models are applied to real ecological data sets. A cohort-dependent
single-year stopover model is applied to data on grey seals, Halichoerus
grypus, where the cohorts are determined by birth year. The integrated stopover
model and integrated multi-state stopover model are used to analyse a data set on
great crested newts, Triturus cristatus. A subset of this data is used to explore closed population models that permit capture probabilities to depend on discrete
state information. The final section of this thesis considers a capture-recapture-recovery
data set relating to Soay sheep, a breed of domestic sheep Ovis aries.
These data contain individual time-varying continuous covariates and raise the issue
of dealing with missing data.
Fri, 24 Jun 2016 00:00:00 GMThttps://hdl.handle.net/10023/182062016-06-24T00:00:00ZWorthington, HannahThis thesis focusses on the analysis of ecological capture-recapture data and the
estimation of population parameters of interest. Many of the common models applied
to such data, for example the Cormack-Jolly-Seber model, condition on the first capture of an individual or on the number of individuals encountered. A consequence
of this conditioning is that it is not possible to estimate the total abundance
directly. Stopover models remove the conditioning on first capture and instead
explicitly model the arrival of individuals into the population. This permits the
estimation of abundance through the likelihood along with other parameters such
as capture and retention probabilities.
We develop an integrated stopover model capable of analysing multiple years of
data within a single likelihood and allowing parameters to be shared across years.
We consider special cases of this model, writing the likelihood using sufficient statistics
as well as utilising the hidden Markov model framework to allow for efficient
evaluation of the likelihood. We further extend this model to an integrated multistate-stopover model which incorporates any available discrete state information.
The new stopover models are applied to real ecological data sets. A cohort-dependent
single-year stopover model is applied to data on grey seals, Halichoerus
grypus, where the cohorts are determined by birth year. The integrated stopover
model and integrated multi-state stopover model are used to analyse a data set on
great crested newts, Triturus cristatus. A subset of this data is used to explore closed population models that permit capture probabilities to depend on discrete
state information. The final section of this thesis considers a capture-recapture-recovery
data set relating to Soay sheep, a breed of domestic sheep Ovis aries.
These data contain individual time-varying continuous covariates and raise the issue
of dealing with missing data.Mathematical modelling of tumour-immune competition tumour growth : discrete and continuum approaches
https://hdl.handle.net/10023/18194
The ability of the human immune system to detect and remove cancer cells is exploited in the development of immunotherapy techniques. However, further understanding of these mechanisms is required and can be achieved through the use of mathematical models. In this thesis, we develop a simple individual-based model of cell movement and illustrate the ability of our model to qualitatively reproduce the migration patterns of immune cells that have been observed in single cell tracking experiments. We then extend the model to describe the spatio-temporal interactions between dendritic cells, cytotoxic T cells and a solid tumour. Through further extension of the model, we explicitly consider the immune recognition of evolving tumour antigens. Computational simulations of our models further clarify the conditions for the onset of a successful immune action against cancer cells and may suggest possible targets to improve the efficacy of immunotherapy. Mathematically, individual-based models can be limited in their amenability to different analysis techniques which are better suited to continuum models. To overcome this, we aim to derive the continuum version of our described individual-based models. However, due to the complexity of the biological mechanisms included, we first consider a simpler biological situation. We develop an individual-based model describing the spatial dynamics of multicellular systems whereby cells undergo pressure-driven movement and pressure-dependent proliferation. From this, we formally derive nonlinear partial differential equations that are commonly used to model the spatial dynamics of growing cell populations. Through systematic comparison of both models, we demonstrate that the results of computational simulations of the individual-based model faithfully mirror the qualitative and quantitative properties of the solutions to the corresponding partial differential equations. This method could be adapted to more complex individual-based models, such as those we describe in this work.
Tue, 03 Dec 2019 00:00:00 GMThttps://hdl.handle.net/10023/181942019-12-03T00:00:00ZMacfarlane, Fiona RuthThe ability of the human immune system to detect and remove cancer cells is exploited in the development of immunotherapy techniques. However, further understanding of these mechanisms is required and can be achieved through the use of mathematical models. In this thesis, we develop a simple individual-based model of cell movement and illustrate the ability of our model to qualitatively reproduce the migration patterns of immune cells that have been observed in single cell tracking experiments. We then extend the model to describe the spatio-temporal interactions between dendritic cells, cytotoxic T cells and a solid tumour. Through further extension of the model, we explicitly consider the immune recognition of evolving tumour antigens. Computational simulations of our models further clarify the conditions for the onset of a successful immune action against cancer cells and may suggest possible targets to improve the efficacy of immunotherapy. Mathematically, individual-based models can be limited in their amenability to different analysis techniques which are better suited to continuum models. To overcome this, we aim to derive the continuum version of our described individual-based models. However, due to the complexity of the biological mechanisms included, we first consider a simpler biological situation. We develop an individual-based model describing the spatial dynamics of multicellular systems whereby cells undergo pressure-driven movement and pressure-dependent proliferation. From this, we formally derive nonlinear partial differential equations that are commonly used to model the spatial dynamics of growing cell populations. Through systematic comparison of both models, we demonstrate that the results of computational simulations of the individual-based model faithfully mirror the qualitative and quantitative properties of the solutions to the corresponding partial differential equations. This method could be adapted to more complex individual-based models, such as those we describe in this work.Assouad type dimensions and dimension spectra
https://hdl.handle.net/10023/18157
In the first part of this thesis we introduce a new dimension spectrum motivated by the Assouad dimension; a familiar notion of dimension which, for a given metric space, returns the minimal exponent α ≥ 0 such that for any pair of scales 0 < r < R, any ball of radius R may be covered by a constant times (R/r)ᵅ balls of radius r. To each 𝛩 ∈ (0,1), we associate the appropriate analogue of the Assouad dimension with the restriction that the two scales r and R used in the definition satisfy log R/log r = 𝛩. The resulting 'dimension spectrum' (as a function of 𝛩) thus gives finer geometric information regarding the scaling structure of the space and, in some precise sense, interpolates between the upper box dimension and the Assouad dimension. This latter point is particularly useful because the spectrum is generally better behaved than the Assouad dimension. We also consider the corresponding 'lower spectrum', motivated by the lower dimension, which acts as a dual to the Assouad spectrum. We conduct a detailed study of these dimension spectra; including analytic and geometric properties. We also compute the spectra explicitly for some common examples of fractals including decreasing sequences with decreasing gaps and spirals with sub-exponential and monotonic winding. We also give several applications of our results, including: dimension distortion estimates under bi-Hölder maps for Assouad dimension. We compute the spectrum explicitly for a range of well-studied fractal sets, including: the self-affine carpets of Bedford and McMullen, self-similar and self-conformal sets with overlaps, Mandelbrot percolation, and Moran constructions. We find that the spectrum behaves differently for each of these models and can take on a rich variety of forms. We also consider some applications, including the provision of new bi-Lipschitz invariants and bounds on a family of 'tail densities' defined for subsets of the integers.
In the second part of this thesis, we study the Assouad dimension of sets of integers and deduce a weak solution to the Erdős-Turán conjecture. Let 𝐹 ⊂ ℕ. If $\sum_{n\in F}n^{-1}=\infty$ then 𝐹 "asymptotically" contains arbitrarily long arithmetic progressions.
Tue, 03 Dec 2019 00:00:00 GMThttps://hdl.handle.net/10023/181572019-12-03T00:00:00ZYu, HanIn the first part of this thesis we introduce a new dimension spectrum motivated by the Assouad dimension; a familiar notion of dimension which, for a given metric space, returns the minimal exponent α ≥ 0 such that for any pair of scales 0 < r < R, any ball of radius R may be covered by a constant times (R/r)ᵅ balls of radius r. To each 𝛩 ∈ (0,1), we associate the appropriate analogue of the Assouad dimension with the restriction that the two scales r and R used in the definition satisfy log R/log r = 𝛩. The resulting 'dimension spectrum' (as a function of 𝛩) thus gives finer geometric information regarding the scaling structure of the space and, in some precise sense, interpolates between the upper box dimension and the Assouad dimension. This latter point is particularly useful because the spectrum is generally better behaved than the Assouad dimension. We also consider the corresponding 'lower spectrum', motivated by the lower dimension, which acts as a dual to the Assouad spectrum. We conduct a detailed study of these dimension spectra; including analytic and geometric properties. We also compute the spectra explicitly for some common examples of fractals including decreasing sequences with decreasing gaps and spirals with sub-exponential and monotonic winding. We also give several applications of our results, including: dimension distortion estimates under bi-Hölder maps for Assouad dimension. We compute the spectrum explicitly for a range of well-studied fractal sets, including: the self-affine carpets of Bedford and McMullen, self-similar and self-conformal sets with overlaps, Mandelbrot percolation, and Moran constructions. We find that the spectrum behaves differently for each of these models and can take on a rich variety of forms. We also consider some applications, including the provision of new bi-Lipschitz invariants and bounds on a family of 'tail densities' defined for subsets of the integers.
In the second part of this thesis, we study the Assouad dimension of sets of integers and deduce a weak solution to the Erdős-Turán conjecture. Let 𝐹 ⊂ ℕ. If $\sum_{n\in F}n^{-1}=\infty$ then 𝐹 "asymptotically" contains arbitrarily long arithmetic progressions.On the emergence and evolution of jets and vortices in turbulent planetary atmospheres
https://hdl.handle.net/10023/17924
This thesis investigates the formation and evolution of jets and vortices in turbulent planetary atmospheres using a dual approach of high-resolution numerical simulations and
novel laboratory experiments. A two-layer quasi-geostrophic beta-channel shallow water
model is used for the numerical study. As in Panetta (1988), a vertical shear is implemented
to represent a spatially-mean latitudinal temperature gradient, which is partially
maintained by thermal relaxation. Baroclinic instabilities work to erode the temperature
gradient, while thermal relaxation acts to restore it. As the basic state vertical shear is
unstable, the thermal relaxation cannot lead to a full recovery, thus modifying subsequent
instabilities and leading to rich nonlinear dynamical behaviour.
First, we consider flow over a flat bottom, and model convective motions like those
thought to occur on Jupiter by pairs of cyclones/anti-cyclones or ‘hetons’ as in Thomson
(2016). We thereby obtain predominantly baroclinic jets, oscillating between quiescent
phases, when jets are zonal and the energy is nearly stationary, and turbulent phases,
when the flow loses its zonality, vortices pinch off from the meandering jets, and zonal
energy components drop while eddy energy components increase. These turbulent phases
typically last for a thermal relaxation period. The impacts of vertical shear
(baroclinicity), thermal relaxation and heton forcing are comprehensively investigated
by considering the energy transfers occurring between kinetic and potential energy, their
barotropic and baroclinic parts as well as their zonal and eddy parts. This leads to a
rethinking of the classic paradigm of energy transfer presented by Salmon (1982), as this
paradigm is too simplistic to explain the results found.
Then, we consider the effect of large-scale bottom topography, as a first approach to
understanding the role of topography in jet and vortex formation. We use the same model
as in the first study but include a linearly sloping topography which has the advantage
of being characterised by a single parameter, the slope. We omit the heton forcing and
instead perturb the flow with a small amplitude Rossby wave initially. The main effect
of heton forcing is actually to act as a kind of damping: energy fluctuations are consistently
less extreme than when no forcing is used. A linear stability analysis is carried
out to motivate a series of nonlinear simulations investigating the effect of topography,
in particular, differences from the flat bottom case previously examined. We find that
destabilising topography makes the jets more dynamic.
In the experimental part, a two-layer salt-stratified fluid is used in a rotating tank with a differentially rotating lid to generate the shear across the interface. We consider
a baroclinically unstable front in the regime of amplitude vacillation, which is found to
be characterised by the sequential emergence and disappearance of a large-scale vortex.
Analysing two similar experiments at the limit of geostrophy, with different Rossby numbers
Ro=0.4 and Ro=0.6, shows surprisingly different behaviours, with a baroclinic dipole
for small, and a barotropic vortex for the large Rossby number. The small-scale wave activity is explored using different methods, and the results suggest small, spontaneously arising inertia-gravity waves preceding the emergence of the vortex which stirs the interface, thus having an impact on the mixing between the two layers. The recovery period of the amplitude vacillation, as well as the intensity of the vortex, increases with the Rossby
number.
For further research on fronts at two-layer immiscible interfaces, a very accurate novel
optical method has been developed to detect the height and slope, based on the refractive
laws of optics. The associated theoretical equations are solved numerically and validated
in various idealised situations.
Tue, 25 Jun 2019 00:00:00 GMThttps://hdl.handle.net/10023/179242019-06-25T00:00:00ZJougla, ThibaultThis thesis investigates the formation and evolution of jets and vortices in turbulent planetary atmospheres using a dual approach of high-resolution numerical simulations and
novel laboratory experiments. A two-layer quasi-geostrophic beta-channel shallow water
model is used for the numerical study. As in Panetta (1988), a vertical shear is implemented
to represent a spatially-mean latitudinal temperature gradient, which is partially
maintained by thermal relaxation. Baroclinic instabilities work to erode the temperature
gradient, while thermal relaxation acts to restore it. As the basic state vertical shear is
unstable, the thermal relaxation cannot lead to a full recovery, thus modifying subsequent
instabilities and leading to rich nonlinear dynamical behaviour.
First, we consider flow over a flat bottom, and model convective motions like those
thought to occur on Jupiter by pairs of cyclones/anti-cyclones or ‘hetons’ as in Thomson
(2016). We thereby obtain predominantly baroclinic jets, oscillating between quiescent
phases, when jets are zonal and the energy is nearly stationary, and turbulent phases,
when the flow loses its zonality, vortices pinch off from the meandering jets, and zonal
energy components drop while eddy energy components increase. These turbulent phases
typically last for a thermal relaxation period. The impacts of vertical shear
(baroclinicity), thermal relaxation and heton forcing are comprehensively investigated
by considering the energy transfers occurring between kinetic and potential energy, their
barotropic and baroclinic parts as well as their zonal and eddy parts. This leads to a
rethinking of the classic paradigm of energy transfer presented by Salmon (1982), as this
paradigm is too simplistic to explain the results found.
Then, we consider the effect of large-scale bottom topography, as a first approach to
understanding the role of topography in jet and vortex formation. We use the same model
as in the first study but include a linearly sloping topography which has the advantage
of being characterised by a single parameter, the slope. We omit the heton forcing and
instead perturb the flow with a small amplitude Rossby wave initially. The main effect
of heton forcing is actually to act as a kind of damping: energy fluctuations are consistently
less extreme than when no forcing is used. A linear stability analysis is carried
out to motivate a series of nonlinear simulations investigating the effect of topography,
in particular, differences from the flat bottom case previously examined. We find that
destabilising topography makes the jets more dynamic.
In the experimental part, a two-layer salt-stratified fluid is used in a rotating tank with a differentially rotating lid to generate the shear across the interface. We consider
a baroclinically unstable front in the regime of amplitude vacillation, which is found to
be characterised by the sequential emergence and disappearance of a large-scale vortex.
Analysing two similar experiments at the limit of geostrophy, with different Rossby numbers
Ro=0.4 and Ro=0.6, shows surprisingly different behaviours, with a baroclinic dipole
for small, and a barotropic vortex for the large Rossby number. The small-scale wave activity is explored using different methods, and the results suggest small, spontaneously arising inertia-gravity waves preceding the emergence of the vortex which stirs the interface, thus having an impact on the mixing between the two layers. The recovery period of the amplitude vacillation, as well as the intensity of the vortex, increases with the Rossby
number.
For further research on fronts at two-layer immiscible interfaces, a very accurate novel
optical method has been developed to detect the height and slope, based on the refractive
laws of optics. The associated theoretical equations are solved numerically and validated
in various idealised situations.Three dimensional MHD simulations of the dynamics and energetics of coronal flux tubes
https://hdl.handle.net/10023/17512
In this thesis we present the results of three-dimensional MHD simulations of the evolution of magnetic flux tubes within the solar atmosphere. We consider the dynamics and energetics of coronal loops that are perturbed from an equilibrium state by wave motions or driven continuously by an imposed velocity field. In each case, we investigate the dissipation of magnetic and kinetic energy and evaluate the implications for the heating of coronal plasma.
We present models of transversely oscillating flux tubes which experience rapid damping as kink mode energy is transferred into azimuthal Alfven modes. This mode conversion is typically associated with a density enhancement within the flux tube, however, we demonstrate that it can also proceed with an increased internal magnetic field strength. In either regime, the azimuthal wave modes are subject to dissipation through phase mixing and may promote the development of the magnetic Kelvin-Helmholtz instability. This is associated with the generation of further small scales in the magnetic and velocity fields and, in a non-ideal regime, will enhance the rate of wave dissipation. We show that the growth rate of the instability is sensitive to the implemented transport coefficients and the presence of helical magnetic field.
Additionally, we consider the effects of thermal conduction and optically thin radiation on the evolution of a flux tube tectonics model. We present the results of simulations in which two magnetic flux tubes are twisted around each other by the action of rotational drivers imposed at the loop foot points. Large currents develop at the interface of the flux tubes and magnetic reconnection is triggered as the braiding progresses. The inclusion of conduction and optically thin radiation reduces the high temperatures and gas pressures observed in the centre of the numerical domain. As a result, these processes modify the reconnection outflows, distribution of plasma and the evolution of the magnetic field.
Tue, 25 Jun 2019 00:00:00 GMThttps://hdl.handle.net/10023/175122019-06-25T00:00:00ZHowson, Thomas AlexanderIn this thesis we present the results of three-dimensional MHD simulations of the evolution of magnetic flux tubes within the solar atmosphere. We consider the dynamics and energetics of coronal loops that are perturbed from an equilibrium state by wave motions or driven continuously by an imposed velocity field. In each case, we investigate the dissipation of magnetic and kinetic energy and evaluate the implications for the heating of coronal plasma.
We present models of transversely oscillating flux tubes which experience rapid damping as kink mode energy is transferred into azimuthal Alfven modes. This mode conversion is typically associated with a density enhancement within the flux tube, however, we demonstrate that it can also proceed with an increased internal magnetic field strength. In either regime, the azimuthal wave modes are subject to dissipation through phase mixing and may promote the development of the magnetic Kelvin-Helmholtz instability. This is associated with the generation of further small scales in the magnetic and velocity fields and, in a non-ideal regime, will enhance the rate of wave dissipation. We show that the growth rate of the instability is sensitive to the implemented transport coefficients and the presence of helical magnetic field.
Additionally, we consider the effects of thermal conduction and optically thin radiation on the evolution of a flux tube tectonics model. We present the results of simulations in which two magnetic flux tubes are twisted around each other by the action of rotational drivers imposed at the loop foot points. Large currents develop at the interface of the flux tubes and magnetic reconnection is triggered as the braiding progresses. The inclusion of conduction and optically thin radiation reduces the high temperatures and gas pressures observed in the centre of the numerical domain. As a result, these processes modify the reconnection outflows, distribution of plasma and the evolution of the magnetic field.Semigroup congruences : computational techniques and theoretical applications
https://hdl.handle.net/10023/17350
Computational semigroup theory is an area of research that is subject to growing interest. The development of semigroup algorithms allows for new theoretical results to be discovered, which in turn informs the creation of yet more algorithms. Groups have benefitted from this cycle since before the invention of electronic computers, and the popularity of computational group theory has resulted in a rich and detailed literature. Computational semigroup theory is a less developed field, but recent work has resulted in a variety of algorithms, and some important pieces of software such as the Semigroups package for GAP.
Congruences are an important part of semigroup theory. A semigroup’s congruences determine its homomorphic images in a manner analogous to a group’s normal subgroups. Prior to the work described here, there existed few practical algorithms for computing with semigroup congruences. However, a number of results about alternative representations for congruences, as well as existing algorithms that can be borrowed from group theory, make congruences a fertile area for improvement. In this thesis, we first consider computational techniques that can be applied to the study of congruences, and then present some results that have been produced or precipitated by applying these techniques to interesting examples.
After some preliminary theory, we present a new parallel approach to computing with congruences specified by generating pairs. We then consider alternative ways of representing a congruence, using intermediate objects such as linked triples. We also present an algorithm for computing the entire congruence lattice of a finite semigroup. In the second part of the thesis, we classify the congruences of several monoids of bipartitions, as well as the principal factors of several monoids of partial transformations. Finally, we consider how many congruences a finite semigroup can have, and examine those on semigroups with up to seven elements.
Tue, 25 Jun 2019 00:00:00 GMThttps://hdl.handle.net/10023/173502019-06-25T00:00:00ZTorpey, MichaelComputational semigroup theory is an area of research that is subject to growing interest. The development of semigroup algorithms allows for new theoretical results to be discovered, which in turn informs the creation of yet more algorithms. Groups have benefitted from this cycle since before the invention of electronic computers, and the popularity of computational group theory has resulted in a rich and detailed literature. Computational semigroup theory is a less developed field, but recent work has resulted in a variety of algorithms, and some important pieces of software such as the Semigroups package for GAP.
Congruences are an important part of semigroup theory. A semigroup’s congruences determine its homomorphic images in a manner analogous to a group’s normal subgroups. Prior to the work described here, there existed few practical algorithms for computing with semigroup congruences. However, a number of results about alternative representations for congruences, as well as existing algorithms that can be borrowed from group theory, make congruences a fertile area for improvement. In this thesis, we first consider computational techniques that can be applied to the study of congruences, and then present some results that have been produced or precipitated by applying these techniques to interesting examples.
After some preliminary theory, we present a new parallel approach to computing with congruences specified by generating pairs. We then consider alternative ways of representing a congruence, using intermediate objects such as linked triples. We also present an algorithm for computing the entire congruence lattice of a finite semigroup. In the second part of the thesis, we classify the congruences of several monoids of bipartitions, as well as the principal factors of several monoids of partial transformations. Finally, we consider how many congruences a finite semigroup can have, and examine those on semigroups with up to seven elements.Maximal subsemigroups of finite transformation and diagram monoids
https://hdl.handle.net/10023/17110
We describe and count the maximal subsemigroups of many well-known transformation monoids, and diagram monoids, using a new unified framework that allows the treatment of several classes of monoids simultaneously. The problem of determining the maximal subsemigroups of a finite monoid of transformations has been extensively studied in the literature. To our knowledge, every existing result in the literature is a special case of the approach we present. In particular, our technique can be used to determine the maximal subsemigroups of the full spectrum of monoids of order- or orientation-preserving transformations and partial permutations considered by I. Dimitrova, V. H. Fernandes, and co-authors. We only present details for the transformation monoids whose maximal subsemigroups were not previously known; and for certain diagram monoids, such as the partition, Brauer, Jones, and Motzkin monoids. The technique we present is based on a specialised version of an algorithm for determining the maximal subsemigroups of any finite semigroup, developed by the third and fourth authors, and available in the Semigroups package for GAP, an open source computer algebra system. This allows us to concisely present the descriptions of the maximal subsemigroups, and to clearly see their common features.
The first author gratefully acknowledges the support of the Glasgow Learning, Teaching, and Research Fund in partially funding his visit to the third author in July, 2014. The second author wishes to acknowledge the support of research initiation grant [0076|2016] provided by BITS Pilani, Pilani. The fourth author wishes to acknowledge the support of his Carnegie Ph.D. Scholarship from the Carnegie Trust for the Universities of Scotland.
Fri, 15 Jun 2018 00:00:00 GMThttps://hdl.handle.net/10023/171102018-06-15T00:00:00ZEast, JamesKumar, JitenderMitchell, James D.Wilson, Wilf A.We describe and count the maximal subsemigroups of many well-known transformation monoids, and diagram monoids, using a new unified framework that allows the treatment of several classes of monoids simultaneously. The problem of determining the maximal subsemigroups of a finite monoid of transformations has been extensively studied in the literature. To our knowledge, every existing result in the literature is a special case of the approach we present. In particular, our technique can be used to determine the maximal subsemigroups of the full spectrum of monoids of order- or orientation-preserving transformations and partial permutations considered by I. Dimitrova, V. H. Fernandes, and co-authors. We only present details for the transformation monoids whose maximal subsemigroups were not previously known; and for certain diagram monoids, such as the partition, Brauer, Jones, and Motzkin monoids. The technique we present is based on a specialised version of an algorithm for determining the maximal subsemigroups of any finite semigroup, developed by the third and fourth authors, and available in the Semigroups package for GAP, an open source computer algebra system. This allows us to concisely present the descriptions of the maximal subsemigroups, and to clearly see their common features.Computing maximal subsemigroups of a finite semigroup
https://hdl.handle.net/10023/17072
A proper subsemigroup of a semigroup is maximal if it is not contained in any other proper subsemigroup. A maximal subsemigroup of a finite semigroup has one of a small number of forms, as described in a paper of Graham, Graham, and Rhodes. Determining which of these forms arise in a given finite semigroup is difficult, and no practical mechanism for doing so appears in the literature. We present an algorithm for computing the maximal subsemigroups of a finite semigroup S given knowledge of the Green's structure of S, and the ability to determine maximal subgroups of certain subgroups of S, namely its group H-classes. In the case of a finite semigroup S represented by a generating set X, in many examples, if it is practical to compute the Green's structure of S from X, then it is also practical to find the maximal subsemigroups of S using the algorithm we present. In such examples, the time taken to determine the Green's structure of S is comparable to that taken to find the maximal subsemigroups. The generating set X for S may consist, for example, of transformations, or partial permutations, of a finite set, or of matrices over a semiring. Algorithms for computing the Green's structure of S from X include the Froidure–Pin Algorithm, and an algorithm of the second author based on the Schreier–Sims algorithm for permutation groups. The worst case complexity of these algorithms is polynomial in |S|, which for, say, transformation semigroups is exponential in the number of points on which they act. Certain aspects of the problem of finding maximal subsemigroups reduce to other well-known computational problems, such as finding all maximal cliques in a graph and computing the maximal subgroups in a group. The algorithm presented comprises two parts. One part relates to computing the maximal subsemigroups of a special class of semigroups, known as Rees 0-matrix semigroups. The other part involves a careful analysis of certain graphs associated to the semigroup S, which, roughly speaking, capture the essential information about the action of S on its J-classes.
The third author wishes to acknowledge the support of his Carnegie Ph.D. Scholarship from the Carnegie Trust for the Universities of Scotland.
Sun, 01 Jul 2018 00:00:00 GMThttps://hdl.handle.net/10023/170722018-07-01T00:00:00ZDonoven, C. R.Mitchell, J. D.Wilson, W. A.A proper subsemigroup of a semigroup is maximal if it is not contained in any other proper subsemigroup. A maximal subsemigroup of a finite semigroup has one of a small number of forms, as described in a paper of Graham, Graham, and Rhodes. Determining which of these forms arise in a given finite semigroup is difficult, and no practical mechanism for doing so appears in the literature. We present an algorithm for computing the maximal subsemigroups of a finite semigroup S given knowledge of the Green's structure of S, and the ability to determine maximal subgroups of certain subgroups of S, namely its group H-classes. In the case of a finite semigroup S represented by a generating set X, in many examples, if it is practical to compute the Green's structure of S from X, then it is also practical to find the maximal subsemigroups of S using the algorithm we present. In such examples, the time taken to determine the Green's structure of S is comparable to that taken to find the maximal subsemigroups. The generating set X for S may consist, for example, of transformations, or partial permutations, of a finite set, or of matrices over a semiring. Algorithms for computing the Green's structure of S from X include the Froidure–Pin Algorithm, and an algorithm of the second author based on the Schreier–Sims algorithm for permutation groups. The worst case complexity of these algorithms is polynomial in |S|, which for, say, transformation semigroups is exponential in the number of points on which they act. Certain aspects of the problem of finding maximal subsemigroups reduce to other well-known computational problems, such as finding all maximal cliques in a graph and computing the maximal subgroups in a group. The algorithm presented comprises two parts. One part relates to computing the maximal subsemigroups of a special class of semigroups, known as Rees 0-matrix semigroups. The other part involves a careful analysis of certain graphs associated to the semigroup S, which, roughly speaking, capture the essential information about the action of S on its J-classes.Randomness as a computational strategy : on matrix and tensor decompositions
https://hdl.handle.net/10023/16693
Matrix and tensor decompositions are fundamental tools for finding structure
and data processing. In particular, the efficient computation of
low-rank matrix approximations is an ubiquitous problem in the area
of machine learning and elsewhere. However, massive data arrays pose
a computational challenge for these techniques, placing significant constraints
on both memory and processing power. Recently, the fascinating
and powerful concept of randomness has been introduced as a strategy to
ease the computational load of deterministic matrix and data algorithms.
The basic idea of these algorithms is to employ a degree of randomness as
part of the logic in order to derive from a high-dimensional input matrix
a smaller matrix, which captures the essential information of the original
data matrix. Subsequently, the smaller matrix is then used to efficiently
compute a near-optimal low-rank approximation. Randomized algorithms
have been shown to be robust, highly reliable, and computationally efficient,
yet simple to implement. In particular, the development of the
randomized singular value decomposition can be seen as a milestone in the
era of ‘big data’. Building up on the great success of this probabilistic strategy
to compute low-rank matrix decompositions, this thesis introduces
a set of new randomized algorithms. Specifically, we present a randomized
algorithm to compute the dynamic mode decomposition, which is
a modern dimension reduction technique designed to extract dynamic
information from dynamical systems. Then, we advocate the randomized
dynamic mode decomposition for background modeling of surveillance
video feeds. Further, we show that randomized algorithms are embarrassingly
parallel by design and that graphics processing units (GPUs)
can be utilized to substantially accelerate the computations. Finally, the
concept of randomized algorithms is generalized for tensors in order to
compute the canonical CANDECOMP/PARAFAC (CP) decomposition.
Mon, 20 Nov 2017 00:00:00 GMThttps://hdl.handle.net/10023/166932017-11-20T00:00:00ZErichson, N. BenjaminMatrix and tensor decompositions are fundamental tools for finding structure
and data processing. In particular, the efficient computation of
low-rank matrix approximations is an ubiquitous problem in the area
of machine learning and elsewhere. However, massive data arrays pose
a computational challenge for these techniques, placing significant constraints
on both memory and processing power. Recently, the fascinating
and powerful concept of randomness has been introduced as a strategy to
ease the computational load of deterministic matrix and data algorithms.
The basic idea of these algorithms is to employ a degree of randomness as
part of the logic in order to derive from a high-dimensional input matrix
a smaller matrix, which captures the essential information of the original
data matrix. Subsequently, the smaller matrix is then used to efficiently
compute a near-optimal low-rank approximation. Randomized algorithms
have been shown to be robust, highly reliable, and computationally efficient,
yet simple to implement. In particular, the development of the
randomized singular value decomposition can be seen as a milestone in the
era of ‘big data’. Building up on the great success of this probabilistic strategy
to compute low-rank matrix decompositions, this thesis introduces
a set of new randomized algorithms. Specifically, we present a randomized
algorithm to compute the dynamic mode decomposition, which is
a modern dimension reduction technique designed to extract dynamic
information from dynamical systems. Then, we advocate the randomized
dynamic mode decomposition for background modeling of surveillance
video feeds. Further, we show that randomized algorithms are embarrassingly
parallel by design and that graphics processing units (GPUs)
can be utilized to substantially accelerate the computations. Finally, the
concept of randomized algorithms is generalized for tensors in order to
compute the canonical CANDECOMP/PARAFAC (CP) decomposition.Phases of physics in J.D. Forbes’ Dissertation Sixth for the Encyclopaedia Britannica (1856)
https://hdl.handle.net/10023/16618
This paper takes James David Forbes’ Encyclopaedia Britannica entry, Dissertation Sixth, as a lens to examine physics as a cognitive, practical, and social, enterprise. Forbes wrote this survey of eighteenth- and nineteenth-century mathematical and physical sciences, in 1852-6, when British “physics” was at a pivotal point in its history, situated between a discipline identified by its mathematical methods – originating in France - and one identified by its university laboratory institutions. Contemporary encyclopaedias provided a nexus for publishers, the book trade, readers, and men of science, in the formation of physics as a field. Forbes was both a witness, whose account of the progress of physics or natural philosophy can be explored at face value, and an agent, who exploited the opportunity offered by the Encyclopaedia Britannica in the mid nineteenth century to enrol the broadly educated public, and scientific collective, illuminating the connection between the definition of physics and its forms of social practice. Forbes used the terms “physics” and “natural philosophy” interchangeably. He portrayed the field as progressed by the natural genius of great men, who curated the discipline within an associational culture that engendered true intellectual spirit. Although this societal mechanism was becoming ineffective, Forbes did not see university institutions as the way forward. Instead, running counter to his friend William Whewell, he advocated inclusion of the mechanical arts (engineering), and a strictly limited role for mathematics. He revealed tensions when the widely accepted discovery-based historiography conflicted with intellectual and moral worth, reflecting a nineteenth-century concern with spirit that cuts across twentieth-century questions about discipline and field.
Mon, 03 Dec 2018 00:00:00 GMThttps://hdl.handle.net/10023/166182018-12-03T00:00:00ZFalconer, Isobel JessieThis paper takes James David Forbes’ Encyclopaedia Britannica entry, Dissertation Sixth, as a lens to examine physics as a cognitive, practical, and social, enterprise. Forbes wrote this survey of eighteenth- and nineteenth-century mathematical and physical sciences, in 1852-6, when British “physics” was at a pivotal point in its history, situated between a discipline identified by its mathematical methods – originating in France - and one identified by its university laboratory institutions. Contemporary encyclopaedias provided a nexus for publishers, the book trade, readers, and men of science, in the formation of physics as a field. Forbes was both a witness, whose account of the progress of physics or natural philosophy can be explored at face value, and an agent, who exploited the opportunity offered by the Encyclopaedia Britannica in the mid nineteenth century to enrol the broadly educated public, and scientific collective, illuminating the connection between the definition of physics and its forms of social practice. Forbes used the terms “physics” and “natural philosophy” interchangeably. He portrayed the field as progressed by the natural genius of great men, who curated the discipline within an associational culture that engendered true intellectual spirit. Although this societal mechanism was becoming ineffective, Forbes did not see university institutions as the way forward. Instead, running counter to his friend William Whewell, he advocated inclusion of the mechanical arts (engineering), and a strictly limited role for mathematics. He revealed tensions when the widely accepted discovery-based historiography conflicted with intellectual and moral worth, reflecting a nineteenth-century concern with spirit that cuts across twentieth-century questions about discipline and field.Computational techniques in finite semigroup theory
https://hdl.handle.net/10023/16521
A semigroup is simply a set with an associative binary operation; computational semigroup theory is the branch of mathematics concerned with developing techniques for computing with semigroups, as well as investigating semigroups with the help of computers. This thesis explores both sides of computational semigroup theory, across several topics, especially in the finite case.
The central focus of this thesis is computing and describing maximal subsemigroups of finite semigroups. A maximal subsemigroup of a semigroup is a proper subsemigroup that is contained in no other proper subsemigroup. We present novel and useful algorithms for computing the maximal subsemigroups of an arbitrary finite semigroup, building on the paper of Graham, Graham, and Rhodes from 1968. In certain cases, the algorithms reduce to computing maximal subgroups of finite groups, and analysing graphs that capture information about the regular ℐ-classes of a semigroup. We use the framework underpinning these algorithms to describe the maximal subsemigroups of many families of finite transformation and diagram monoids. This reproduces and greatly extends a large amount of existing work in the literature, and allows us to easily see the common features between these maximal subsemigroups.
This thesis is also concerned with direct products of semigroups, and with a special class of semigroups known as Rees 0-matrix semigroups. We extend known results concerning the generating sets of direct products of semigroups; in doing so, we propose techniques for computing relatively small generating sets for certain kinds of direct products. Additionally, we characterise several features of Rees 0-matrix semigroups in terms of their underlying semigroups and matrices, such as their Green's relations and generating sets, and whether they are inverse. In doing so, we suggest new methods for computing Rees 0-matrix semigroups.
Tue, 25 Jun 2019 00:00:00 GMThttps://hdl.handle.net/10023/165212019-06-25T00:00:00ZWilson, Wilf A.A semigroup is simply a set with an associative binary operation; computational semigroup theory is the branch of mathematics concerned with developing techniques for computing with semigroups, as well as investigating semigroups with the help of computers. This thesis explores both sides of computational semigroup theory, across several topics, especially in the finite case.
The central focus of this thesis is computing and describing maximal subsemigroups of finite semigroups. A maximal subsemigroup of a semigroup is a proper subsemigroup that is contained in no other proper subsemigroup. We present novel and useful algorithms for computing the maximal subsemigroups of an arbitrary finite semigroup, building on the paper of Graham, Graham, and Rhodes from 1968. In certain cases, the algorithms reduce to computing maximal subgroups of finite groups, and analysing graphs that capture information about the regular ℐ-classes of a semigroup. We use the framework underpinning these algorithms to describe the maximal subsemigroups of many families of finite transformation and diagram monoids. This reproduces and greatly extends a large amount of existing work in the literature, and allows us to easily see the common features between these maximal subsemigroups.
This thesis is also concerned with direct products of semigroups, and with a special class of semigroups known as Rees 0-matrix semigroups. We extend known results concerning the generating sets of direct products of semigroups; in doing so, we propose techniques for computing relatively small generating sets for certain kinds of direct products. Additionally, we characterise several features of Rees 0-matrix semigroups in terms of their underlying semigroups and matrices, such as their Green's relations and generating sets, and whether they are inverse. In doing so, we suggest new methods for computing Rees 0-matrix semigroups.Incorporating animal movement with distance sampling and spatial capture-recapture
https://hdl.handle.net/10023/16467
Distance sampling and spatial capture-recapture are statistical methods to estimate the
number of animals in a wild population based on encounters between these animals and
scientific detectors. Both methods estimate the probability an animal is detected during a
survey, but do not explicitly model animal movement.
The primary challenge is that animal movement in these surveys is unobserved; one must
average over all possible paths each animal could have travelled during the survey. In this
thesis, a general statistical model, with distance sampling and spatial capture-recapture
as special cases, is presented that explicitly incorporates animal movement. An efficient
algorithm to integrate over all possible movement paths, based on quadrature and hidden
Markov modelling, is given to overcome the computational obstacles.
For distance sampling, simulation studies and case studies show that incorporating animal
movement can reduce the bias in estimated abundance found in conventional models and
expand application of distance sampling to surveys that violate the assumption of no animal
movement. For spatial capture-recapture, continuous-time encounter records are used to
make detailed inference on where animals spend their time during the survey. In surveys
conducted in discrete occasions, maximum likelihood models that allow for mobile activity
centres are presented to account for transience, dispersal, and heterogeneous space use.
These methods provide an alternative when animal movement causes bias in standard methods and the opportunity to gain richer inference on how animals move, where they spend
their time, and how they interact.
Thu, 06 Dec 2018 00:00:00 GMThttps://hdl.handle.net/10023/164672018-12-06T00:00:00ZGlennie, RichardDistance sampling and spatial capture-recapture are statistical methods to estimate the
number of animals in a wild population based on encounters between these animals and
scientific detectors. Both methods estimate the probability an animal is detected during a
survey, but do not explicitly model animal movement.
The primary challenge is that animal movement in these surveys is unobserved; one must
average over all possible paths each animal could have travelled during the survey. In this
thesis, a general statistical model, with distance sampling and spatial capture-recapture
as special cases, is presented that explicitly incorporates animal movement. An efficient
algorithm to integrate over all possible movement paths, based on quadrature and hidden
Markov modelling, is given to overcome the computational obstacles.
For distance sampling, simulation studies and case studies show that incorporating animal
movement can reduce the bias in estimated abundance found in conventional models and
expand application of distance sampling to surveys that violate the assumption of no animal
movement. For spatial capture-recapture, continuous-time encounter records are used to
make detailed inference on where animals spend their time during the survey. In surveys
conducted in discrete occasions, maximum likelihood models that allow for mobile activity
centres are presented to account for transience, dispersal, and heterogeneous space use.
These methods provide an alternative when animal movement causes bias in standard methods and the opportunity to gain richer inference on how animals move, where they spend
their time, and how they interact.Surveying abundance and stand type associations of Formica aquilonia and F. lugubris (Hymenoptera: Formicidae) nest mounds over an extensive area : Trialing a novel method
https://hdl.handle.net/10023/16260
Red wood ants are ecologically important members of woodland communities, and some species are of conservation concern. They occur commonly only in certain habitats in Britain, but there is limited knowledge of their numbers and distribution. This study provided baseline information at a key locality (Abernethy Forest, 37 km2) in the central Highlands of Scotland and trialed a new method of surveying red wood ant density and stand type associations: a distance sampling line transect survey of nests. This method is efficient because it allows an observer to quickly survey a large area either side of transect lines, without having to assume that all nests are detected. Instead, data collected on the distance of nests from the line are used to estimate probability of detection and the effective transect width, using the free software "Distance". Surveys took place in August and September 2003 along a total of 71.2 km of parallel, equally-spaced transects. One hundred and forty-four red wood ant nests were located, comprising 89 F. aquilonia (Yarrow, 1955) and 55 F. lugubris (Zetterstedt, 1838) nests. Estimated densities were 1.13 nests per hectare (95% CI 0.74-1.73) for F. aquilonia and 0.83 nests per hectare (95% CI 0.32-2.17) for F. lugubris. These translated to total estimated nest numbers of 4,200 (95% CI 2,700-6,400) and 3,100 (95% CI 1,200-8,100), respectively, for the whole forest. Indices of stand selection indicated that F. aquilonia had some positive association with old-growth and F. lugubris with younger stands (stem exclusion stage). No nests were found in areas that had been clear-felled, and ploughed and planted in the 1970s-1990s. The pattern of stand type association and hence distribution of F. aquilonia and F. lugubris may be due to the differing ability to disperse (F. lugubris is the faster disperser) and compete (F. aquilonia is competitively superior). We recommend using line transect sampling for extensive surveys of ants that construct nest mounds to estimate abundance and stand type association.
Tue, 03 Jan 2012 00:00:00 GMThttps://hdl.handle.net/10023/162602012-01-03T00:00:00ZBorkin, KerrySummers, RonThomas, LenRed wood ants are ecologically important members of woodland communities, and some species are of conservation concern. They occur commonly only in certain habitats in Britain, but there is limited knowledge of their numbers and distribution. This study provided baseline information at a key locality (Abernethy Forest, 37 km2) in the central Highlands of Scotland and trialed a new method of surveying red wood ant density and stand type associations: a distance sampling line transect survey of nests. This method is efficient because it allows an observer to quickly survey a large area either side of transect lines, without having to assume that all nests are detected. Instead, data collected on the distance of nests from the line are used to estimate probability of detection and the effective transect width, using the free software "Distance". Surveys took place in August and September 2003 along a total of 71.2 km of parallel, equally-spaced transects. One hundred and forty-four red wood ant nests were located, comprising 89 F. aquilonia (Yarrow, 1955) and 55 F. lugubris (Zetterstedt, 1838) nests. Estimated densities were 1.13 nests per hectare (95% CI 0.74-1.73) for F. aquilonia and 0.83 nests per hectare (95% CI 0.32-2.17) for F. lugubris. These translated to total estimated nest numbers of 4,200 (95% CI 2,700-6,400) and 3,100 (95% CI 1,200-8,100), respectively, for the whole forest. Indices of stand selection indicated that F. aquilonia had some positive association with old-growth and F. lugubris with younger stands (stem exclusion stage). No nests were found in areas that had been clear-felled, and ploughed and planted in the 1970s-1990s. The pattern of stand type association and hence distribution of F. aquilonia and F. lugubris may be due to the differing ability to disperse (F. lugubris is the faster disperser) and compete (F. aquilonia is competitively superior). We recommend using line transect sampling for extensive surveys of ants that construct nest mounds to estimate abundance and stand type association.Crambled : a Shiny application to enable intuitive resolution of conflicting cellularity estimates
https://hdl.handle.net/10023/16248
It is now commonplace to investigate tumour samples using whole-genome sequencing, and some commonly performed tasks are the estimation of cellularity (or sample purity), the genome-wide profiling of copy numbers, and the assessment of sub-clonal behaviours. Several tools are available to undertake these tasks, but often give conflicting results - not least because there is often genuine uncertainty due to a lack of model identifiability. Presented here is a tool, "Crambled", that allows for an intuitive visual comparison of the conflicting solutions. Crambled is implemented as a Shiny application within R, and is accompanied by example images from two use cases (one tumour sample with matched normal sequencing, and one standalone cell line example) as well as functions to generate the necessary images from any sequencing data set. Through the use of Crambled, a user may gain insight into why each tool has offered its given solution and combined with a knowledge of the disease being studied can choose between the competing solutions in an informed manner.
Mon, 07 Dec 2015 00:00:00 GMThttps://hdl.handle.net/10023/162482015-12-07T00:00:00ZLynch, AndyIt is now commonplace to investigate tumour samples using whole-genome sequencing, and some commonly performed tasks are the estimation of cellularity (or sample purity), the genome-wide profiling of copy numbers, and the assessment of sub-clonal behaviours. Several tools are available to undertake these tasks, but often give conflicting results - not least because there is often genuine uncertainty due to a lack of model identifiability. Presented here is a tool, "Crambled", that allows for an intuitive visual comparison of the conflicting solutions. Crambled is implemented as a Shiny application within R, and is accompanied by example images from two use cases (one tumour sample with matched normal sequencing, and one standalone cell line example) as well as functions to generate the necessary images from any sequencing data set. Through the use of Crambled, a user may gain insight into why each tool has offered its given solution and combined with a knowledge of the disease being studied can choose between the competing solutions in an informed manner.Some group presentations with few defining relations
https://hdl.handle.net/10023/15964
We consider two classes of groups with two generators and three relations. One class has a similar presentation to groups considered in the paper by C.M. Campbell and R.M. Thomas, ‘On (2,n)-Groups related to Fibonacci Groups’, (Israel J. Math., 58), with one generator of order three instead of order two . We attempt to find the order of these groups and in one case find groups which have the alternating group A₅ as a subgroup of index equal to the order of the second generator of the group. Questions remain as to the order of some of the other groups.
The second class has already been considered in the paper 'Some families of finite groups having two generators and two relations' by C.M. Campbell , H.S.M. Coxeter and E.F. Robertson, (Proc. R. Soc. London A. 357, 423-438 (1977)), in which a formula for the orders of these groups was found. We attempt to find simpler formulae based on recurrence relations for subclasses and write Maple programs to enable us to do this. We also find a formula, again based on recurrence relations, for an upper bound for the orders of the groups.
Mon, 01 Jan 1990 00:00:00 GMThttps://hdl.handle.net/10023/159641990-01-01T00:00:00ZGill, David MichaelWe consider two classes of groups with two generators and three relations. One class has a similar presentation to groups considered in the paper by C.M. Campbell and R.M. Thomas, ‘On (2,n)-Groups related to Fibonacci Groups’, (Israel J. Math., 58), with one generator of order three instead of order two . We attempt to find the order of these groups and in one case find groups which have the alternating group A₅ as a subgroup of index equal to the order of the second generator of the group. Questions remain as to the order of some of the other groups.
The second class has already been considered in the paper 'Some families of finite groups having two generators and two relations' by C.M. Campbell , H.S.M. Coxeter and E.F. Robertson, (Proc. R. Soc. London A. 357, 423-438 (1977)), in which a formula for the orders of these groups was found. We attempt to find simpler formulae based on recurrence relations for subclasses and write Maple programs to enable us to do this. We also find a formula, again based on recurrence relations, for an upper bound for the orders of the groups.Gaussian Markov random fields and structural additive regression : applications in freshwater fisheries management
https://hdl.handle.net/10023/15909
In this thesis structural additive regression (STAR) models are constructed for two
applications in freshwater fisheries management 1) large scale modelling of fish
abundance using electrofishing removal data and 2) assessing the effect on stream
temperature of tree felling. Both approaches take advantage of the central role
Gaussian Markov random fields (GMRFs) play in the construction of structured
additive regression components.
The R package mgcv can fit, in principle, any STAR model. In practice, however,
several extensions are required to allow a non-specialised user to access this functionality,
and a large part of this thesis is the developement of software to allow a
general user ready access to a wide range of GMRF models within the familiar mgcv
framework. All models are fitted making use of this extension where possible (and
practical).
The thesis is divided into three main chapters. Chapter 2 serves to provide background
and insight into penalised regression and STAR models and the role that GMRFs
play in smoothing. Also presented are the extensions required to fit GMRF models
in mgcv.
Chapter 3 presents a two stage model for fish density using electrofishing removal
data. The first stage of this model estimates fish capture probability and is not a
STAR model, but can utilise aspects of GMRFs through low rank approximations;
software to make this available is developed and presented. The second stage is a
Poisson STAR model and can therefore be fitted in the extended mgcv framework.
Finally, Chapter 4 presents a model for the impact of a clear felling event on stream
temperature. This model utilises cyclic smoothers applied to the functional principal
components of daily temperature curves. This allows for a detailed assessment of the
effects of felling on stream temperature that is not possible when modelling daily
summaries alone.
Fri, 23 Jun 2017 00:00:00 GMThttps://hdl.handle.net/10023/159092017-06-23T00:00:00ZMillar, Colin PearsonIn this thesis structural additive regression (STAR) models are constructed for two
applications in freshwater fisheries management 1) large scale modelling of fish
abundance using electrofishing removal data and 2) assessing the effect on stream
temperature of tree felling. Both approaches take advantage of the central role
Gaussian Markov random fields (GMRFs) play in the construction of structured
additive regression components.
The R package mgcv can fit, in principle, any STAR model. In practice, however,
several extensions are required to allow a non-specialised user to access this functionality,
and a large part of this thesis is the developement of software to allow a
general user ready access to a wide range of GMRF models within the familiar mgcv
framework. All models are fitted making use of this extension where possible (and
practical).
The thesis is divided into three main chapters. Chapter 2 serves to provide background
and insight into penalised regression and STAR models and the role that GMRFs
play in smoothing. Also presented are the extensions required to fit GMRF models
in mgcv.
Chapter 3 presents a two stage model for fish density using electrofishing removal
data. The first stage of this model estimates fish capture probability and is not a
STAR model, but can utilise aspects of GMRFs through low rank approximations;
software to make this available is developed and presented. The second stage is a
Poisson STAR model and can therefore be fitted in the extended mgcv framework.
Finally, Chapter 4 presents a model for the impact of a clear felling event on stream
temperature. This model utilises cyclic smoothers applied to the functional principal
components of daily temperature curves. This allows for a detailed assessment of the
effects of felling on stream temperature that is not possible when modelling daily
summaries alone.Decision problems in groups of homeomorphisms of Cantor space
https://hdl.handle.net/10023/15885
The Thompson groups $F, T$ and $V$ are important groups in geometric group theory: $T$ and $V$ being the first discovered examples of finitely presented infinite simple groups. There are many generalisations of these groups including, for $n$ and $r$ natural numbers and $1 < r < n$, the groups $F_{n}$, $T_{n,r}$ and $G_{n,r}$ ($T ≅ T_{2,1}$ and $V ≅ G_{2,1}$). Automorphisms of $F$ and $T$ were characterised in the seminal paper of Brin ([16]) and, later on, Brin and Guzman ([17]) investigate automorphisms of $T_{n, n-1}$ and $F_{n}$ for $n>2$. However, their techniques give no information about automorphisms of $G_{n,r}$.
The second chapter of this thesis is dedicated to characterising the automorphisms of $G_{n,r}$. Presenting results of the author's article [10], we show that automorphisms of $G_{n,r}$ are homeomorphisms of Cantor space induced by transducers (finite state machines) which satisfy a strong synchronizing condition.
In the rest of Chapter 2 and early sections of Chapter 3 we investigate the group $\out{G_{n,r}}$ of outer automorphisms of $G_{n,r}$. Presenting results of the forthcoming article [6] of the author's, we show that there is a subgroup $\hn{n}$ of $\out{G_{n,r}}$, independent of $r$, which is isomorphic to the group of automorphisms of the one-sided shift dynamical system. Most of Chapter 3 is devoted to the order problem in $\hn{n}$ and is based on [44]. We give necessary and sufficient conditions for an element of $\hn{n}$ to have finite order, although these do not yield a decision procedure.
Given an automorphism $\phi$ of a group $G$, two elements $f, g ∈ G$ are said to be $\phi$-twisted conjugate to one another if for some $h ∈ G$, $g = h⁻¹ f (h)\phi$. This defines an equivalence relation on $G$ and $G$ is said to have the $\rfty$ property if it has infinitely many $\phi$-twisted conjugacy classes for all automorphisms $\phi ∈ \aut{G}$. In the final chapter we show, using the description of $\aut{G_{n,r}}$, that for certain automorphisms, $G_{n,r}$ has infinitely many twisted conjugacy classes. We also show that for certain $\phi ∈ \aut{G_{2,1}}$ the problem of deciding when two elements of $G_{2,1}$ are $\phi$-twisted conjugate to one another is soluble.
Thu, 06 Dec 2018 00:00:00 GMThttps://hdl.handle.net/10023/158852018-12-06T00:00:00ZOlukoya, FeyisayoThe Thompson groups $F, T$ and $V$ are important groups in geometric group theory: $T$ and $V$ being the first discovered examples of finitely presented infinite simple groups. There are many generalisations of these groups including, for $n$ and $r$ natural numbers and $1 < r < n$, the groups $F_{n}$, $T_{n,r}$ and $G_{n,r}$ ($T ≅ T_{2,1}$ and $V ≅ G_{2,1}$). Automorphisms of $F$ and $T$ were characterised in the seminal paper of Brin ([16]) and, later on, Brin and Guzman ([17]) investigate automorphisms of $T_{n, n-1}$ and $F_{n}$ for $n>2$. However, their techniques give no information about automorphisms of $G_{n,r}$.
The second chapter of this thesis is dedicated to characterising the automorphisms of $G_{n,r}$. Presenting results of the author's article [10], we show that automorphisms of $G_{n,r}$ are homeomorphisms of Cantor space induced by transducers (finite state machines) which satisfy a strong synchronizing condition.
In the rest of Chapter 2 and early sections of Chapter 3 we investigate the group $\out{G_{n,r}}$ of outer automorphisms of $G_{n,r}$. Presenting results of the forthcoming article [6] of the author's, we show that there is a subgroup $\hn{n}$ of $\out{G_{n,r}}$, independent of $r$, which is isomorphic to the group of automorphisms of the one-sided shift dynamical system. Most of Chapter 3 is devoted to the order problem in $\hn{n}$ and is based on [44]. We give necessary and sufficient conditions for an element of $\hn{n}$ to have finite order, although these do not yield a decision procedure.
Given an automorphism $\phi$ of a group $G$, two elements $f, g ∈ G$ are said to be $\phi$-twisted conjugate to one another if for some $h ∈ G$, $g = h⁻¹ f (h)\phi$. This defines an equivalence relation on $G$ and $G$ is said to have the $\rfty$ property if it has infinitely many $\phi$-twisted conjugacy classes for all automorphisms $\phi ∈ \aut{G}$. In the final chapter we show, using the description of $\aut{G_{n,r}}$, that for certain automorphisms, $G_{n,r}$ has infinitely many twisted conjugacy classes. We also show that for certain $\phi ∈ \aut{G_{2,1}}$ the problem of deciding when two elements of $G_{2,1}$ are $\phi$-twisted conjugate to one another is soluble.Adaptive multivariate global testing
https://hdl.handle.net/10023/15760
We present a methodology for dealing with recent challenges in testing global hypotheses using multivariate observations. The proposed tests target situations, often arising in emerging applications of neuroimaging, where the sample size n is relatively small compared with the observations' dimension K. We employ adaptive designs allowing for sequential modifications of the test statistics adapting to accumulated data. The adaptations are optimal in the sense of maximizing the predictive power of the test at each interim analysis while still controlling the Type I error. Optimality is obtained by a general result applicable to typical adaptive design settings. Further, we prove that the potentially high-dimensional design space of the tests can be reduced to a low-dimensional projection space enabling us to perform simpler power analysis studies, including comparisons to alternative tests. We illustrate the substantial improvement in efficiency that the proposed tests can make over standard tests, especially in the case of n smaller or slightly larger than K. The methods are also studied empirically using both simulated data and data from an EEG study, where the use of prior knowledge substantially increases the power of the test. Supplementary materials for this article are available online.
Sun, 01 Jun 2014 00:00:00 GMThttps://hdl.handle.net/10023/157602014-06-01T00:00:00ZMinas, GiorgosAston, John A DStallard, NigelWe present a methodology for dealing with recent challenges in testing global hypotheses using multivariate observations. The proposed tests target situations, often arising in emerging applications of neuroimaging, where the sample size n is relatively small compared with the observations' dimension K. We employ adaptive designs allowing for sequential modifications of the test statistics adapting to accumulated data. The adaptations are optimal in the sense of maximizing the predictive power of the test at each interim analysis while still controlling the Type I error. Optimality is obtained by a general result applicable to typical adaptive design settings. Further, we prove that the potentially high-dimensional design space of the tests can be reduced to a low-dimensional projection space enabling us to perform simpler power analysis studies, including comparisons to alternative tests. We illustrate the substantial improvement in efficiency that the proposed tests can make over standard tests, especially in the case of n smaller or slightly larger than K. The methods are also studied empirically using both simulated data and data from an EEG study, where the use of prior knowledge substantially increases the power of the test. Supplementary materials for this article are available online.ReTrOS : a MATLAB toolbox for reconstructing transcriptional activity from gene and protein expression data
https://hdl.handle.net/10023/15759
BACKGROUND: Given the development of high-throughput experimental techniques, an increasing number of whole genome transcription profiling time series data sets, with good temporal resolution, are becoming available to researchers. The ReTrOS toolbox (Reconstructing Transcription Open Software) provides MATLAB-based implementations of two related methods, namely ReTrOS-Smooth and ReTrOS-Switch, for reconstructing the temporal transcriptional activity profile of a gene from given mRNA expression time series or protein reporter time series. The methods are based on fitting a differential equation model incorporating the processes of transcription, translation and degradation. RESULTS: The toolbox provides a framework for model fitting along with statistical analyses of the model with a graphical interface and model visualisation. We highlight several applications of the toolbox, including the reconstruction of the temporal cascade of transcriptional activity inferred from mRNA expression data and protein reporter data in the core circadian clock in Arabidopsis thaliana, and how such reconstructed transcription profiles can be used to study the effects of different cell lines and conditions. CONCLUSIONS: The ReTrOS toolbox allows users to analyse gene and/or protein expression time series where, with appropriate formulation of prior information about a minimum of kinetic parameters, in particular rates of degradation, users are able to infer timings of changes in transcriptional activity. Data from any organism and obtained from a range of technologies can be used as input due to the flexible and generic nature of the model and implementation. The output from this software provides a useful analysis of time series data and can be incorporated into further modelling approaches or in hypothesis generation.
This work was supported through providing funds by the Biotechnology and Biological Sciences Research Council [BB/F005806/1, BB/F005237/1]; and the Engineering and Physical Sciences Research Council [EP/C544587/1 to DAR].
Mon, 26 Jun 2017 00:00:00 GMThttps://hdl.handle.net/10023/157592017-06-26T00:00:00ZMinas, GiorgosMomiji, HiroshiJenkins, Dafyd JCosta, Maria JRand, David AFinkenstädt, BärbelBACKGROUND: Given the development of high-throughput experimental techniques, an increasing number of whole genome transcription profiling time series data sets, with good temporal resolution, are becoming available to researchers. The ReTrOS toolbox (Reconstructing Transcription Open Software) provides MATLAB-based implementations of two related methods, namely ReTrOS-Smooth and ReTrOS-Switch, for reconstructing the temporal transcriptional activity profile of a gene from given mRNA expression time series or protein reporter time series. The methods are based on fitting a differential equation model incorporating the processes of transcription, translation and degradation. RESULTS: The toolbox provides a framework for model fitting along with statistical analyses of the model with a graphical interface and model visualisation. We highlight several applications of the toolbox, including the reconstruction of the temporal cascade of transcriptional activity inferred from mRNA expression data and protein reporter data in the core circadian clock in Arabidopsis thaliana, and how such reconstructed transcription profiles can be used to study the effects of different cell lines and conditions. CONCLUSIONS: The ReTrOS toolbox allows users to analyse gene and/or protein expression time series where, with appropriate formulation of prior information about a minimum of kinetic parameters, in particular rates of degradation, users are able to infer timings of changes in transcriptional activity. Data from any organism and obtained from a range of technologies can be used as input due to the flexible and generic nature of the model and implementation. The output from this software provides a useful analysis of time series data and can be incorporated into further modelling approaches or in hypothesis generation.Numerical modelling of ultra low frequency waves in Earth's magnetosphere
https://hdl.handle.net/10023/15663
Ultra Low Frequency (ULF) waves are a ubiquitous feature of Earth's outer atmosphere, known as the magnetosphere, having been observed on the ground for almost two centuries, and in space over the last 50 years. These waves represent small oscillations in Earth's magnetic field, most often as a response to the external influence of the solar wind. They are important for the transfer of energy throughout the magnetosphere and for coupling different regions together. In this thesis, various features of these oscillations are considered. A detailed background on the history and previous study of ULF waves relevant to our work is given in the introductory chapter. In the following chapters, we predominantly use numerical methods to model ULF waves, which are carefully developed and thoroughly tested. We consider the application of these methods to reports on ground and spaced based observations, which allows a more in depth study of the data. In one case, the simulation results provide evidence for an alternative explanation of the data to the original report, which displays the power of theoretical modelling. An analytical model is also constructed, which is tested on simulation data, to identify the incidence and reflection of a class of ULF wave in the flank magnetosphere. This technique is developed with the aim of future applications to satellite data. Further to this, we develop models both in Cartesian and dipole geometries to investigate some of the theoretical aspects of the coupling between various waves modes. New light is shed on the coupling of compressional (fast) and transverse (Alfvén) magnetohydrodynamic (MHD) wave modes in a 3D dipole geometry. Overall, this thesis aims to develop useful numerical models, which can be used to aid in the interpretation of ULF wave observations, as well as probing new aspects of the existing wave theory.
Fri, 24 Jun 2016 00:00:00 GMThttps://hdl.handle.net/10023/156632016-06-24T00:00:00ZElsden, TomUltra Low Frequency (ULF) waves are a ubiquitous feature of Earth's outer atmosphere, known as the magnetosphere, having been observed on the ground for almost two centuries, and in space over the last 50 years. These waves represent small oscillations in Earth's magnetic field, most often as a response to the external influence of the solar wind. They are important for the transfer of energy throughout the magnetosphere and for coupling different regions together. In this thesis, various features of these oscillations are considered. A detailed background on the history and previous study of ULF waves relevant to our work is given in the introductory chapter. In the following chapters, we predominantly use numerical methods to model ULF waves, which are carefully developed and thoroughly tested. We consider the application of these methods to reports on ground and spaced based observations, which allows a more in depth study of the data. In one case, the simulation results provide evidence for an alternative explanation of the data to the original report, which displays the power of theoretical modelling. An analytical model is also constructed, which is tested on simulation data, to identify the incidence and reflection of a class of ULF wave in the flank magnetosphere. This technique is developed with the aim of future applications to satellite data. Further to this, we develop models both in Cartesian and dipole geometries to investigate some of the theoretical aspects of the coupling between various waves modes. New light is shed on the coupling of compressional (fast) and transverse (Alfvén) magnetohydrodynamic (MHD) wave modes in a 3D dipole geometry. Overall, this thesis aims to develop useful numerical models, which can be used to aid in the interpretation of ULF wave observations, as well as probing new aspects of the existing wave theory.Eruptions and jets in the Sun
https://hdl.handle.net/10023/15648
Magnetic flux emergence is a fundamental process in the Sun, during which magnetic fields emerge from the solar interior to the surface, to build up active regions and give onset to spectacular dynamic phenomena, such as eruptions and jets. In this thesis, we performed 3D, resistive MHD simulations to study the emergence and the associated magnetic activity of a quadrupolar region in the Sun. Our aim behind the setup of this initial condition (i.e. a quadrupolar region) was to study a magnetic field configuration, which has not been studied in detail before, although it has been repeatedly observed in the Sun and it has been shown that it can host intense magnetic activity (e.g. in the form of jets, flares and eruptions).
The results of our experiments showed that the internal dynamics of such regions leads to the onset of eruptions in the form of twisted magnetic flux tubes (flux ropes). These eruptions are recurrent but they cannot escape the outermost field of the emerging flux (envelope field). They remain confined within the envelope field, as the downward tension of the outermost field lines overwhelms the upward Lorentz force of the erupting field. When we add an ambient magnetic field in the solar atmosphere, external reconnection between the emerging and the ambient field triggers the emission of (standard) reconnection jets. The external reconnection also releases the tension of the ambient field lines and, thus, the eruptions move in an ejective way towards the outer space. Namely, the confined eruptions become ejective eruptions, which escape from the numerical domain. These ejective eruptions drive a newly observed class of jets, the so called "blowout" jets. Our experiments reproduce some of the main observed characteristics of the "blowout" jets. We showed that "blowout" jets emit hot and cool plasma into the outer solar atmosphere simultaneously, and they undergo untwisting motion due to the relaxation of twist during their ejection. We found that the untwisting motion of the "blowout" jets is associated with the propagation of torsional Alfvén waves. Finally, we performed a parametric study to explore the effect of the ambient field strength on the onset and dynamics of the eruptive events. We found that one of the main effects is that the stronger ambient field suppresses the vertical expansion of the magnetic envelope of the quadrupolar region due to the higher magnetic pressure above it. This result has an effect on the emission of jets, which are emitted due to reconnection between the two fields. When the ambient field is relatively weak, it is pushed away from the strong emerging field and reconnection between them is not so persistent. On the other hand, when the ambient field is relatively strong, we find that more jets are ejected due to more efficient and more frequent reconnection between the two flux systems. As a consequence, we find that more mass and flux is being transferred into the solar corona by the reconnection jets. Also, we find that there are more eruptions when the ambient field is stronger. The study of the total energy flux carried by the jets showed that it is sufficient to provide the energy required to accelerate the high speed solar wind. This indicates that the "blowout" jets may play an important role in driving the solar wind.
Fri, 23 Jun 2017 00:00:00 GMThttps://hdl.handle.net/10023/156482017-06-23T00:00:00ZLee, Eon JuiMagnetic flux emergence is a fundamental process in the Sun, during which magnetic fields emerge from the solar interior to the surface, to build up active regions and give onset to spectacular dynamic phenomena, such as eruptions and jets. In this thesis, we performed 3D, resistive MHD simulations to study the emergence and the associated magnetic activity of a quadrupolar region in the Sun. Our aim behind the setup of this initial condition (i.e. a quadrupolar region) was to study a magnetic field configuration, which has not been studied in detail before, although it has been repeatedly observed in the Sun and it has been shown that it can host intense magnetic activity (e.g. in the form of jets, flares and eruptions).
The results of our experiments showed that the internal dynamics of such regions leads to the onset of eruptions in the form of twisted magnetic flux tubes (flux ropes). These eruptions are recurrent but they cannot escape the outermost field of the emerging flux (envelope field). They remain confined within the envelope field, as the downward tension of the outermost field lines overwhelms the upward Lorentz force of the erupting field. When we add an ambient magnetic field in the solar atmosphere, external reconnection between the emerging and the ambient field triggers the emission of (standard) reconnection jets. The external reconnection also releases the tension of the ambient field lines and, thus, the eruptions move in an ejective way towards the outer space. Namely, the confined eruptions become ejective eruptions, which escape from the numerical domain. These ejective eruptions drive a newly observed class of jets, the so called "blowout" jets. Our experiments reproduce some of the main observed characteristics of the "blowout" jets. We showed that "blowout" jets emit hot and cool plasma into the outer solar atmosphere simultaneously, and they undergo untwisting motion due to the relaxation of twist during their ejection. We found that the untwisting motion of the "blowout" jets is associated with the propagation of torsional Alfvén waves. Finally, we performed a parametric study to explore the effect of the ambient field strength on the onset and dynamics of the eruptive events. We found that one of the main effects is that the stronger ambient field suppresses the vertical expansion of the magnetic envelope of the quadrupolar region due to the higher magnetic pressure above it. This result has an effect on the emission of jets, which are emitted due to reconnection between the two fields. When the ambient field is relatively weak, it is pushed away from the strong emerging field and reconnection between them is not so persistent. On the other hand, when the ambient field is relatively strong, we find that more jets are ejected due to more efficient and more frequent reconnection between the two flux systems. As a consequence, we find that more mass and flux is being transferred into the solar corona by the reconnection jets. Also, we find that there are more eruptions when the ambient field is stronger. The study of the total energy flux carried by the jets showed that it is sufficient to provide the energy required to accelerate the high speed solar wind. This indicates that the "blowout" jets may play an important role in driving the solar wind.On plausible counterexamples to Lehnert's conjecture
https://hdl.handle.net/10023/15631
A group whose co-word problem is a context free language is called co𝐶𝐹 . Lehnert's conjecture states that a group 𝐺 is co𝐶𝐹 if and only if 𝐺 embeds as a finitely generated subgroup of R. Thompson's group V . In this thesis we explore a class of groups, Faug, proposed by Berns-Zieze, Fry, Gillings, Hoganson, and Mathews to contain potential counterexamples to Lehnert's conjecture. We create infinite and finite presentations for such groups and go on to prove that a certain subclass of 𝓕𝑎𝑢𝑔 consists of groups that do embed into 𝑉.
By Anisimov a group has regular word problem if and only if it is finite. It is also known
that a group 𝐺 is finite if and only if there exists an embedding of 𝐺 into 𝑉 such that
its natural action on 𝕮₂:= {0, 1}[super]𝜔 is free on the whole space. We show that the class of
groups with a context free word problem, the class of 𝐶𝐹 groups, is precisely the class of finitely generated demonstrable groups for 𝑉 . A demonstrable group for V is a group 𝐺 which is isomorphic to a subgroup in 𝑉 whose natural action on 𝕮₂ acts freely on an open subset. Thus our result extends the correspondence between language theoretic properties of groups and dynamical properties of subgroups of V . Additionally, our result also shows that the final condition of the four known closure properties of the class of co𝐶𝐹 groups also holds for the set of finitely generated subgroups of 𝑉.
Mon, 01 Jan 2018 00:00:00 GMThttps://hdl.handle.net/10023/156312018-01-01T00:00:00ZBennett, DanielA group whose co-word problem is a context free language is called co𝐶𝐹 . Lehnert's conjecture states that a group 𝐺 is co𝐶𝐹 if and only if 𝐺 embeds as a finitely generated subgroup of R. Thompson's group V . In this thesis we explore a class of groups, Faug, proposed by Berns-Zieze, Fry, Gillings, Hoganson, and Mathews to contain potential counterexamples to Lehnert's conjecture. We create infinite and finite presentations for such groups and go on to prove that a certain subclass of 𝓕𝑎𝑢𝑔 consists of groups that do embed into 𝑉.
By Anisimov a group has regular word problem if and only if it is finite. It is also known
that a group 𝐺 is finite if and only if there exists an embedding of 𝐺 into 𝑉 such that
its natural action on 𝕮₂:= {0, 1}[super]𝜔 is free on the whole space. We show that the class of
groups with a context free word problem, the class of 𝐶𝐹 groups, is precisely the class of finitely generated demonstrable groups for 𝑉 . A demonstrable group for V is a group 𝐺 which is isomorphic to a subgroup in 𝑉 whose natural action on 𝕮₂ acts freely on an open subset. Thus our result extends the correspondence between language theoretic properties of groups and dynamical properties of subgroups of V . Additionally, our result also shows that the final condition of the four known closure properties of the class of co𝐶𝐹 groups also holds for the set of finitely generated subgroups of 𝑉.Incorporating animal movement into circular plot and point transect surveys of wildlife abundance
https://hdl.handle.net/10023/15612
Estimating wildlife abundance is fundamental for its effective management and conservation.
A range of methods exist: total counts, plot sampling, distance sampling and
capture-recapture based approaches. Methods have assumptions and their failure can
lead to substantial bias. Current research in the field is focused not on establishing new
methods but in extending existing methods to deal with their assumptions' violation.
This thesis focus on incorporating animal movement into circular plot sampling (CPS)
and point transect sampling (PTS), where a key assumption is that animals do not move
while within detection range, i.e., the survey is a snapshot in time. While targeting this
goal, we found some unexpected bias in PTS when animals were still and model selection
was used to choose among different candidate models for the detection function (the
model describing how detectability changes with observer-animal distance). Using a simulation
study, we found that, although PTS estimators are asymptotically unbiased, for
the recommended sample sizes the bias depended on the form of the true detection function.
We then extended the simulation study to include animal movement, and found this
led to further bias in CPS and PTS. We present novel methods that incorporate animal
movement with constant speed into estimates of abundance. First, in CPS, we present
an analytic expression to correct for the bias given linear movement. When movement
is de ned by a diffusion process, a simulation based approach, modelling the probability
of animal presence in the circular plot, results in less than 3% bias in the abundance
estimates. For PTS we introduce an estimator composed of two linked submodels: the
movement (animals moving linearly) and the detection model. The performance of the
proposed method is assessed via simulation. Despite being biased, the new estimator
yields improved results compared to ignoring animal movement using conventional PTS.
Mon, 01 Jan 2018 00:00:00 GMThttps://hdl.handle.net/10023/156122018-01-01T00:00:00ZPrieto González, RocíoEstimating wildlife abundance is fundamental for its effective management and conservation.
A range of methods exist: total counts, plot sampling, distance sampling and
capture-recapture based approaches. Methods have assumptions and their failure can
lead to substantial bias. Current research in the field is focused not on establishing new
methods but in extending existing methods to deal with their assumptions' violation.
This thesis focus on incorporating animal movement into circular plot sampling (CPS)
and point transect sampling (PTS), where a key assumption is that animals do not move
while within detection range, i.e., the survey is a snapshot in time. While targeting this
goal, we found some unexpected bias in PTS when animals were still and model selection
was used to choose among different candidate models for the detection function (the
model describing how detectability changes with observer-animal distance). Using a simulation
study, we found that, although PTS estimators are asymptotically unbiased, for
the recommended sample sizes the bias depended on the form of the true detection function.
We then extended the simulation study to include animal movement, and found this
led to further bias in CPS and PTS. We present novel methods that incorporate animal
movement with constant speed into estimates of abundance. First, in CPS, we present
an analytic expression to correct for the bias given linear movement. When movement
is de ned by a diffusion process, a simulation based approach, modelling the probability
of animal presence in the circular plot, results in less than 3% bias in the abundance
estimates. For PTS we introduce an estimator composed of two linked submodels: the
movement (animals moving linearly) and the detection model. The performance of the
proposed method is assessed via simulation. Despite being biased, the new estimator
yields improved results compared to ignoring animal movement using conventional PTS.A continuous-time formulation for spatial capture-recapture models
https://hdl.handle.net/10023/15596
Spatial capture-recapture (SCR) models are relatively new but have become the
standard approach used to estimate animal density from capture-recapture data. It
has in the past been impractical to obtain sufficient data for analysis on species that
are very difficult to capture such as elusive carnivores that occur at low density and
range very widely. Advances in technology have led to alternative ways to virtually
“capture" individuals without having to physically hold them. Some examples of
these new non-invasive sampling methods include scat or hair collection for genetic
analysis, acoustic detection and camera trapping.
In traditional capture-recapture (CR) and SCR studies populations are sampled
at discrete points in time leading to clear and well defined occasions whereas the
new detector types mentioned above sample populations continuously in time. Researchers
with data collected continuously currently need to define an appropriate
occasion and aggregate their data accordingly thereby imposing an artificial construct
on their data for analytical convenience.
This research develops a continuous-time (CT) framework for SCR models by
treating detections as a temporal non homogeneous Poisson process (NHPP) and
replacing the usual SCR detection function with a continuous detection hazard function.
The general CT likelihood is first developed for data from passive (also called
“proximity") detectors like camera traps that do not physically hold individuals. The
likelihood is then modified to produce a likelihood for single-catch traps (traps that
are taken out of action by capturing an animal) that has proven difficult to develop
with a discrete-occasion approach.
The lack of a suitable single-catch trap likelihood has led to researchers using
a discrete-time (DT) multi-catch trap estimator to analyse single-catch trap data.
Previous work has found the DT multi-catch estimator to be robust despite the fact
that it is known to be based on the wrong model for single-catch traps (it assumes
that the traps continue operating after catching an individual). Simulation studies in
this work confirm that the multi-catch estimator is robust for estimating density when
density is constant or does not vary much in space. However, there are scenarios with
non-constant density surfaces when the multi-catch estimator is not able to correctly
identify regions of high density. Furthermore, the multi-catch estimator is known
to be negatively biased for the intercept parameter of SCR detection functions and
there may be interest in the detection function in its own right. On the other hand
the CT single-catch estimator is unbiased or nearly so for all parameters of interest
including those in the detection function and those in the model for density.
When one assumes that the detection hazard is constant through time there is
no impact of ignoring capture times and using only the detection frequencies. This
is of course a special case and in reality detection hazards will tend to vary in time.
However when one assumes that the effects of time and distance in the time-varying
hazard are independent, then similarly there is no information in the capture times
about density and detection function parameters. The work here uses a detection
hazard that assumes independence between time and distance. Different forms for
the detection hazard are explored with the most flexible choice being that of a cyclic
regression spline.
Extensive simulation studies suggest as expected that a DT proximity estimator is
unbiased for the estimation of density even when the detection hazard varies though
time. However there are indirect benefits of incorporating capture times because
doing so will lead to a better fitting detection component of the model, and this can
prevent unexplained variation being erroneously attributed to the wrong covariate.
The analysis of two real datasets supports this assertion because the models with the
best fitting detection hazard have different effects to the other models. In addition,
modelling the detection process in continuous-time leads to a more parsimonious
approach compared to using DT models when the detection hazard varies in time.
The underlying process is occurring in continuous-time and so using CT models
allows inferences to be drawn about the underlying process, for example the timevarying
detection hazard can be viewed as a proxy for animal activity. The CT
formulation is able to model the underlying detection hazard accurately and provides
a formal modelling framework to explore different hypotheses about activity patterns.
There is scope to integrate the CT models developed here with models for space usage
and landscape connectivity to explore these processes on a finer temporal scale.
SCR models are experiencing a rapid growth in both application and method
development. The data generating process occurs in CT and hence a CT modelling
approach is a natural fit and opens up several opportunities that are not possible
with a DT formulation. The work here makes a contribution by developing and
exploring the utility of such a CT SCR formulation.
Sun, 01 Jan 2017 00:00:00 GMThttps://hdl.handle.net/10023/155962017-01-01T00:00:00ZDistiller, GregSpatial capture-recapture (SCR) models are relatively new but have become the
standard approach used to estimate animal density from capture-recapture data. It
has in the past been impractical to obtain sufficient data for analysis on species that
are very difficult to capture such as elusive carnivores that occur at low density and
range very widely. Advances in technology have led to alternative ways to virtually
“capture" individuals without having to physically hold them. Some examples of
these new non-invasive sampling methods include scat or hair collection for genetic
analysis, acoustic detection and camera trapping.
In traditional capture-recapture (CR) and SCR studies populations are sampled
at discrete points in time leading to clear and well defined occasions whereas the
new detector types mentioned above sample populations continuously in time. Researchers
with data collected continuously currently need to define an appropriate
occasion and aggregate their data accordingly thereby imposing an artificial construct
on their data for analytical convenience.
This research develops a continuous-time (CT) framework for SCR models by
treating detections as a temporal non homogeneous Poisson process (NHPP) and
replacing the usual SCR detection function with a continuous detection hazard function.
The general CT likelihood is first developed for data from passive (also called
“proximity") detectors like camera traps that do not physically hold individuals. The
likelihood is then modified to produce a likelihood for single-catch traps (traps that
are taken out of action by capturing an animal) that has proven difficult to develop
with a discrete-occasion approach.
The lack of a suitable single-catch trap likelihood has led to researchers using
a discrete-time (DT) multi-catch trap estimator to analyse single-catch trap data.
Previous work has found the DT multi-catch estimator to be robust despite the fact
that it is known to be based on the wrong model for single-catch traps (it assumes
that the traps continue operating after catching an individual). Simulation studies in
this work confirm that the multi-catch estimator is robust for estimating density when
density is constant or does not vary much in space. However, there are scenarios with
non-constant density surfaces when the multi-catch estimator is not able to correctly
identify regions of high density. Furthermore, the multi-catch estimator is known
to be negatively biased for the intercept parameter of SCR detection functions and
there may be interest in the detection function in its own right. On the other hand
the CT single-catch estimator is unbiased or nearly so for all parameters of interest
including those in the detection function and those in the model for density.
When one assumes that the detection hazard is constant through time there is
no impact of ignoring capture times and using only the detection frequencies. This
is of course a special case and in reality detection hazards will tend to vary in time.
However when one assumes that the effects of time and distance in the time-varying
hazard are independent, then similarly there is no information in the capture times
about density and detection function parameters. The work here uses a detection
hazard that assumes independence between time and distance. Different forms for
the detection hazard are explored with the most flexible choice being that of a cyclic
regression spline.
Extensive simulation studies suggest as expected that a DT proximity estimator is
unbiased for the estimation of density even when the detection hazard varies though
time. However there are indirect benefits of incorporating capture times because
doing so will lead to a better fitting detection component of the model, and this can
prevent unexplained variation being erroneously attributed to the wrong covariate.
The analysis of two real datasets supports this assertion because the models with the
best fitting detection hazard have different effects to the other models. In addition,
modelling the detection process in continuous-time leads to a more parsimonious
approach compared to using DT models when the detection hazard varies in time.
The underlying process is occurring in continuous-time and so using CT models
allows inferences to be drawn about the underlying process, for example the timevarying
detection hazard can be viewed as a proxy for animal activity. The CT
formulation is able to model the underlying detection hazard accurately and provides
a formal modelling framework to explore different hypotheses about activity patterns.
There is scope to integrate the CT models developed here with models for space usage
and landscape connectivity to explore these processes on a finer temporal scale.
SCR models are experiencing a rapid growth in both application and method
development. The data generating process occurs in CT and hence a CT modelling
approach is a natural fit and opens up several opportunities that are not possible
with a DT formulation. The work here makes a contribution by developing and
exploring the utility of such a CT SCR formulation.Equilibrium states, pressure and escape for multimodal maps with holes
https://hdl.handle.net/10023/15214
For a class of non-uniformly hyperbolic interval maps, we study rates of escape with respect to conformal measures associated with a family of geometric potentials. We establish the existence of physically relevant conditionally invariant measures and equilibrium states and prove a relation between the rate of escape and pressure with respect to these potentials. As a consequence, we obtain a Bowen formula: we express the Hausdorff dimension of the set of points which never exit through the hole in terms of the relevant pressure function. Finally, we obtain an expression for the derivative of the escape rate in the zero-hole limit.
MD was partially supported by NSF grants DMS 1101572 and DMS 1362420. MT was partially supported by NSF grants DMS 0606343 and DMS 0908093.
Fri, 01 Sep 2017 00:00:00 GMThttps://hdl.handle.net/10023/152142017-09-01T00:00:00ZDemers, Mark F.Todd, MikeFor a class of non-uniformly hyperbolic interval maps, we study rates of escape with respect to conformal measures associated with a family of geometric potentials. We establish the existence of physically relevant conditionally invariant measures and equilibrium states and prove a relation between the rate of escape and pressure with respect to these potentials. As a consequence, we obtain a Bowen formula: we express the Hausdorff dimension of the set of points which never exit through the hole in terms of the relevant pressure function. Finally, we obtain an expression for the derivative of the escape rate in the zero-hole limit.Nonlinear partial differential equations on fractals
https://hdl.handle.net/10023/15180
The study of nonlinear partial differential equations on fractals is a burgeoning inter-disciplinary topic, allowing dynamic properties on fractals to be investigated. In this thesis we will investigate nonlinear PDEs of three basic types on bounded and unbounded fractals. We first review the definition of post-critically finite (p.c.f.) self-similar fractals with regular harmonic structure. A Dirichlet form exists on such a fractal; thus we may define a weak version of the Laplacian. The Sobolev-type inequality, established on p.c.f. self-similar fractals satisfying the separation condition, plays a crucial role in the analysis of PDEs on p.c.f. self-similar fractals. We use the classical approach to study the linear eigenvalue problem on p.c.f. self-similar fractals, which depends on the Sobolev-type inequality. Fundamental solutions such as Green's function, wave propagator and heat kernel are then explicitly expressed in terms of eigenvalues and eigenfunctions. The main aim of the thesis is to study nonlinear PDEs on fractals. We begin with nonlinear elliptic equations on p.c.f. self-similar fractals. We prove the existence of non-trivial solutions to elliptic equations with zero Dirichlet boundary conditions using the mountain pass theorem and the saddle point theorem. For nonlinear wave equations on p.c.f. self-similar fractals, we show the existence of global solutions for appropriate initial and boundary data. We also examine blow up at finite time which may occur for certain initial data. Finally, we consider nonlinear diffusion equations on p.c.f. self-similar fractals and unbounded fractals. Using the upper-lower solution technique, we prove the global existence of solutions of the nonlinear diffusion equation with initial value and boundary conditions on p.c.f. self-similar fractals. For unbounded fractals, starting with a heat kernel satisfying certain assumptions, we prove that the diffusion equation with a nonlinear term of the form uᵖ possesses a global solution if the initial data is small and p > 1 + ds/2, while solutions blow up if p ≤ 1 + ds/2 even for small initial data, where dg is the spectral dimension of the fractal. We investigate smoothness and Holder continuity of solutions when they exist.
Mon, 01 Jan 2001 00:00:00 GMThttps://hdl.handle.net/10023/151802001-01-01T00:00:00ZHu, JiaxinThe study of nonlinear partial differential equations on fractals is a burgeoning inter-disciplinary topic, allowing dynamic properties on fractals to be investigated. In this thesis we will investigate nonlinear PDEs of three basic types on bounded and unbounded fractals. We first review the definition of post-critically finite (p.c.f.) self-similar fractals with regular harmonic structure. A Dirichlet form exists on such a fractal; thus we may define a weak version of the Laplacian. The Sobolev-type inequality, established on p.c.f. self-similar fractals satisfying the separation condition, plays a crucial role in the analysis of PDEs on p.c.f. self-similar fractals. We use the classical approach to study the linear eigenvalue problem on p.c.f. self-similar fractals, which depends on the Sobolev-type inequality. Fundamental solutions such as Green's function, wave propagator and heat kernel are then explicitly expressed in terms of eigenvalues and eigenfunctions. The main aim of the thesis is to study nonlinear PDEs on fractals. We begin with nonlinear elliptic equations on p.c.f. self-similar fractals. We prove the existence of non-trivial solutions to elliptic equations with zero Dirichlet boundary conditions using the mountain pass theorem and the saddle point theorem. For nonlinear wave equations on p.c.f. self-similar fractals, we show the existence of global solutions for appropriate initial and boundary data. We also examine blow up at finite time which may occur for certain initial data. Finally, we consider nonlinear diffusion equations on p.c.f. self-similar fractals and unbounded fractals. Using the upper-lower solution technique, we prove the global existence of solutions of the nonlinear diffusion equation with initial value and boundary conditions on p.c.f. self-similar fractals. For unbounded fractals, starting with a heat kernel satisfying certain assumptions, we prove that the diffusion equation with a nonlinear term of the form uᵖ possesses a global solution if the initial data is small and p > 1 + ds/2, while solutions blow up if p ≤ 1 + ds/2 even for small initial data, where dg is the spectral dimension of the fractal. We investigate smoothness and Holder continuity of solutions when they exist.Adaptive distance sampling
https://hdl.handle.net/10023/15176
We investigate mechanisms to improve efficiency for line and point transect surveys of clustered populations by combining the distance methods with adaptive sampling. In adaptive sampling, survey effort is increased when areas of high animal density are located, thereby increasing the number of observations. We begin by building on existing adaptive sampling techniques, to create both point and line transect adaptive estimators, these are then extended to allow the inclusion of covariates in the detection function estimator. However, the methods are limited, as the total effort required cannot be forecast at the start of a survey, and so a new fixed total effort adaptive approach is developed. A key difference in the new method is that it does not require the calculation of the inclusion probabilities typically used by existing adaptive estimators. The fixed effort method is primarily aimed at line transect sampling, but point transect derivations are also provided. We evaluate the new methodology by computer simulation, and report on surveys of harbour porpoise in the Gulf of Maine, in which the approach was compared with conventional line transect sampling. Line transect simulation results for a clustered population showed up to a 6% improvement in the adaptive density variance estimate over the conventional, whilst when there was no clustering the adaptive estimate was 1% less efficient than the conventional. For the harbour porpoise survey, the adaptive density estimate cvs showed improvements of 8% for individual porpoise density and 14% for school density over the conventional estimates. The primary benefit of the fixed effort method is the potential to improve survey coverage, allowing a survey to complete within a fixed time and effort; an important feature if expensive survey resources are involved, such as an aircraft, crew and observers.
Tue, 01 Jan 2002 00:00:00 GMThttps://hdl.handle.net/10023/151762002-01-01T00:00:00ZPollard, JohnWe investigate mechanisms to improve efficiency for line and point transect surveys of clustered populations by combining the distance methods with adaptive sampling. In adaptive sampling, survey effort is increased when areas of high animal density are located, thereby increasing the number of observations. We begin by building on existing adaptive sampling techniques, to create both point and line transect adaptive estimators, these are then extended to allow the inclusion of covariates in the detection function estimator. However, the methods are limited, as the total effort required cannot be forecast at the start of a survey, and so a new fixed total effort adaptive approach is developed. A key difference in the new method is that it does not require the calculation of the inclusion probabilities typically used by existing adaptive estimators. The fixed effort method is primarily aimed at line transect sampling, but point transect derivations are also provided. We evaluate the new methodology by computer simulation, and report on surveys of harbour porpoise in the Gulf of Maine, in which the approach was compared with conventional line transect sampling. Line transect simulation results for a clustered population showed up to a 6% improvement in the adaptive density variance estimate over the conventional, whilst when there was no clustering the adaptive estimate was 1% less efficient than the conventional. For the harbour porpoise survey, the adaptive density estimate cvs showed improvements of 8% for individual porpoise density and 14% for school density over the conventional estimates. The primary benefit of the fixed effort method is the potential to improve survey coverage, allowing a survey to complete within a fixed time and effort; an important feature if expensive survey resources are involved, such as an aircraft, crew and observers.Contractive Markov systems
https://hdl.handle.net/10023/15173
We introduce a theory of contractive Markov systems (CMS) which provides a unifying framework in so-called "fractal" geometry. It extends the known theory of iterated function systems (IFS) with place dependent probabilities [1][8] in a way that it also covers graph directed constructions of "fractal" sets [18]. Such systems naturally extend finite Markov chains and inherit some of their properties. In Chapter 1, we consider iterations of a Markov system and show that they preserve the essential structure of it. In Chapter 2, we show that the Markov operator defined by such a system has a unique invariant probability measure in the irreducible case and an attractive probability measure in the aperiodic case if the restrictions of the probability functions on their vertex sets are Dini-continuous and bounded away from zero, and the system satisfies a condition of a contractiveness on average. This generalizes a result from [1]. Furthermore, we show that the rate of convergence to the stationary state is exponential in the aperiodic case with constant probabilities and a compact state space. In Chapter 3, we construct a coding map for a contractive Markov system. In Chapter 4, we calculate Kolmogorov-Sinai entropy of the generalized Markov shift. In Chapter 5, we prove an ergodic theorem for Markov chains associated with the contractive Markov systems. It generalizes the ergodic theorem of Elton [8].
Thu, 01 Jan 2004 00:00:00 GMThttps://hdl.handle.net/10023/151732004-01-01T00:00:00ZWerner, IvanWe introduce a theory of contractive Markov systems (CMS) which provides a unifying framework in so-called "fractal" geometry. It extends the known theory of iterated function systems (IFS) with place dependent probabilities [1][8] in a way that it also covers graph directed constructions of "fractal" sets [18]. Such systems naturally extend finite Markov chains and inherit some of their properties. In Chapter 1, we consider iterations of a Markov system and show that they preserve the essential structure of it. In Chapter 2, we show that the Markov operator defined by such a system has a unique invariant probability measure in the irreducible case and an attractive probability measure in the aperiodic case if the restrictions of the probability functions on their vertex sets are Dini-continuous and bounded away from zero, and the system satisfies a condition of a contractiveness on average. This generalizes a result from [1]. Furthermore, we show that the rate of convergence to the stationary state is exponential in the aperiodic case with constant probabilities and a compact state space. In Chapter 3, we construct a coding map for a contractive Markov system. In Chapter 4, we calculate Kolmogorov-Sinai entropy of the generalized Markov shift. In Chapter 5, we prove an ergodic theorem for Markov chains associated with the contractive Markov systems. It generalizes the ergodic theorem of Elton [8].Finding "small' matrices P,Q such that PDQ = S
https://hdl.handle.net/10023/15171
Given an integer matrix A, there is a unique matrix S of a particular form, called the Smith Normal Form, and non-unique unimodular matrices P and Q such that PAQ = S. It is often the case that these matrices P and Q will be used for further calculation, and as such it is desirable to find P and Q with small entries. In this thesis we address the problem of finding such P and Q with small entries, in particular in the case where A is a diagonal matrix, which arises as a final step in many published algorithms. Heuristic algorithms are developed which appear to do well in practice and some theory is developed to explain this behaviour. We also give an account of the implementation of an alternative algorithm which bypasses this intermediary diagonal form. The basic theoretical development of this is work by Storjohan.
Tue, 01 Jan 2002 00:00:00 GMThttps://hdl.handle.net/10023/151712002-01-01T00:00:00ZWainwright, Robert J.Given an integer matrix A, there is a unique matrix S of a particular form, called the Smith Normal Form, and non-unique unimodular matrices P and Q such that PAQ = S. It is often the case that these matrices P and Q will be used for further calculation, and as such it is desirable to find P and Q with small entries. In this thesis we address the problem of finding such P and Q with small entries, in particular in the case where A is a diagonal matrix, which arises as a final step in many published algorithms. Heuristic algorithms are developed which appear to do well in practice and some theory is developed to explain this behaviour. We also give an account of the implementation of an alternative algorithm which bypasses this intermediary diagonal form. The basic theoretical development of this is work by Storjohan.Automatic S-acts and inverse semigroup presentations
https://hdl.handle.net/10023/15123
To provide a general framework for the theory of automatic groups and semigroups, we introduce the notion of an automatic semigroup act. This notion gives rise to a variety of definitions for automaticity depending on the set chosen as a semigroup act. Namely, we obtain the notions of automaticity, Schutzenberger automaticity, R- and L-class automaticity, etc. We discuss the basic properties of automatic semigroup acts. We show that if S is a semigroup with local right identities, then automaticty of a semigroup act is independent of the choice of both the generators of S and the generators of the semigroup act. We also discuss the equality problem of automatic semigroup acts. To give a geometric approach, we associate a directed labelled graph to each S-act and introduce the notion of the fellow traveller property in the associated graph. We verify that if S is a regular semigroup with finitely many idempotents, then Schutzenberger automaticity is characterized by the fellow traveller property of the Schutzenberger graph. We also verify that a Schutzenberger automatic regular semigroup with finitely many idempotents is finitely presented. We end Chapter 3 by proving that an inverse free product of Schutzenberger automatic inverse semigroups is Schutzenberger automatic. In Chapter 4, we first introduce the notion of finite generation and finite presentability with respect to a semigroup action. With the help of these concepts we give a necessary and sufficient condition for a semidirect product of a semilattice by a group to be finitely generated and finitely presented as an inverse semigroup. We end Chapter 4 by giving a necessary and sufficient condition for the semidirect product of a semilattice by a group to be Schutzenberger automatic. Chapter 5 is devoted to the study of HNN extensions of inverse semigroups from finite generation and finite presentability point of view. Namely, we give necessary and sufficient conditions for finite presentability of Gilbert's and Yamamura's HNN extension of inverse semigroups. The majority of the results contained in Chapter 5 are the result of a joint work with N.D. Gilbert and N. Ruskuc.
Sat, 01 Jan 2005 00:00:00 GMThttps://hdl.handle.net/10023/151232005-01-01T00:00:00ZDombi, Erzsebet RitaTo provide a general framework for the theory of automatic groups and semigroups, we introduce the notion of an automatic semigroup act. This notion gives rise to a variety of definitions for automaticity depending on the set chosen as a semigroup act. Namely, we obtain the notions of automaticity, Schutzenberger automaticity, R- and L-class automaticity, etc. We discuss the basic properties of automatic semigroup acts. We show that if S is a semigroup with local right identities, then automaticty of a semigroup act is independent of the choice of both the generators of S and the generators of the semigroup act. We also discuss the equality problem of automatic semigroup acts. To give a geometric approach, we associate a directed labelled graph to each S-act and introduce the notion of the fellow traveller property in the associated graph. We verify that if S is a regular semigroup with finitely many idempotents, then Schutzenberger automaticity is characterized by the fellow traveller property of the Schutzenberger graph. We also verify that a Schutzenberger automatic regular semigroup with finitely many idempotents is finitely presented. We end Chapter 3 by proving that an inverse free product of Schutzenberger automatic inverse semigroups is Schutzenberger automatic. In Chapter 4, we first introduce the notion of finite generation and finite presentability with respect to a semigroup action. With the help of these concepts we give a necessary and sufficient condition for a semidirect product of a semilattice by a group to be finitely generated and finitely presented as an inverse semigroup. We end Chapter 4 by giving a necessary and sufficient condition for the semidirect product of a semilattice by a group to be Schutzenberger automatic. Chapter 5 is devoted to the study of HNN extensions of inverse semigroups from finite generation and finite presentability point of view. Namely, we give necessary and sufficient conditions for finite presentability of Gilbert's and Yamamura's HNN extension of inverse semigroups. The majority of the results contained in Chapter 5 are the result of a joint work with N.D. Gilbert and N. Ruskuc.Automatic semigroups : constructions and subsemigroups
https://hdl.handle.net/10023/15122
In this thesis we start by considering conditions under which some standard semigroup constructions preserve automaticity. We first consider Rees matrix semigroups over a semigroup, which we call the base, and work on the following questions: (i) If the base is automatic is the Rees matrix semigroup automatic? (ii) If the Rees matrix semigroup is automatic must the base be automatic as well? We also consider similar questions for Bruck-Reilly extensions of monoids and wreath products of semigroups. Then we consider subsemigroups of free products of semigroups and we study conditions that guarantee them to be automatic. Finally we obtain a description of the subsemigroups of the bicyclic monoid that allow us to study some of their properties, which include finite generation, automaticity and finite presentability.
Tue, 01 Jan 2002 00:00:00 GMThttps://hdl.handle.net/10023/151222002-01-01T00:00:00ZDescalco, L.In this thesis we start by considering conditions under which some standard semigroup constructions preserve automaticity. We first consider Rees matrix semigroups over a semigroup, which we call the base, and work on the following questions: (i) If the base is automatic is the Rees matrix semigroup automatic? (ii) If the Rees matrix semigroup is automatic must the base be automatic as well? We also consider similar questions for Bruck-Reilly extensions of monoids and wreath products of semigroups. Then we consider subsemigroups of free products of semigroups and we study conditions that guarantee them to be automatic. Finally we obtain a description of the subsemigroups of the bicyclic monoid that allow us to study some of their properties, which include finite generation, automaticity and finite presentability.Presentations for subsemigroups of groups
https://hdl.handle.net/10023/15119
This thesis studies subsemigroups of groups from three perspectives: automatic structures, ordinary semigroup presentations, and Malcev presentaions. [A Malcev presentation is a presentation of a special type for a semigroup that can be embedded into a group. A group-embeddable semigroup is Malcev coherent if all of its finitely generated subsemigroups admit finite Malcev presentations.] The theory of synchronous and asynchronous automatic structures for semigroups is expounded, particularly for group-embeddable semigroups. In particular, automatic semigroups embeddable into groups are shown to inherit many of the pleasant geometric properties of automatic groups. It is proved that group- embeddable automatic semigroups admit finite Malcev presentations, and such presentations can be found effectively. An algorithm is exhibited to test whether an automatic semigroup is a free semigroup. Cancellativity of automatic semigroups is proved to be undecidable. Study is made of several classes of groups: virtually free groups; groups that satisfy semigroup laws (in particular [virtually] nilpotent and [virtually] abelian groups); polycyclic groups; free and direct products of certain groups; and one-relator groups. For each of these classes, the question of Malcev coherence is considered, together with the problems of whether finitely generated subsemigroups are finitely presented or automatic. This study yields closure and containment results regarding the class of Malcev coherent groups. The property of having a finite Malcev presentation is shown to be preserved under finite Rees index extensions and subsemigroups. Other concepts of index are also studied.
Sat, 01 Jan 2005 00:00:00 GMThttps://hdl.handle.net/10023/151192005-01-01T00:00:00ZCain, Alan JamesThis thesis studies subsemigroups of groups from three perspectives: automatic structures, ordinary semigroup presentations, and Malcev presentaions. [A Malcev presentation is a presentation of a special type for a semigroup that can be embedded into a group. A group-embeddable semigroup is Malcev coherent if all of its finitely generated subsemigroups admit finite Malcev presentations.] The theory of synchronous and asynchronous automatic structures for semigroups is expounded, particularly for group-embeddable semigroups. In particular, automatic semigroups embeddable into groups are shown to inherit many of the pleasant geometric properties of automatic groups. It is proved that group- embeddable automatic semigroups admit finite Malcev presentations, and such presentations can be found effectively. An algorithm is exhibited to test whether an automatic semigroup is a free semigroup. Cancellativity of automatic semigroups is proved to be undecidable. Study is made of several classes of groups: virtually free groups; groups that satisfy semigroup laws (in particular [virtually] nilpotent and [virtually] abelian groups); polycyclic groups; free and direct products of certain groups; and one-relator groups. For each of these classes, the question of Malcev coherence is considered, together with the problems of whether finitely generated subsemigroups are finitely presented or automatic. This study yields closure and containment results regarding the class of Malcev coherent groups. The property of having a finite Malcev presentation is shown to be preserved under finite Rees index extensions and subsemigroups. Other concepts of index are also studied.Finiteness conditions of wreath products of semigroups and related properties of diagonal acts
https://hdl.handle.net/10023/15117
The purpose of this thesis is to consider finite generation, finite presentability and related properties of restricted wreath products of semigroups. We show that the wreath product Awr B of two monoids is finitely generated if and only if A and B are finitely generated and the action by right multiplication on B of the group of units of B has only finitely many orbits. Also we show that the wreath product AwrB of two non-trivial monoids is finitely presented if and only if A is finitely presented and B is finite. The situation is more complicated in the case of the wreath product SₑwrT of two semigroups with respect to an idempotent e ϵ S. We give a complete characterization for finite generation in the case where T is finite. This result depends on the properties of the diagonal action of S on S x S. We also prove that if this action is not finitely generated, then SₑwrT (with S infinite and T finite) is finitely presented if and only if S x S is finitely presented and T is the direct product of a monoid and a left zero semigroup. In the case where T is infinite, we prove that S must be a monoid in order for SwrT to be finitely generated. We show that the finiteness properties of periodicity and local finiteness are preserved under the wreath product construction. We conclude the thesis with a systematic investigation into the properties of diagonal acts of semigroups, and make some interesting connections between diagonal acts and power semigroups.
Mon, 01 Jan 2001 00:00:00 GMThttps://hdl.handle.net/10023/151172001-01-01T00:00:00ZThomson, Michael R.The purpose of this thesis is to consider finite generation, finite presentability and related properties of restricted wreath products of semigroups. We show that the wreath product Awr B of two monoids is finitely generated if and only if A and B are finitely generated and the action by right multiplication on B of the group of units of B has only finitely many orbits. Also we show that the wreath product AwrB of two non-trivial monoids is finitely presented if and only if A is finitely presented and B is finite. The situation is more complicated in the case of the wreath product SₑwrT of two semigroups with respect to an idempotent e ϵ S. We give a complete characterization for finite generation in the case where T is finite. This result depends on the properties of the diagonal action of S on S x S. We also prove that if this action is not finitely generated, then SₑwrT (with S infinite and T finite) is finitely presented if and only if S x S is finitely presented and T is the direct product of a monoid and a left zero semigroup. In the case where T is infinite, we prove that S must be a monoid in order for SwrT to be finitely generated. We show that the finiteness properties of periodicity and local finiteness are preserved under the wreath product construction. We conclude the thesis with a systematic investigation into the properties of diagonal acts of semigroups, and make some interesting connections between diagonal acts and power semigroups.Todd-Coxeter methods for inverse monoids
https://hdl.handle.net/10023/15052
Let P be the inverse monoid presentation (X|U) for the inverse monoid M, let π be the set of generators for a right congruence on M and let u Є M. Using the work of J. Stephen [15], the current work demonstrates a coset enumeration technique for the R-class Rᵤ similar to the coset enumeration algorithm developed by J. A. Todd and H. S. M. Coxeter for groups. Furthermore it is demonstrated how to test whether Rᵤ = Rᵥ, for u, v Є M and so a technique for enumerating inverse monoids is described. This technique is generalised to enumerate the H-classes of M. The algorithms have been implemented in GAP 3.4.4 [25], and have been used to analyse some examples given in Chapter 6. The thesis concludes by a related discussion of normal forms and automaticity of free inverse semigroups.
Mon, 01 Jan 2001 00:00:00 GMThttps://hdl.handle.net/10023/150522001-01-01T00:00:00ZCutting, AndrewLet P be the inverse monoid presentation (X|U) for the inverse monoid M, let π be the set of generators for a right congruence on M and let u Є M. Using the work of J. Stephen [15], the current work demonstrates a coset enumeration technique for the R-class Rᵤ similar to the coset enumeration algorithm developed by J. A. Todd and H. S. M. Coxeter for groups. Furthermore it is demonstrated how to test whether Rᵤ = Rᵥ, for u, v Є M and so a technique for enumerating inverse monoids is described. This technique is generalised to enumerate the H-classes of M. The algorithms have been implemented in GAP 3.4.4 [25], and have been used to analyse some examples given in Chapter 6. The thesis concludes by a related discussion of normal forms and automaticity of free inverse semigroups.Fibonacci length and efficiency in group presentations
https://hdl.handle.net/10023/15048
In this thesis we shall consider two topics that are contained in combinatorial group theory and concern properties of finitely presented groups. The first problem we examine is that of calculating the Fibonacci length of certain families of finitely presented groups. In pursuing this we come across ideas and unsolved problems from number theory. We mainly concentrate on finding the Fibonacci length of powers of dihedral groups, certain Fibonacci groups and a family of metacyclic groups. The second problem we investigate in this thesis is finding if the group PGL(2, p), for p a prime, is efficient on a minimal generating set. We find various presentations that define PGL(2,p) or C₂ x PSL(2,p) and direct products of these groups. As in the previous sections we come across number theoretic problems. We also have occasion to use results from tensor theory and homological algebra in order to obtain our results.
Wed, 01 Jan 2003 00:00:00 GMThttps://hdl.handle.net/10023/150482003-01-01T00:00:00ZCampbell, Peter P.In this thesis we shall consider two topics that are contained in combinatorial group theory and concern properties of finitely presented groups. The first problem we examine is that of calculating the Fibonacci length of certain families of finitely presented groups. In pursuing this we come across ideas and unsolved problems from number theory. We mainly concentrate on finding the Fibonacci length of powers of dihedral groups, certain Fibonacci groups and a family of metacyclic groups. The second problem we investigate in this thesis is finding if the group PGL(2, p), for p a prime, is efficient on a minimal generating set. We find various presentations that define PGL(2,p) or C₂ x PSL(2,p) and direct products of these groups. As in the previous sections we come across number theoretic problems. We also have occasion to use results from tensor theory and homological algebra in order to obtain our results.Automated theorem proving for mathematics : real analysis in PVS
https://hdl.handle.net/10023/15046
Computer Algebra Systems (CASs), such as Maple and Mathematica, are now widely used in both industry and education. In many areas of mathematics they perform well. However, many well-established methods in mathematics, such as definite integration via the fundamental theorem of calculus, rely on analytic side conditions which CASs in general do not support. This thesis presents our work with automatic, formal mathematics using the theorem prover PVS. Based on an existing real analysis library for PVS, we have implemented transcendental functions such as exp, cos, sin, tan and their inverses, and we have provided strategies to prove that a function is continuous at a given point. In general, this is undecidable, but using certain restrictions we can still provide proofs for a large collection of functions. Similarly, we can prove that a function has a limit at a point. We illustrate how the extended library may be used with Maple to provide correct results where Maple's are incorrect. We present a case study of definite integration in the CASs axiom. Maple, Mathematica and Matlab. The case study clearly shows that apart from axiom the systems do not fully check the necessary conditions for the definite integral to exist, thus giving results varying from plain incorrect to correct, even if the latter is difficult to detect without manipulating the result. The extension and correction of the PVS library consists of around 1000 theorems proven by around 18000 PVS proof commands. We also have a test suite of 88 lemmas for the automatic checks for continuity and existence of limits. Thus we have devised and tested automatic computational logic support for the use of formal mathematics in applications, particularly computer algebra.
Tue, 01 Jan 2002 00:00:00 GMThttps://hdl.handle.net/10023/150462002-01-01T00:00:00ZGottliebsen, HanneComputer Algebra Systems (CASs), such as Maple and Mathematica, are now widely used in both industry and education. In many areas of mathematics they perform well. However, many well-established methods in mathematics, such as definite integration via the fundamental theorem of calculus, rely on analytic side conditions which CASs in general do not support. This thesis presents our work with automatic, formal mathematics using the theorem prover PVS. Based on an existing real analysis library for PVS, we have implemented transcendental functions such as exp, cos, sin, tan and their inverses, and we have provided strategies to prove that a function is continuous at a given point. In general, this is undecidable, but using certain restrictions we can still provide proofs for a large collection of functions. Similarly, we can prove that a function has a limit at a point. We illustrate how the extended library may be used with Maple to provide correct results where Maple's are incorrect. We present a case study of definite integration in the CASs axiom. Maple, Mathematica and Matlab. The case study clearly shows that apart from axiom the systems do not fully check the necessary conditions for the definite integral to exist, thus giving results varying from plain incorrect to correct, even if the latter is difficult to detect without manipulating the result. The extension and correction of the PVS library consists of around 1000 theorems proven by around 18000 PVS proof commands. We also have a test suite of 88 lemmas for the automatic checks for continuity and existence of limits. Thus we have devised and tested automatic computational logic support for the use of formal mathematics in applications, particularly computer algebra.Commutativity and free products in Thompson's group V
https://hdl.handle.net/10023/14652
We broaden the theory of dynamical interpretation, investigate the property of commutativity and explore the subject of subgroups forming free products in Thompson's group V.
We expand Brin's terminology for a revealing pair to an any tree pair. We use it to analyse the dynamical behaviour of an arbitrary tree pair which cannot occur in a revealing pair. Hence, we design a series of algorithms generating Brin's revealing pair from any tree pair, by successively eliminating the undesirable structures. To detect patterns and transitioning between tree pairs, we introduce a new combinatorial object called the chains graph. A newly defined, unique and symmetrical type of a tree pair, called a balanced tree pair, stems from the use of the chains graphs.
The main theorem of Bleak et al. in "Centralizers in the R. Thompson's Group V_n" states the necessary structure of the centraliser of an element of V. We provide a converse to this theorem, by proving that each of the predicted structures is realisable. Hence we obtain a complete classification of centralisers in V. We give an explicit construction of an element of V with prescribed centraliser. The underlying concept is to embed a Cayley graph of a finite group into the flow graph (introduced in Bleak et al.) of the desired element. To reflect the symmetry, we present the resulting element in terms of a balanced tree pair.
The group V is conjectured to be a universal coCF group, which generates interest in studying its subgroups. We develop a better understanding of embeddings into V by providing a necessary and sufficient dynamical condition for two subgroups (not both torsion) to form a free product in V. For this, we use the properties, explored in Bleak and Salazar-Díaz "Free Products in Thompson's Group V", of sets of so--called important points, and the Ping-Pong action induced on them.
Tue, 26 Jun 2018 00:00:00 GMThttps://hdl.handle.net/10023/146522018-06-26T00:00:00ZBieniecka, EwaWe broaden the theory of dynamical interpretation, investigate the property of commutativity and explore the subject of subgroups forming free products in Thompson's group V.
We expand Brin's terminology for a revealing pair to an any tree pair. We use it to analyse the dynamical behaviour of an arbitrary tree pair which cannot occur in a revealing pair. Hence, we design a series of algorithms generating Brin's revealing pair from any tree pair, by successively eliminating the undesirable structures. To detect patterns and transitioning between tree pairs, we introduce a new combinatorial object called the chains graph. A newly defined, unique and symmetrical type of a tree pair, called a balanced tree pair, stems from the use of the chains graphs.
The main theorem of Bleak et al. in "Centralizers in the R. Thompson's Group V_n" states the necessary structure of the centraliser of an element of V. We provide a converse to this theorem, by proving that each of the predicted structures is realisable. Hence we obtain a complete classification of centralisers in V. We give an explicit construction of an element of V with prescribed centraliser. The underlying concept is to embed a Cayley graph of a finite group into the flow graph (introduced in Bleak et al.) of the desired element. To reflect the symmetry, we present the resulting element in terms of a balanced tree pair.
The group V is conjectured to be a universal coCF group, which generates interest in studying its subgroups. We develop a better understanding of embeddings into V by providing a necessary and sufficient dynamical condition for two subgroups (not both torsion) to form a free product in V. For this, we use the properties, explored in Bleak and Salazar-Díaz "Free Products in Thompson's Group V", of sets of so--called important points, and the Ping-Pong action induced on them.The dynamic topology of the solar corona : mapping the Sun’s three dimensional magnetic skeleton
https://hdl.handle.net/10023/14637
Observations of the surface of the Sun reveal multi-scaled, mixed magnetic features
that carpet the entire solar surface. Not surprisingly, the global magnetic fields
extrapolated from these observations are highly complex. This thesis explores the
topology of the Sun’s global coronal magnetic fields. The magnetic skeleton of a
magnetic field provides us with a way of examining the magnetic field and
quantifying its complexity.
Using specialised codes to find the magnetic skeletons which were written during the
course of this work, we first examine potential field extrapolations of the global solar
coronal magnetic field determined from observed synoptic magnetograms from the
Heliospheric Magnetic Imager on the Solar Dynamics Observatory. The resolution of
the PFSS models is found to be very important for discovering the true nature of the
global magnetic skeleton. By increasing the maximum number of harmonics used in
the potential field extrapolations and, therefore, the grid resolution, 60 times more
null points may be found in the coronal magnetic field. These high resolution fields
also have a large global separator network which connects the coronal magnetic field
over large distances and involves between 40 % and 60 % of all the null points in the
solar atmosphere. This global separator network exists at both solar minimum and
solar maximum and has separators that reach high into the solar atmosphere
(> 1R☉) even though they connect null points close to the solar surface.
These potential field extrapolations are then compared with magnetohydrostatic
(MHS) extrapolations of the coronal magnetic field which also provide us with
information about the plasma in the corona. With a small component of electric
current density in the direction perpendicular to the radial direction, these MHS fields
are found to have a plasma beta and pressure typical of the corona. As this small
component of electric current density grows, the heliospheric current sheet is warped
significantly and the magnetic field, plasma beta and pressure become unphysical.
Torsional spine reconnection is also studied local to a single null point. First using a
dynamical relaxation of a spiral null point under non-resistive magnetohydrodynamics
(MHD) to a MHS equilibrium is form in which a current layer has built up around
the spine lines. Then the reconnection under resistive MHD in this current sheet is
studied. The current about the spine lines is dissipated and the magnetic energy is
mainly converted into heat directly as the field lines untwist about the spine line.
Tue, 26 Jun 2018 00:00:00 GMThttps://hdl.handle.net/10023/146372018-06-26T00:00:00ZWilliams, Benjamin MatthewObservations of the surface of the Sun reveal multi-scaled, mixed magnetic features
that carpet the entire solar surface. Not surprisingly, the global magnetic fields
extrapolated from these observations are highly complex. This thesis explores the
topology of the Sun’s global coronal magnetic fields. The magnetic skeleton of a
magnetic field provides us with a way of examining the magnetic field and
quantifying its complexity.
Using specialised codes to find the magnetic skeletons which were written during the
course of this work, we first examine potential field extrapolations of the global solar
coronal magnetic field determined from observed synoptic magnetograms from the
Heliospheric Magnetic Imager on the Solar Dynamics Observatory. The resolution of
the PFSS models is found to be very important for discovering the true nature of the
global magnetic skeleton. By increasing the maximum number of harmonics used in
the potential field extrapolations and, therefore, the grid resolution, 60 times more
null points may be found in the coronal magnetic field. These high resolution fields
also have a large global separator network which connects the coronal magnetic field
over large distances and involves between 40 % and 60 % of all the null points in the
solar atmosphere. This global separator network exists at both solar minimum and
solar maximum and has separators that reach high into the solar atmosphere
(> 1R☉) even though they connect null points close to the solar surface.
These potential field extrapolations are then compared with magnetohydrostatic
(MHS) extrapolations of the coronal magnetic field which also provide us with
information about the plasma in the corona. With a small component of electric
current density in the direction perpendicular to the radial direction, these MHS fields
are found to have a plasma beta and pressure typical of the corona. As this small
component of electric current density grows, the heliospheric current sheet is warped
significantly and the magnetic field, plasma beta and pressure become unphysical.
Torsional spine reconnection is also studied local to a single null point. First using a
dynamical relaxation of a spiral null point under non-resistive magnetohydrodynamics
(MHD) to a MHS equilibrium is form in which a current layer has built up around
the spine lines. Then the reconnection under resistive MHD in this current sheet is
studied. The current about the spine lines is dissipated and the magnetic energy is
mainly converted into heat directly as the field lines untwist about the spine line.Modelling chromospheric evaporation in response to coronal heating
https://hdl.handle.net/10023/14630
This thesis presents a new computationally efficient method for modelling the response of the solar corona to the release of energy. During impulsive heating events, the coronal temperature increases which leads to a downward heat flux into the transition region (TR). The plasma is unable to radiate this excess conductive heating and so the gas pressure increases locally. The resulting pressure gradient drives an upflow of dense material, creating an increase in the coronal density. This density increase is often called chromospheric evaporation. A process which is highly sensitive to the TR resolution in numerical simulations. If the resolution is not adequate, then the downward heat flux jumps over the TR and deposits the heat in the chromosphere, where it is radiated away. The outcome is that with an under-resolved TR, major errors occur in simulating the coronal density evolution. We address this problem by treating the lower transition region as a discontinuity that responds to changing coronal conditions through the imposition of a jump condition that is derived from an integrated form of energy conservation. In this thesis, it is shown that this method permits fast and accurate numerical solutions in both one-dimensional and multi-dimensional simulations. By modelling the TR with this appropriate jump condition, we remove the influence of poor numerical resolution and obtain the correct evaporative response to coronal heating, even when using resolutions that are compatible with multi-dimensional magnetohydrodynamic simulations.
Tue, 26 Jun 2018 00:00:00 GMThttps://hdl.handle.net/10023/146302018-06-26T00:00:00ZJohnston, Craig DavidThis thesis presents a new computationally efficient method for modelling the response of the solar corona to the release of energy. During impulsive heating events, the coronal temperature increases which leads to a downward heat flux into the transition region (TR). The plasma is unable to radiate this excess conductive heating and so the gas pressure increases locally. The resulting pressure gradient drives an upflow of dense material, creating an increase in the coronal density. This density increase is often called chromospheric evaporation. A process which is highly sensitive to the TR resolution in numerical simulations. If the resolution is not adequate, then the downward heat flux jumps over the TR and deposits the heat in the chromosphere, where it is radiated away. The outcome is that with an under-resolved TR, major errors occur in simulating the coronal density evolution. We address this problem by treating the lower transition region as a discontinuity that responds to changing coronal conditions through the imposition of a jump condition that is derived from an integrated form of energy conservation. In this thesis, it is shown that this method permits fast and accurate numerical solutions in both one-dimensional and multi-dimensional simulations. By modelling the TR with this appropriate jump condition, we remove the influence of poor numerical resolution and obtain the correct evaporative response to coronal heating, even when using resolutions that are compatible with multi-dimensional magnetohydrodynamic simulations.Photometry of star clusters
https://hdl.handle.net/10023/14283
The suitability of the Cassegrain Schmidt telescopes at St. Andrews University Observatory for the measurement of stellar magnitudes and colours by in-focus multicolour photography has been examined. A major requirement is that the photographic plate should coincide with the focal surface. Thermal effects in the Scott Lang Telescope and optical and mechanical problems in the James Gregory Telescope cause difficulty in, attaining this. These difficulties have been overcome in the case of the Scott Lang Telescope but no certain method for focussing the James Gregory Telescope was found. The photometric field limited by field error, is approximately one degree in diameter in each case. Colour equations between the instrumental and standard B, V systems depend on magnitude and, in the case of the Scott Lang Telescope, on exposure time as well. The methods used to measure UBV magnitudes and colours with the Radcliffe 74-inch reflector are described and the accuracy of the results discussed. Magnitudes and colours of stars brighter than V - 15.5 in the open cluster IC 2581 have been measured, together with MK spectral types for a few of the brighter stars. The interstellar absorption provides a criterion for the recognition of cluster members. A discrepancy between the shape of the cluster main sequence and that of the zero age main sequence is attributed to an error in the derivation of the standard zero age main sequence. The cluster is found to be at a distance of 2500 parsecs and may form part of the Carinae complex. The positions of the brightest stars in the colour magnitude diagram are discussed in the light of modern theories of stellar evolution and an age of approximately 10 million years is deduced. The colour magnitude diagram of the open cluster NGC 6383 has been obtained for stars brighter than V - 18.1; the limiting magnitudes in B and U are 19.7 and 17.9, respectively. MK spectral types have permitted the cluster membership of several bright B stars to be established; some stars of later type are non-members. The observation of this cluster are more complete than for most young clusters studied to date, but the poorness of the cluster and the unfavourable distribution of interstellar absorption with distance make it impossible to be certain of the membership of stars fainter than V = 13. The lack of stars fainter than V = 12.8 on the zero age main sequence indicates a contraction age of 5 million years. The distance is 1300 parsecs, like those of other young groups in the vicinity. The dense dust clouds which divide the Milky Way in Scorpius are immediately beyond this. Several faint variable stars may be of the T Tauri type.
Mon, 01 Jan 1968 00:00:00 GMThttps://hdl.handle.net/10023/142831968-01-01T00:00:00ZEvans, Thomas Harry Hope LloydThe suitability of the Cassegrain Schmidt telescopes at St. Andrews University Observatory for the measurement of stellar magnitudes and colours by in-focus multicolour photography has been examined. A major requirement is that the photographic plate should coincide with the focal surface. Thermal effects in the Scott Lang Telescope and optical and mechanical problems in the James Gregory Telescope cause difficulty in, attaining this. These difficulties have been overcome in the case of the Scott Lang Telescope but no certain method for focussing the James Gregory Telescope was found. The photometric field limited by field error, is approximately one degree in diameter in each case. Colour equations between the instrumental and standard B, V systems depend on magnitude and, in the case of the Scott Lang Telescope, on exposure time as well. The methods used to measure UBV magnitudes and colours with the Radcliffe 74-inch reflector are described and the accuracy of the results discussed. Magnitudes and colours of stars brighter than V - 15.5 in the open cluster IC 2581 have been measured, together with MK spectral types for a few of the brighter stars. The interstellar absorption provides a criterion for the recognition of cluster members. A discrepancy between the shape of the cluster main sequence and that of the zero age main sequence is attributed to an error in the derivation of the standard zero age main sequence. The cluster is found to be at a distance of 2500 parsecs and may form part of the Carinae complex. The positions of the brightest stars in the colour magnitude diagram are discussed in the light of modern theories of stellar evolution and an age of approximately 10 million years is deduced. The colour magnitude diagram of the open cluster NGC 6383 has been obtained for stars brighter than V - 18.1; the limiting magnitudes in B and U are 19.7 and 17.9, respectively. MK spectral types have permitted the cluster membership of several bright B stars to be established; some stars of later type are non-members. The observation of this cluster are more complete than for most young clusters studied to date, but the poorness of the cluster and the unfavourable distribution of interstellar absorption with distance make it impossible to be certain of the membership of stars fainter than V = 13. The lack of stars fainter than V = 12.8 on the zero age main sequence indicates a contraction age of 5 million years. The distance is 1300 parsecs, like those of other young groups in the vicinity. The dense dust clouds which divide the Milky Way in Scorpius are immediately beyond this. Several faint variable stars may be of the T Tauri type.Solar intense magnetic fields
https://hdl.handle.net/10023/14277
The nature of motions in intense magnetic fields is investigated. For a flux tube in a uniform atmosphere a dispersion relation is derived for the modes of vibration and analytic approximations are obtained for a slender tube. In a stratified atmosphere an expansion procedure is used to derive an equation for the vertical velocity perturbation. The behaviour of motions within the flux tube is shown to depend upon a transition frequency 𝜔[sub]v such that vertically propagating waves are possible only for frequencies greater than 𝜔[sub]v. Also, the nature of convective instability in a slender magnetic flux tube is explored. A sufficient condition for stability is derived for the case of an arbitrary temperature profile in the external medium. For a tube of infinite depth, with a uniform-temperature gradient inside the tube equal to that in the exterior, a necessary and sufficient condition for convective stability to occur inside the tube is derived. Under the assumptions of the model, intense flux tubes are convectively stable if sufficiently shallow (with depths 1 - 2 x 10³ km or less). Tubes that extend deeper into the convection zone are potentially (convectively) unstable, but may be stabilised for sufficiently strong magnetic fields. Radiative damping of waves is important in the upper photosphere and the effect of radiative relaxation on the propagation of waves in an intense flux tube is examined both for a uniform and stratified atmosphere. The cut-off frequency is generalized to include the effects of radiative relaxation. The phase-shift between velocity oscillations at two different levels and the phase difference between temperature and velocity perturbations are derived and compared with the available observations. Finally, the consequences of the observed steady downflow are discussed.
Tue, 01 Jan 1980 00:00:00 GMThttps://hdl.handle.net/10023/142771980-01-01T00:00:00ZWebb, Andrew RobertThe nature of motions in intense magnetic fields is investigated. For a flux tube in a uniform atmosphere a dispersion relation is derived for the modes of vibration and analytic approximations are obtained for a slender tube. In a stratified atmosphere an expansion procedure is used to derive an equation for the vertical velocity perturbation. The behaviour of motions within the flux tube is shown to depend upon a transition frequency 𝜔[sub]v such that vertically propagating waves are possible only for frequencies greater than 𝜔[sub]v. Also, the nature of convective instability in a slender magnetic flux tube is explored. A sufficient condition for stability is derived for the case of an arbitrary temperature profile in the external medium. For a tube of infinite depth, with a uniform-temperature gradient inside the tube equal to that in the exterior, a necessary and sufficient condition for convective stability to occur inside the tube is derived. Under the assumptions of the model, intense flux tubes are convectively stable if sufficiently shallow (with depths 1 - 2 x 10³ km or less). Tubes that extend deeper into the convection zone are potentially (convectively) unstable, but may be stabilised for sufficiently strong magnetic fields. Radiative damping of waves is important in the upper photosphere and the effect of radiative relaxation on the propagation of waves in an intense flux tube is examined both for a uniform and stratified atmosphere. The cut-off frequency is generalized to include the effects of radiative relaxation. The phase-shift between velocity oscillations at two different levels and the phase difference between temperature and velocity perturbations are derived and compared with the available observations. Finally, the consequences of the observed steady downflow are discussed.Nonlinear stability of flows over rigid and flexible boundaries
https://hdl.handle.net/10023/14273
This work assesses the importance of nonlinearity in the stability of flows over compliant and rigid walls, and comprises three main parts. The first part considers inviscid flow with a free surface over a flexible boundary. The dispersion relation is obtained, and the conditions for linear instability investigated. The linear dispersion relation is then used to show that the conditions for nonlinear three-wave resonance are often met. In some circumstances, the resonance may be of 'explosive' sort, involving waves of opposite energy sign; but non-explosive resonant configurations are most common. Next, the wave- amplitude evolution equations for three-wave resonance are derived, firstly by a 'direct' approach, and then via a variational (averaged Lagrangian) method. Results agree with those of Case & Chiu (1977) for capillary-gravity waves, and Craik & Adam (1979), for three-layer fluid flow, on taking the appropriate limits. We also consider a nonlinear model for the flexible boundary. In the second part, stability of Blasius flow over a compliant surface is studied. This extension of rigid-wall work of Craik (1971) and Hendriks (appendix to Usher & Craik 1975) determines the quadratic interaction coefficients of three-wave resonance, and complements the linear analysis of Carpenter & Garrad (1985, 1986) and others. First, the linear eigenvalue spectrum is investigated for various values of the wall parameters. Then, resonant triads are located and the quadratic interaction coefficients determined numerically. By way of introduction some rigid-wall results are also presented, extending those of Hendriks.
Mon, 01 Jan 1990 00:00:00 GMThttps://hdl.handle.net/10023/142731990-01-01T00:00:00ZThomas, Michael DThis work assesses the importance of nonlinearity in the stability of flows over compliant and rigid walls, and comprises three main parts. The first part considers inviscid flow with a free surface over a flexible boundary. The dispersion relation is obtained, and the conditions for linear instability investigated. The linear dispersion relation is then used to show that the conditions for nonlinear three-wave resonance are often met. In some circumstances, the resonance may be of 'explosive' sort, involving waves of opposite energy sign; but non-explosive resonant configurations are most common. Next, the wave- amplitude evolution equations for three-wave resonance are derived, firstly by a 'direct' approach, and then via a variational (averaged Lagrangian) method. Results agree with those of Case & Chiu (1977) for capillary-gravity waves, and Craik & Adam (1979), for three-layer fluid flow, on taking the appropriate limits. We also consider a nonlinear model for the flexible boundary. In the second part, stability of Blasius flow over a compliant surface is studied. This extension of rigid-wall work of Craik (1971) and Hendriks (appendix to Usher & Craik 1975) determines the quadratic interaction coefficients of three-wave resonance, and complements the linear analysis of Carpenter & Garrad (1985, 1986) and others. First, the linear eigenvalue spectrum is investigated for various values of the wall parameters. Then, resonant triads are located and the quadratic interaction coefficients determined numerically. By way of introduction some rigid-wall results are also presented, extending those of Hendriks.Time dependent heating of the solar corona
https://hdl.handle.net/10023/14267
The problem of how the Sun's corona is heated is of central importance in Solar Physics research. In this thesis, a model is constructed of a typical coronal magnetic loop in order to investigate the response of coronal plasma to a time-dependent heating source. It is not the aim of the research to study in detail a particular heating mechanism but rather to understand the important features arising from time-dependent heating in general. A time-varying energy input into the coronal loop is required because it is likely that none of the suggested theoretical heating methods can provide a constant supply of heat to the corona. The magnetic field is taken to be strong enough that the loop dynamics reduce to a one-dimensional problem along the field. In addition, it is assumed that the radiative timescale in the corona is much longer than the sound travel time and thus, the plasma evolves isobarically. The thermal equilibria profiles along the coronal loop are then investigated for a simplified form of the optically thin radiation. Initially, a heating function that displays a regular, sinusoidal variation in time is introduced and it is found that there is a critical heating frequency above which a hot coronal loop solution can be maintained and below which the plasma temperature cools to chromospheric values. Pulse heating and the deposition of random-sized energy quanta in a loop are also investigated. An evaluation of the isobaric assumption to the corona is presented by allowing sound waves to propagate back and forth along the loop. It is found that the system can exhibit isobaric-like behaviour provided the acoustic timescale is short enough. Possible extensions of the developed loop model are discussed as well as the implications of time-dependent heating upon observations from the SOHO satellite.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/142671996-01-01T00:00:00ZWalsh, Robert WilliamThe problem of how the Sun's corona is heated is of central importance in Solar Physics research. In this thesis, a model is constructed of a typical coronal magnetic loop in order to investigate the response of coronal plasma to a time-dependent heating source. It is not the aim of the research to study in detail a particular heating mechanism but rather to understand the important features arising from time-dependent heating in general. A time-varying energy input into the coronal loop is required because it is likely that none of the suggested theoretical heating methods can provide a constant supply of heat to the corona. The magnetic field is taken to be strong enough that the loop dynamics reduce to a one-dimensional problem along the field. In addition, it is assumed that the radiative timescale in the corona is much longer than the sound travel time and thus, the plasma evolves isobarically. The thermal equilibria profiles along the coronal loop are then investigated for a simplified form of the optically thin radiation. Initially, a heating function that displays a regular, sinusoidal variation in time is introduced and it is found that there is a critical heating frequency above which a hot coronal loop solution can be maintained and below which the plasma temperature cools to chromospheric values. Pulse heating and the deposition of random-sized energy quanta in a loop are also investigated. An evaluation of the isobaric assumption to the corona is presented by allowing sound waves to propagate back and forth along the loop. It is found that the system can exhibit isobaric-like behaviour provided the acoustic timescale is short enough. Possible extensions of the developed loop model are discussed as well as the implications of time-dependent heating upon observations from the SOHO satellite.Time-dependent MHD wave coupling in non-uniform media
https://hdl.handle.net/10023/14264
This thesis studies the time dependent evolution of MHD waves in cold, fully compressible non-uniform plasmas. We used a 1-D box model (e.g., Southwood (1974)) to study wave mode coupling, and concentrate upon developing an understanding of the underlying physics that governs waves in the Earth's magnetosphere. We begin by discussing the form of the (often singular) governing eigenmodes of the system, and subsequently use these eigenmodes as a basis with which to construct the solution to a variety of initial value problems. We consider a detailed analysis of both the widths and the internal length scales developed by cavity mode driven held line resonances (FLRs), and compare our results to observations presented in the literature. We find that (especially asymptotically in time) the coupled waves derive their dominant characteristics from the form of undriven decoupled toroidal Alfvén eigenmodes. Ideal numerical solutions show that fine spatial scales are developed across the background magnetic field, and we demonstrate that this is accurately estimated as the decoupled phase mixing length
L[sub]p[sub]h = 2π/𝜔ⁱ[sub]A = d 𝜔[sub]A/dx
We also discuss the likely ionospheric and kinetic modifications to our theory. Later, we consider the evolution of poloidal Alfvén waves having large azimuthal wavenumber (𝜆). We find that the 𝜆 → ∞ decoupled poloidal Alfvén wave evaluation (Dungey, 1967) is modified for finite 𝜆 lambda, approaching decoupled toroidal field line oscillations for large t. We define a poloidal lifetime 𝛵, when toroidal and poloidal displacements become equal, and demonstrate that this is when the phase mixing length is equal to 2pi/lambda. We examine numerically the poloidal Alfvén wave evolution for 𝜆 ≫ k[sub]z, and k[sub]≳ lambda, when k[sub]x(x,t = 0) ≪ lambda or k[sub]z. We interpret the lambda ≪ kz results (applicable to the Earth's magnetosphere) in the context of poloidal Alfvén wave observations, and compare our study to the numerical analysis of Ding et al. (1995). We conclude the thesis by undertaking an asymptotic derivation of the large 𝜆 solutions by using the method of multiple time scales. We find our analytic solutions are in excellent agreement with those determined numerically. A central result of the thesis is the importance and dominance of the phase mixing length for time dependent solutions, irrespective of the value of 𝜆.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/142641996-01-01T00:00:00ZMann, Ian R.This thesis studies the time dependent evolution of MHD waves in cold, fully compressible non-uniform plasmas. We used a 1-D box model (e.g., Southwood (1974)) to study wave mode coupling, and concentrate upon developing an understanding of the underlying physics that governs waves in the Earth's magnetosphere. We begin by discussing the form of the (often singular) governing eigenmodes of the system, and subsequently use these eigenmodes as a basis with which to construct the solution to a variety of initial value problems. We consider a detailed analysis of both the widths and the internal length scales developed by cavity mode driven held line resonances (FLRs), and compare our results to observations presented in the literature. We find that (especially asymptotically in time) the coupled waves derive their dominant characteristics from the form of undriven decoupled toroidal Alfvén eigenmodes. Ideal numerical solutions show that fine spatial scales are developed across the background magnetic field, and we demonstrate that this is accurately estimated as the decoupled phase mixing length
L[sub]p[sub]h = 2π/𝜔ⁱ[sub]A = d 𝜔[sub]A/dx
We also discuss the likely ionospheric and kinetic modifications to our theory. Later, we consider the evolution of poloidal Alfvén waves having large azimuthal wavenumber (𝜆). We find that the 𝜆 → ∞ decoupled poloidal Alfvén wave evaluation (Dungey, 1967) is modified for finite 𝜆 lambda, approaching decoupled toroidal field line oscillations for large t. We define a poloidal lifetime 𝛵, when toroidal and poloidal displacements become equal, and demonstrate that this is when the phase mixing length is equal to 2pi/lambda. We examine numerically the poloidal Alfvén wave evolution for 𝜆 ≫ k[sub]z, and k[sub]≳ lambda, when k[sub]x(x,t = 0) ≪ lambda or k[sub]z. We interpret the lambda ≪ kz results (applicable to the Earth's magnetosphere) in the context of poloidal Alfvén wave observations, and compare our study to the numerical analysis of Ding et al. (1995). We conclude the thesis by undertaking an asymptotic derivation of the large 𝜆 solutions by using the method of multiple time scales. We find our analytic solutions are in excellent agreement with those determined numerically. A central result of the thesis is the importance and dominance of the phase mixing length for time dependent solutions, irrespective of the value of 𝜆.Magnetic neutral points and nonuniform reconnection
https://hdl.handle.net/10023/14250
Ever since the first recorded observation of a solar flare in September 1859, it has been a key question - for physics as a whole and for astrophsics in particular - to ask what mechanism lies behind the sudden, violent release of energy from the sun. It has become increasingly apparent that the complex structure of the solar magnetic field lies at the heart of the answer. The process of magnetic reconnection has, over the years, become the accepted explanation by which magnetic energy can be released on both large and small scales in astrophysical and laboratory plasmas. The results of reconnection can be seen, for instance, in star formation, solar flares and the earth's aurorae; indeed the 1859 flare was followed by exceptional auroral activity. The mechanism of magnetic reconnection was first postulated by Giovanelli (1947) as a way of releasing the magnetic energy stored in the Sun. He, and later Dungey (1953), realised that the behaviour of the plasma in the vicinity of a magnetic neutral or null point, where the field disappears, is quite different from other regions of space. In this thesis the nature of magnetic neutral points and their role in the process of reconnection is investigated. Firstly, a general classification of magnetic neutral points is presented. The chapter includes equilibrium and steady-state solutions for two-dimensional magnetic neutral points. The differences in the field behaviour close to each type of neutral point are explained and criteria for the existence of steady-state solutions and equilibria involving pressure balance are presented. In the last section, a self-similar solution for a collapsed X-point is explored. The X-point necessarily becomes cusp-like in nature if shearing is applied in the ignorable direction. Two reconnection models are considered. The first is an extension of the Priest-Lee model (1990). It incorporates large pressure gradients in the inflow corresponding to the Forbes-Priest Almost-Uniform Model. The investigation includes both analytical and numerical solutions and a study of the separatrix jet. In the numerical study, current spikes are found at the end of the current sheets and a much increased reconnection rate is found analytically in the extreme flux file-up limit. The second reconnection model presented is also based on the Priest-Lee configuration. A uniform field is imposed on the basic structure producing a cusp-point with a non-zero field strength as the neutral point is approached from above. This results in the removal of the singularity in the flow above the separatrix. A non-singular solution is found analytically for a double-cusp. A much larger reconnection rate is found and a numerical solution is presented.
Sat, 01 Jan 1994 00:00:00 GMThttps://hdl.handle.net/10023/142501994-01-01T00:00:00ZStrachan, N. R.Ever since the first recorded observation of a solar flare in September 1859, it has been a key question - for physics as a whole and for astrophsics in particular - to ask what mechanism lies behind the sudden, violent release of energy from the sun. It has become increasingly apparent that the complex structure of the solar magnetic field lies at the heart of the answer. The process of magnetic reconnection has, over the years, become the accepted explanation by which magnetic energy can be released on both large and small scales in astrophysical and laboratory plasmas. The results of reconnection can be seen, for instance, in star formation, solar flares and the earth's aurorae; indeed the 1859 flare was followed by exceptional auroral activity. The mechanism of magnetic reconnection was first postulated by Giovanelli (1947) as a way of releasing the magnetic energy stored in the Sun. He, and later Dungey (1953), realised that the behaviour of the plasma in the vicinity of a magnetic neutral or null point, where the field disappears, is quite different from other regions of space. In this thesis the nature of magnetic neutral points and their role in the process of reconnection is investigated. Firstly, a general classification of magnetic neutral points is presented. The chapter includes equilibrium and steady-state solutions for two-dimensional magnetic neutral points. The differences in the field behaviour close to each type of neutral point are explained and criteria for the existence of steady-state solutions and equilibria involving pressure balance are presented. In the last section, a self-similar solution for a collapsed X-point is explored. The X-point necessarily becomes cusp-like in nature if shearing is applied in the ignorable direction. Two reconnection models are considered. The first is an extension of the Priest-Lee model (1990). It incorporates large pressure gradients in the inflow corresponding to the Forbes-Priest Almost-Uniform Model. The investigation includes both analytical and numerical solutions and a study of the separatrix jet. In the numerical study, current spikes are found at the end of the current sheets and a much increased reconnection rate is found analytically in the extreme flux file-up limit. The second reconnection model presented is also based on the Priest-Lee configuration. A uniform field is imposed on the basic structure producing a cusp-point with a non-zero field strength as the neutral point is approached from above. This results in the removal of the singularity in the flow above the separatrix. A non-singular solution is found analytically for a double-cusp. A much larger reconnection rate is found and a numerical solution is presented.Aspects of the MHD stability of coronal and laboratory plasmas
https://hdl.handle.net/10023/14248
The magnetohydrodynamic (MHD) model is a simple mathematical model that treats a plasma as a perfectly conducting fluid acted upon by magnetic and pressure-driven forces. Many instabilities in plasmas can be predicted using this model. In this Thesis, aspects of the linear stability of solar and laboratory plasmas are studied using the MHD model. Firstly, we investigate the thermal instability of coronal plasmas with line-tied magnetic fields and with anisotropical heat conduction, using an analytical analysis which concentrates on isobaric perturbations, and a time-dependent numerical code. We find that including perpendicular thermal conduction means that condensations are restricted to a narrow layer around the region where the local isobaric growth rate is largest and that, while the growth rate of the thermal mode is largely unaffected by perpendicular thermal conduction, this may be an important factor in determining the lengthscale for the width of condensations. Secondly, the effect of a finitely conducting wall on the linear stability of Spheromak and Reversed Field Finch equilibria is investigated. We find growth rates for the modes that are present because of the finite resistivity of the wall, which grow proportionally to the "long" time constant of the wall. Finally, we apply a tractable method, derived by De Bruyne (1990), for investigating the stability of 2-D line-tied magnetic fields, to cylindrically symmetric spheromak equilibria. The method involves the solution of two sets of ordinary differential equations, integrated along the field lines, which give necessary and sufficient conditions for stability. The role of plasma pressure and of the width of the entrance region are investigated.
Fri, 01 Jan 1993 00:00:00 GMThttps://hdl.handle.net/10023/142481993-01-01T00:00:00ZClifford, Leo J.The magnetohydrodynamic (MHD) model is a simple mathematical model that treats a plasma as a perfectly conducting fluid acted upon by magnetic and pressure-driven forces. Many instabilities in plasmas can be predicted using this model. In this Thesis, aspects of the linear stability of solar and laboratory plasmas are studied using the MHD model. Firstly, we investigate the thermal instability of coronal plasmas with line-tied magnetic fields and with anisotropical heat conduction, using an analytical analysis which concentrates on isobaric perturbations, and a time-dependent numerical code. We find that including perpendicular thermal conduction means that condensations are restricted to a narrow layer around the region where the local isobaric growth rate is largest and that, while the growth rate of the thermal mode is largely unaffected by perpendicular thermal conduction, this may be an important factor in determining the lengthscale for the width of condensations. Secondly, the effect of a finitely conducting wall on the linear stability of Spheromak and Reversed Field Finch equilibria is investigated. We find growth rates for the modes that are present because of the finite resistivity of the wall, which grow proportionally to the "long" time constant of the wall. Finally, we apply a tractable method, derived by De Bruyne (1990), for investigating the stability of 2-D line-tied magnetic fields, to cylindrically symmetric spheromak equilibria. The method involves the solution of two sets of ordinary differential equations, integrated along the field lines, which give necessary and sufficient conditions for stability. The role of plasma pressure and of the width of the entrance region are investigated.Ducted magnetoacoustic waves in the solar corona
https://hdl.handle.net/10023/14246
This thesis investigates the ducting of magnetoacoustic waves in coronal structures. The propagation of waves in current sheets and coronal loops has been examined in order to understand wave ducting in structured plasmas, and to provide an explanation of the observed oscillatory behaviour in the solar corona. Firstly a comprehensive review of the observations of loops and oscillations in the corona is given. An investigation into how the curvature of the loop alters the ducting of magnetoacoustic waves is then presented by studying the effect of the length, width and the density enhancement of the loop and also the frequency of oscillation. The effect of the curvature is to generate wave leakage from the loop. The guiding of magnetoacoustic waves by a current sheet is also considered. An investigation into the type of modes which may propagate and the time scales of oscillation is performed. Impulsively generated waves exhibit similar temporal signatures to observations of X-ray and radio emission. Periods of oscillation for all the ducted wave models are in good agreement with reported observations. The effect of a random boundary motion on a magnetospheric cavity is examined through numerical simulations. A broadband driving spectrum excites the quasi-monochromatic fast modes whose frequencies lie within the driving spectrum. These fast modes couple to an Alfvén mode if the frequency lies within the Alfvén continuum. The position of the resonant field lines and the Alfvén mode eigenfunction may be accurately calculated by assuming a periodic boundary motion. To conclude the work in this thesis the three-dimensional magnetic topologies surrounding neutral points are studied. The local linear magnetic structure about the null is found to depend only on a 3 X 3 matrix containing four parameters. The type of topology is dependent upon the nature of the eigenvalues and eigenvectors of this matrix.
Wed, 01 Jan 1997 00:00:00 GMThttps://hdl.handle.net/10023/142461997-01-01T00:00:00ZSmith, Jason M.This thesis investigates the ducting of magnetoacoustic waves in coronal structures. The propagation of waves in current sheets and coronal loops has been examined in order to understand wave ducting in structured plasmas, and to provide an explanation of the observed oscillatory behaviour in the solar corona. Firstly a comprehensive review of the observations of loops and oscillations in the corona is given. An investigation into how the curvature of the loop alters the ducting of magnetoacoustic waves is then presented by studying the effect of the length, width and the density enhancement of the loop and also the frequency of oscillation. The effect of the curvature is to generate wave leakage from the loop. The guiding of magnetoacoustic waves by a current sheet is also considered. An investigation into the type of modes which may propagate and the time scales of oscillation is performed. Impulsively generated waves exhibit similar temporal signatures to observations of X-ray and radio emission. Periods of oscillation for all the ducted wave models are in good agreement with reported observations. The effect of a random boundary motion on a magnetospheric cavity is examined through numerical simulations. A broadband driving spectrum excites the quasi-monochromatic fast modes whose frequencies lie within the driving spectrum. These fast modes couple to an Alfvén mode if the frequency lies within the Alfvén continuum. The position of the resonant field lines and the Alfvén mode eigenfunction may be accurately calculated by assuming a periodic boundary motion. To conclude the work in this thesis the three-dimensional magnetic topologies surrounding neutral points are studied. The local linear magnetic structure about the null is found to depend only on a 3 X 3 matrix containing four parameters. The type of topology is dependent upon the nature of the eigenvalues and eigenvectors of this matrix.Inverse polarity prominence equilibria
https://hdl.handle.net/10023/14243
It has been supposed since the middle of this century that it is the global magnetic field surrounding a quiescent prominence that provides the force to prevent its collapse due to the sun’s gravitational field. Many theoretical models, assuming that the prominence plasma is supported in a dip in the magnetic field lines associated by the magnetic tension force, have since been put forward. The aim of this thesis is to propose further models of quiescent prominences to widen our understanding and knowledge of these remarkable features.
A short overview over the magnetohydrodynamic equations used to describe solar prominences, or most of the solar phenomena for that matter, are discussed in chapter 2, and a short summary of prominence observations and attempts to model them is given in chapter 3.
A brief description of the numerical code used in chapters 5 and 7 is given in chapter 4.
Observations of Kim (1990) and Leroy (1985) have found that most large quiescent prominences are of inverse polarity type for which the magnetic field passes through the prominence in the opposite direction to that expected from the photospheric magnetic field. Many theoretical models have been proposed, but failed. Hence, in chapter 5 we investigate first – without the inclusion of a prominence sheet – when an inverse polarity magnetic field must have the correct topology for an inverse polarity configuration before the formation of the prominence itself. Only very recently, the first basic successful model of an I-type polarity prominence was proposed by Low (1993). In chapter 6 we examine this model and investigate current sheets more complicated and realistic than the one used by Low. These analytical models deal with the force-free solution, which is matched onto an external, unsheared, potential coronal magnetic field. These solutions are mathematically interesting and allow an investigation of different profiles of the current intensity of the magnetic field vector and of the mass density in the sheet. The prominence properties predicted by these models have been examined and have been found to match the observational values. The mathematics of current sheets in general is also briefly discussed.
Chapter 7 deals with numerical solutions of inverse polarity prominences embedded in a force-free magnetic flux tube, matched onto an unsheared potential coronal field. Unfortunately the solutions gained are quite sensitive to the boundary conditions imposed on them through the numerical box, showing a loss of convergence and a tendency for the solution to blow up.
Finally, a short summary as well as possible future work is given in chapter 8.
Sun, 01 Jan 1995 00:00:00 GMThttps://hdl.handle.net/10023/142431995-01-01T00:00:00ZSchönfelder, Apollonia Maria OktaviaIt has been supposed since the middle of this century that it is the global magnetic field surrounding a quiescent prominence that provides the force to prevent its collapse due to the sun’s gravitational field. Many theoretical models, assuming that the prominence plasma is supported in a dip in the magnetic field lines associated by the magnetic tension force, have since been put forward. The aim of this thesis is to propose further models of quiescent prominences to widen our understanding and knowledge of these remarkable features.
A short overview over the magnetohydrodynamic equations used to describe solar prominences, or most of the solar phenomena for that matter, are discussed in chapter 2, and a short summary of prominence observations and attempts to model them is given in chapter 3.
A brief description of the numerical code used in chapters 5 and 7 is given in chapter 4.
Observations of Kim (1990) and Leroy (1985) have found that most large quiescent prominences are of inverse polarity type for which the magnetic field passes through the prominence in the opposite direction to that expected from the photospheric magnetic field. Many theoretical models have been proposed, but failed. Hence, in chapter 5 we investigate first – without the inclusion of a prominence sheet – when an inverse polarity magnetic field must have the correct topology for an inverse polarity configuration before the formation of the prominence itself. Only very recently, the first basic successful model of an I-type polarity prominence was proposed by Low (1993). In chapter 6 we examine this model and investigate current sheets more complicated and realistic than the one used by Low. These analytical models deal with the force-free solution, which is matched onto an external, unsheared, potential coronal magnetic field. These solutions are mathematically interesting and allow an investigation of different profiles of the current intensity of the magnetic field vector and of the mass density in the sheet. The prominence properties predicted by these models have been examined and have been found to match the observational values. The mathematics of current sheets in general is also briefly discussed.
Chapter 7 deals with numerical solutions of inverse polarity prominences embedded in a force-free magnetic flux tube, matched onto an unsheared potential coronal field. Unfortunately the solutions gained are quite sensitive to the boundary conditions imposed on them through the numerical box, showing a loss of convergence and a tendency for the solution to blow up.
Finally, a short summary as well as possible future work is given in chapter 8.The magnetohydrostatic equilibrium of quiescent solar prominences
https://hdl.handle.net/10023/14239
Since the mid 1900's it has been supposed that the global magnetic field surrounding a quiescent prominence provides the force required to prevent its collapse under the influence of the Sun's gravitational field. Many theoretical models of this magnetic field have been produced in which it is assumed that the prominence plasma is supported in a dip in the field lines by the associated magnetic tension force. It is the aim of this thesis to propose further models of the magnetic field in order to extend our knowledge and understanding of prominences. In doing so we present three distinct models. The first is an extension of the twisted flux tube model for prominences proposed by Priest et al. (1989). Here we present analytical solutions to the magnetohydrostatic equilibrium equation within the tube using the so- called generating function method in which we select two distinct functional forms of the longitudinal field component. Unlike the solutions found by Priest et al., we allow for large deviations of the field from cylindrical symmetry. The prominence is represented by a finite vertical sheet of mass and current and we show that it is possible for such a sheet to be in static equilibrium everywhere along its vertical extent. Next we consider the model of van Ballegooijen and Martens in which photospheric motions drive a reconnection process leading to the formation of a helical magnetic structure capable of supporting dense prominence plasma in the low points of the helical windings. Under the assumption of cylindrical symmetry we analyse two methods of solving the magnetohydrostatic equilibrium equation in which the positions of the field line footpoints at the photosphere are imposed. Using a combination of analytical and numerical techniques, we study the quasi-static evolution of the model as the height of the helical axis increases. Unlike the numerical analysis of van Ballegooijen and Martens we are able to produce inverse polarity configurations without the problem of singular field components at the helical axis. Lastly we present an analysis of the interaction of a finite, vertical sheet of mass and current (representing a prominence) with an external constant-current force-free field. We formalise two distinct boundary-value problems in which the distribution of the normal magnetic field component along the photosphere is imposed along with the distribution of either the normal magnetic field component across the prominence or the prominence surface current. In both cases we demonstrate for particular boundary conditions that it is possible for equilibrium solutions to exist of both normal and inverse polarity in which dense material is supported everywhere along the prominence sheet. In particular we are, for the first time, able to produce an inverse polarity equilibrium configuration in which the field components are locally bounded and closed field lines exist above the prominence sheet while an X-type neutral point lies below it.
Wed, 01 Jan 1992 00:00:00 GMThttps://hdl.handle.net/10023/142391992-01-01T00:00:00ZRidgway, ChristopherSince the mid 1900's it has been supposed that the global magnetic field surrounding a quiescent prominence provides the force required to prevent its collapse under the influence of the Sun's gravitational field. Many theoretical models of this magnetic field have been produced in which it is assumed that the prominence plasma is supported in a dip in the field lines by the associated magnetic tension force. It is the aim of this thesis to propose further models of the magnetic field in order to extend our knowledge and understanding of prominences. In doing so we present three distinct models. The first is an extension of the twisted flux tube model for prominences proposed by Priest et al. (1989). Here we present analytical solutions to the magnetohydrostatic equilibrium equation within the tube using the so- called generating function method in which we select two distinct functional forms of the longitudinal field component. Unlike the solutions found by Priest et al., we allow for large deviations of the field from cylindrical symmetry. The prominence is represented by a finite vertical sheet of mass and current and we show that it is possible for such a sheet to be in static equilibrium everywhere along its vertical extent. Next we consider the model of van Ballegooijen and Martens in which photospheric motions drive a reconnection process leading to the formation of a helical magnetic structure capable of supporting dense prominence plasma in the low points of the helical windings. Under the assumption of cylindrical symmetry we analyse two methods of solving the magnetohydrostatic equilibrium equation in which the positions of the field line footpoints at the photosphere are imposed. Using a combination of analytical and numerical techniques, we study the quasi-static evolution of the model as the height of the helical axis increases. Unlike the numerical analysis of van Ballegooijen and Martens we are able to produce inverse polarity configurations without the problem of singular field components at the helical axis. Lastly we present an analysis of the interaction of a finite, vertical sheet of mass and current (representing a prominence) with an external constant-current force-free field. We formalise two distinct boundary-value problems in which the distribution of the normal magnetic field component along the photosphere is imposed along with the distribution of either the normal magnetic field component across the prominence or the prominence surface current. In both cases we demonstrate for particular boundary conditions that it is possible for equilibrium solutions to exist of both normal and inverse polarity in which dense material is supported everywhere along the prominence sheet. In particular we are, for the first time, able to produce an inverse polarity equilibrium configuration in which the field components are locally bounded and closed field lines exist above the prominence sheet while an X-type neutral point lies below it.Wave-particle dynamics in a hot inhomogenous fusion plasma
https://hdl.handle.net/10023/14235
An outstanding problem in the field of nuclear fusion research is the precise mechanism by which a hot, magnetically inhomogeneous plasma is heated when illuminated by a constant beam of small amplitude radio waves matched in frequency to harmonics of the ion Larmor frequency. An accurate model must include microscopic dynamics and inevitably a kinetic theory is required. Highly energetic ions (> 1MeV) born from fusion reactions or powered by gyroresonance have large Larmor radii (> 10cm) which are comparable in size to the wavelength of the incident radiation. In particular we will focus on fast magnetosonic waves. Exact full wave equations describing a thermal plasma in a weakly inhomogeneous field are presently at least fourth order integro-differential equations (Sauter, 1992). These are computationally taxing. Recently a method was proposed to reduce the problem to a second order integro-differential equation at the expense of information related to the propagation of mode-converted waves (Holt, 1992). We present here a generalisation of the theory to allow for arbitrary velocity-dependent equilibria while at the same time retaining a general functional form for the field profile. We consider the specific case of a bi-Maxwellian plasma immersed in a linearly inhomogenous magnetic field. We find that thermal anisotropy produces resonance localisation when the perpendicular ion temperature is greater than that parallel to the ambient field. A study of the symmetry properties of the conductivity tensor reveals that the Onsager reciprocal relations are obeyed only for an isotropic plasma in an inhomogeneous field. This is a generalisation of the result obtained by Nambu (1995). We present a generalisation of the reduction method to include effects due to changes in wave amplitude. We find that we are able to include the odd-order field derivatives responsible for energy conservation. Our numerical study of fundamental Helium-3 gyroresonance in a majority Deuterium plasma reveals that we have > 99.9% energy conservation in all cases. We show that locally-uniform theory can be very inaccurate (≃ 70% in one case presented in our recent paper, Cairns et al., 1995) particularly for higher energy ions whose non-locality is more extreme. We present a representative sample of results for minority heating and mode conversion heating schemes. We report the appearance of an unexpected cut-off on the low field side of the minority gyroresonance which may have important consequences for antennae presently placed on the outside of Tokamaks.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/142351996-01-01T00:00:00ZTaylor, Michael AnthonyAn outstanding problem in the field of nuclear fusion research is the precise mechanism by which a hot, magnetically inhomogeneous plasma is heated when illuminated by a constant beam of small amplitude radio waves matched in frequency to harmonics of the ion Larmor frequency. An accurate model must include microscopic dynamics and inevitably a kinetic theory is required. Highly energetic ions (> 1MeV) born from fusion reactions or powered by gyroresonance have large Larmor radii (> 10cm) which are comparable in size to the wavelength of the incident radiation. In particular we will focus on fast magnetosonic waves. Exact full wave equations describing a thermal plasma in a weakly inhomogeneous field are presently at least fourth order integro-differential equations (Sauter, 1992). These are computationally taxing. Recently a method was proposed to reduce the problem to a second order integro-differential equation at the expense of information related to the propagation of mode-converted waves (Holt, 1992). We present here a generalisation of the theory to allow for arbitrary velocity-dependent equilibria while at the same time retaining a general functional form for the field profile. We consider the specific case of a bi-Maxwellian plasma immersed in a linearly inhomogenous magnetic field. We find that thermal anisotropy produces resonance localisation when the perpendicular ion temperature is greater than that parallel to the ambient field. A study of the symmetry properties of the conductivity tensor reveals that the Onsager reciprocal relations are obeyed only for an isotropic plasma in an inhomogeneous field. This is a generalisation of the result obtained by Nambu (1995). We present a generalisation of the reduction method to include effects due to changes in wave amplitude. We find that we are able to include the odd-order field derivatives responsible for energy conservation. Our numerical study of fundamental Helium-3 gyroresonance in a majority Deuterium plasma reveals that we have > 99.9% energy conservation in all cases. We show that locally-uniform theory can be very inaccurate (≃ 70% in one case presented in our recent paper, Cairns et al., 1995) particularly for higher energy ions whose non-locality is more extreme. We present a representative sample of results for minority heating and mode conversion heating schemes. We report the appearance of an unexpected cut-off on the low field side of the minority gyroresonance which may have important consequences for antennae presently placed on the outside of Tokamaks.Models of X-ray bright points and concelling magnetic features
https://hdl.handle.net/10023/14232
Small brightenings called x-ray bright points (Golub et al, 1974) occur in the solar corona. They are observed with the soft x-ray telescope on Skylab to be approximately 22 Mm in diameter with a brighter inner core of width 4-7 Mm although with the Normal Incidence X-ray Telescope their dimensions are observed to be typically 6 Mm x 9 Mm. By comparison with magnetograms of the photosphere it has been noticed recently that there is a high correlation between the occurrence of x-ray bright points and the mutual reduction of flux between two opposite polarity magnetic fragments. These fragments are originally unconnected magnetically, but move towards each other and simultaneously lose equal amounts of flux (cancel): they are called cancelling magnetic features (Martin et al, 1984). The observations relating to these features were reviewed by Priest et al. (1994) who suggested that they naturally evolve through three phases: the pre-interaction, interaction and cancellation phases. From this evidence qualitative pictures of the magnetic field structure for an x-ray bright point and associated cancelling magnetic feature were established. The aim of this thesis has been to build on the ideas of Priest et al. (1994) to produce a detailed theoretical model of an x-ray bright point and a cancelling magnetic feature. The magnetic field structures are estimated, and the position and lifetime of the bright point are calculated, as is the total amount of energy released during the bright point. This work is also extended to study more complex cancelling configurations representing the main basic types of cancelling magnetic feature. The results of these models determine the factors that affect the lifetime and position of a bright point and indicate which types of cancelling magnetic features are most likely to produce bright points that are long-lived, lie directly above the cancellation site and occur simultaneously with the cancellation phase. The complex structure of a bright point cannot be explained from the above two-dimensional models: thus two recently observed bright points were studied to see if the above model could be extended into three dimensions to explain the structure seen in soft x-ray images. The available observational data was used and leads to reasonable explanations for the complex shapes of both bright points. Finally, a more realistic model for the overlying field was set up involving a model of the field above a supergranule cell field with fragments of finite width. The interaction of an ephemeral region within this field was then studied and led to five different scenarios. The results obtained reaffirmed those found in the previous simpler models and suggest where bright points may appear in a cell relative to the cancelling magnetic feature and for how long the bright points might last. Predictions for the lifetimes of cancelling magnetic features are also made, indicating when the cancelling magnetic feature occurs relative to the bright point.
Sun, 01 Jan 1995 00:00:00 GMThttps://hdl.handle.net/10023/142321995-01-01T00:00:00ZParnell, Clare ElizabethSmall brightenings called x-ray bright points (Golub et al, 1974) occur in the solar corona. They are observed with the soft x-ray telescope on Skylab to be approximately 22 Mm in diameter with a brighter inner core of width 4-7 Mm although with the Normal Incidence X-ray Telescope their dimensions are observed to be typically 6 Mm x 9 Mm. By comparison with magnetograms of the photosphere it has been noticed recently that there is a high correlation between the occurrence of x-ray bright points and the mutual reduction of flux between two opposite polarity magnetic fragments. These fragments are originally unconnected magnetically, but move towards each other and simultaneously lose equal amounts of flux (cancel): they are called cancelling magnetic features (Martin et al, 1984). The observations relating to these features were reviewed by Priest et al. (1994) who suggested that they naturally evolve through three phases: the pre-interaction, interaction and cancellation phases. From this evidence qualitative pictures of the magnetic field structure for an x-ray bright point and associated cancelling magnetic feature were established. The aim of this thesis has been to build on the ideas of Priest et al. (1994) to produce a detailed theoretical model of an x-ray bright point and a cancelling magnetic feature. The magnetic field structures are estimated, and the position and lifetime of the bright point are calculated, as is the total amount of energy released during the bright point. This work is also extended to study more complex cancelling configurations representing the main basic types of cancelling magnetic feature. The results of these models determine the factors that affect the lifetime and position of a bright point and indicate which types of cancelling magnetic features are most likely to produce bright points that are long-lived, lie directly above the cancellation site and occur simultaneously with the cancellation phase. The complex structure of a bright point cannot be explained from the above two-dimensional models: thus two recently observed bright points were studied to see if the above model could be extended into three dimensions to explain the structure seen in soft x-ray images. The available observational data was used and leads to reasonable explanations for the complex shapes of both bright points. Finally, a more realistic model for the overlying field was set up involving a model of the field above a supergranule cell field with fragments of finite width. The interaction of an ephemeral region within this field was then studied and led to five different scenarios. The results obtained reaffirmed those found in the previous simpler models and suggest where bright points may appear in a cell relative to the cancelling magnetic feature and for how long the bright points might last. Predictions for the lifetimes of cancelling magnetic features are also made, indicating when the cancelling magnetic feature occurs relative to the bright point.Aspects of MHD wave propagation in solar atmospheric studies
https://hdl.handle.net/10023/14227
The theme of this thesis is ideal linear MHD wave propagation in structured media, using models relevant to structures in the solar atmosphere. We derive dispersion relations governing the ideal linear MHD modes for stationary states which are discretely structured in velocity and other plasma properties, in a direction transverse to the magnetic field, with field-aligned steady flow; the discrete structures considered are the single interface, uniform slab and uniform cylinder. This represents an extension of earlier models for the static case (Edwin 1984), by the inclusion of structured flows. The basic effects of flow are described, drawing on a discussion of the dispersion relations. The dispersion relations for the case of incompressible surface modes are examined in detail. We identify the qualitative effects of flow, including the onset of instability, by tracing the evolution of stable solutions and their propagation windows, as the relative flow is increased. Our analysis is presented in terms of a general formulation applicable to all three geometries (interface, slab and cylinder), revealing the combined role of dispersion and the ratio of densities in the two media. We go on to consider the relevance of the incompressible approximation to compressible surface modes, with particular reference to the static case of a single interface one side of which is field-free. We present and investigate analytical solutions for several special cases. The properties of the solutions obtained are compared with those for the equivalent incompressible case. Finally, we turn to the topic of global oscillations of quiescent prominences. A uniform slab model (Joarder 1993) yields, under conditions appropriate to the prominence-coronal inhomogeneity with the magnetic field threading the prominence being line-tied in the photosphere, modes which are analogous to the oscillations of a uniform string loaded with a point mass, and a formula approximating the period is given. We investigate the robustness of this formula for various plasma density profiles, assessing the applicability of the results from the uniform slab calculation to more realistic density profiles of the prominence-coronal inhomogeneity.
Thu, 01 Jan 1998 00:00:00 GMThttps://hdl.handle.net/10023/142271998-01-01T00:00:00ZMundie, Cheryl AnnThe theme of this thesis is ideal linear MHD wave propagation in structured media, using models relevant to structures in the solar atmosphere. We derive dispersion relations governing the ideal linear MHD modes for stationary states which are discretely structured in velocity and other plasma properties, in a direction transverse to the magnetic field, with field-aligned steady flow; the discrete structures considered are the single interface, uniform slab and uniform cylinder. This represents an extension of earlier models for the static case (Edwin 1984), by the inclusion of structured flows. The basic effects of flow are described, drawing on a discussion of the dispersion relations. The dispersion relations for the case of incompressible surface modes are examined in detail. We identify the qualitative effects of flow, including the onset of instability, by tracing the evolution of stable solutions and their propagation windows, as the relative flow is increased. Our analysis is presented in terms of a general formulation applicable to all three geometries (interface, slab and cylinder), revealing the combined role of dispersion and the ratio of densities in the two media. We go on to consider the relevance of the incompressible approximation to compressible surface modes, with particular reference to the static case of a single interface one side of which is field-free. We present and investigate analytical solutions for several special cases. The properties of the solutions obtained are compared with those for the equivalent incompressible case. Finally, we turn to the topic of global oscillations of quiescent prominences. A uniform slab model (Joarder 1993) yields, under conditions appropriate to the prominence-coronal inhomogeneity with the magnetic field threading the prominence being line-tied in the photosphere, modes which are analogous to the oscillations of a uniform string loaded with a point mass, and a formula approximating the period is given. We investigate the robustness of this formula for various plasma density profiles, assessing the applicability of the results from the uniform slab calculation to more realistic density profiles of the prominence-coronal inhomogeneity.Magnetohydrodynamic surface waves in the solar atmosphere
https://hdl.handle.net/10023/14225
In this thesis the nature of magnetoacoustic surface waves at a single magnetic interface is examined for the case of parallel propagation. Above the interface is an isothermal medium permeated by a horizontal magnetic field. The lower region is a field-free medium of different density to the magnetic atmosphere. We consider both the incompressible and compressible situations and the effect of including gravity. In each case a transcendental dispersion relation is solved numerically for a range of parameters and the resulting dispersion curves plotted. In the first chapter we provide a general introduction to the work, reviewing previous work in this area and considering applications of surface waves. In the second chapter we consider the existence of surface waves for the case when the media are incompressible either side of the interface. We consider the cases of both uniform and non-uniform distributions of densities and the effect of including gravity. We show that the f-mode exists in a restricted band of horizontal wavenumber. In the subsequent chapters we consider the effect of compressibility on surface waves. The media either side of the interface are taken to be isothermal. In the absence of gravity the interface may support one or two surface modes determined by the relative temperatures and magnetism of the two media. This case is studied in Chapter 3 where phase-speeds and penetration depths of the waves and the associated pressure perturbations are investigated for a variety of field strengths and sound speeds. In Chapters 4 and 5 we consider the effect of gravity on the compressible modes described in Chapter 3. In Chapter 4 an exact dispersion relation is obtained for the case of a constant Alfven speed, whilst in Chapter 5 the case of a uniform magnetic field is considered. In the absence of the magnetic field the transcendental dispersion relation may be reduced to a polynomial. This polynomial possesses two acceptable solutions, only one of which may exist at any given circumstance depending on the densities either side of the interface. If the gas density within the field exceeds that in the field-free medium, then the f-mode may propagate; otherwise, a magnetic surface gravity mode propagates. As in the incompressible case, the f-mode exists in a restricted band of horizontal wavenumber. An analytical form for the wave speed of the f-mode is obtained for small values of the Alfven speed. It is shown that the f-mode is related to the fast magnetoacoustic surface wave, merging into that mode at short wavelengths.
Tue, 01 Jan 1991 00:00:00 GMThttps://hdl.handle.net/10023/142251991-01-01T00:00:00ZMiles, Alan J.In this thesis the nature of magnetoacoustic surface waves at a single magnetic interface is examined for the case of parallel propagation. Above the interface is an isothermal medium permeated by a horizontal magnetic field. The lower region is a field-free medium of different density to the magnetic atmosphere. We consider both the incompressible and compressible situations and the effect of including gravity. In each case a transcendental dispersion relation is solved numerically for a range of parameters and the resulting dispersion curves plotted. In the first chapter we provide a general introduction to the work, reviewing previous work in this area and considering applications of surface waves. In the second chapter we consider the existence of surface waves for the case when the media are incompressible either side of the interface. We consider the cases of both uniform and non-uniform distributions of densities and the effect of including gravity. We show that the f-mode exists in a restricted band of horizontal wavenumber. In the subsequent chapters we consider the effect of compressibility on surface waves. The media either side of the interface are taken to be isothermal. In the absence of gravity the interface may support one or two surface modes determined by the relative temperatures and magnetism of the two media. This case is studied in Chapter 3 where phase-speeds and penetration depths of the waves and the associated pressure perturbations are investigated for a variety of field strengths and sound speeds. In Chapters 4 and 5 we consider the effect of gravity on the compressible modes described in Chapter 3. In Chapter 4 an exact dispersion relation is obtained for the case of a constant Alfven speed, whilst in Chapter 5 the case of a uniform magnetic field is considered. In the absence of the magnetic field the transcendental dispersion relation may be reduced to a polynomial. This polynomial possesses two acceptable solutions, only one of which may exist at any given circumstance depending on the densities either side of the interface. If the gas density within the field exceeds that in the field-free medium, then the f-mode may propagate; otherwise, a magnetic surface gravity mode propagates. As in the incompressible case, the f-mode exists in a restricted band of horizontal wavenumber. An analytical form for the wave speed of the f-mode is obtained for small values of the Alfven speed. It is shown that the f-mode is related to the fast magnetoacoustic surface wave, merging into that mode at short wavelengths.Some aspects of solar flare and prominence theory
https://hdl.handle.net/10023/14222
Solar flares and solar prominences are amongst the best known features of solar activity. Despite this familiarity, however, there are still significant gaps in our knowledge of these phenomena. In this thesis some theoretical aspects of these events are considered. We first consider solar prominences. We propose a model for the static equilibrium of quiescent prominences which will simultaneously explain the support mechanism for the dense prominence material and take account roughly of the required energy balance. This model contains two parameters, namely the coronal plasma beta and the horizontal shear angle 𝜙, that the magnetic fieldlines make with the prominence normal. We obtain limits on both these parameters which, when exceeded, imply that no equilibrium state is possible. The results obtained provide a possible explanation for several prominence features. For the remainder of the thesis we consider one aspect of the solar flare problem, namely the possibility of a trigger mechanism for the rapid release of energy in a flare. One candidate for this mechanism is the sudden release of energy stored in excess of potential by a force-free magnetic field which becomes unstable as a result of photospheric motions. For this reason we seek simple analytic solutions to the force-free field equations which may exhibit such an instability. An alternative trigger mechanism, which requires the presence of a current sheet, is given by the emerging flux model for solar flares. We thus develop a one-dimensional model for current sheets in general, where the conditions within the current sheet are given in terms of several non-dimensional parameters which describe the external conditions. These results are then applied to the emerging flux model.
Tue, 01 Jan 1980 00:00:00 GMThttps://hdl.handle.net/10023/142221980-01-01T00:00:00ZMilne, Alexander MitchellSolar flares and solar prominences are amongst the best known features of solar activity. Despite this familiarity, however, there are still significant gaps in our knowledge of these phenomena. In this thesis some theoretical aspects of these events are considered. We first consider solar prominences. We propose a model for the static equilibrium of quiescent prominences which will simultaneously explain the support mechanism for the dense prominence material and take account roughly of the required energy balance. This model contains two parameters, namely the coronal plasma beta and the horizontal shear angle 𝜙, that the magnetic fieldlines make with the prominence normal. We obtain limits on both these parameters which, when exceeded, imply that no equilibrium state is possible. The results obtained provide a possible explanation for several prominence features. For the remainder of the thesis we consider one aspect of the solar flare problem, namely the possibility of a trigger mechanism for the rapid release of energy in a flare. One candidate for this mechanism is the sudden release of energy stored in excess of potential by a force-free magnetic field which becomes unstable as a result of photospheric motions. For this reason we seek simple analytic solutions to the force-free field equations which may exhibit such an instability. An alternative trigger mechanism, which requires the presence of a current sheet, is given by the emerging flux model for solar flares. We thus develop a one-dimensional model for current sheets in general, where the conditions within the current sheet are given in terms of several non-dimensional parameters which describe the external conditions. These results are then applied to the emerging flux model.Polynomial generated polygons
https://hdl.handle.net/10023/14198
A turtle geometric construction on the plane, called a polynomial generated polygon (PGP) and represented by 𝒫[sub]f,[sub]pᵐ, is generated from the sequence obtained from evaluating f(𝓍) ∈ ℤ [𝓍] over ℤ modulo pᵐ where p is a prime and m ∈ ℕ. Computational methods are developed to pre-calculate the symmetries exhibited by [sub]f,[sub]pᵐ for a given f and pᵐ.
These include procedures to find whether [sub]f,[sub]pᵐ is bounded or unbounded, the degree of rotational symmetry present, whether lines of reflectional symmetry can be observed, and in the case of 𝒫[sub]f,[sub]pᵐ unbounded, whether the PGP has a glide reflection.
Methods are also sought to find a suitable f and pᵐ to produce a desired 'feasible' shape in a PGP construction, and how the same shape might be generated modulo pᵐ⁺ᵏ if it cannot be produced modulo pᵐ.
Fri, 01 Jan 1999 00:00:00 GMThttps://hdl.handle.net/10023/141981999-01-01T00:00:00ZSoares, Benedict J.A turtle geometric construction on the plane, called a polynomial generated polygon (PGP) and represented by 𝒫[sub]f,[sub]pᵐ, is generated from the sequence obtained from evaluating f(𝓍) ∈ ℤ [𝓍] over ℤ modulo pᵐ where p is a prime and m ∈ ℕ. Computational methods are developed to pre-calculate the symmetries exhibited by [sub]f,[sub]pᵐ for a given f and pᵐ.
These include procedures to find whether [sub]f,[sub]pᵐ is bounded or unbounded, the degree of rotational symmetry present, whether lines of reflectional symmetry can be observed, and in the case of 𝒫[sub]f,[sub]pᵐ unbounded, whether the PGP has a glide reflection.
Methods are also sought to find a suitable f and pᵐ to produce a desired 'feasible' shape in a PGP construction, and how the same shape might be generated modulo pᵐ⁺ᵏ if it cannot be produced modulo pᵐ.Nonlinear magnetic reconnection
https://hdl.handle.net/10023/14195
In many astrophysical problems magnetic reconnection plays a major role. Despite this reconnection theory remains incompletely understood, partly due to the strong non-linearity of the governing equations and the resulting difficulties in demonstrating analytical solutions. This thesis examines some fundamental aspects of reconnection theory: namely, the dynamics of driven and spontaneously reconnecting systems. We first consider the dynamics of a driven reconnecting system by numerically modelling a configuration consisting of two oppositely oriented flux systems with a variety of different boundary conditions and internal parameters. The results indicate that the rate of reconnection is chiefly dependent on the magnetic Reynolds number, but that the plasma flow is weakly dependent on this parameter, being more affected by the curvature of Incoming magnetic field. Scaling laws for the dimensions of the diffusion region are derived, and the existence of several reconnection regimes is shown. Using the same computer code we also simulate tearing modes in Cartesian geometry under different boundary conditions. By imposing a suitable perturbation a magnetic island is generated. The plasma flows show marked differences for the different boundary conditions implemented. Lastly, we examine some aspects of the coalescence instability. The usual flux function taken to represent a tearing node Island in the linear growth phase is shown to be erroneous, and we derive a correct expression. We show that under certain conditions there exists a threshold to coalescence that depends on the island wavenumbers and the associated perturbation.
Thu, 01 Jan 1987 00:00:00 GMThttps://hdl.handle.net/10023/141951987-01-01T00:00:00ZColin, A. M.In many astrophysical problems magnetic reconnection plays a major role. Despite this reconnection theory remains incompletely understood, partly due to the strong non-linearity of the governing equations and the resulting difficulties in demonstrating analytical solutions. This thesis examines some fundamental aspects of reconnection theory: namely, the dynamics of driven and spontaneously reconnecting systems. We first consider the dynamics of a driven reconnecting system by numerically modelling a configuration consisting of two oppositely oriented flux systems with a variety of different boundary conditions and internal parameters. The results indicate that the rate of reconnection is chiefly dependent on the magnetic Reynolds number, but that the plasma flow is weakly dependent on this parameter, being more affected by the curvature of Incoming magnetic field. Scaling laws for the dimensions of the diffusion region are derived, and the existence of several reconnection regimes is shown. Using the same computer code we also simulate tearing modes in Cartesian geometry under different boundary conditions. By imposing a suitable perturbation a magnetic island is generated. The plasma flows show marked differences for the different boundary conditions implemented. Lastly, we examine some aspects of the coalescence instability. The usual flux function taken to represent a tearing node Island in the linear growth phase is shown to be erroneous, and we derive a correct expression. We show that under certain conditions there exists a threshold to coalescence that depends on the island wavenumbers and the associated perturbation.The nonlinear thermal evolution of coronal structures
https://hdl.handle.net/10023/14193
The thermal equilibrium and evolution of coronal structure is studied in this thesis. A symmetric and constant cross-sectional coronal loop is considered and, because of the strong magnetic field, the plasma is confined to move along the field lines, so that a one-dimensional problem can be assumed. We begin by giving a brief description of the Sun and corresponding phenomena. Then a discussion of the basic MHD equations is given. Here, it is assumed that the heating function is spatially dependent and the cooling function is due to an optically thin plasma. The thermal equilibrium of uniform-pressure coronal loop is investigated. The effects due to varying the values of the parameters involved in the governing equations are studied. It is found that there is a critical decay length of the heating below which a hot coronal loop does not exist. It is suggested that thermal non-equilibrium occurs, allowing the existence of catastrophic cooling. A study of the stability of the equilibrium up to the second order approximation is presented, and it is found that the response of the structure not only depends on the amplitude of the disturbance, but also on whether the disturbance increases or decreases the static temperature. The thermal evolution of the above structure is also investigated by assuming that the inertial terms are small. The previous results related to the critical heating decay length are verified. The numerical simulation shows that an initial hot plasma evolves to a new equilibrium which has a cool summit. This equilibrium is identified as a prominence-like solution. Further investigations are made in order to show how the structure can either evolve to a hot or a cool summit temperature depending on whether the initial conditions are above or below threshold values. The evolution of a disturbance increasing or decreasing an initial equilibrium temperature is followed numerically verifying the prediction made in the stability analysis. Furthermore, the effect of gravity is considered in the thermal equilibrium of loop. Similar results were found as studied for a constant-pressure loop. However, it was found that the critical values in which thermal non-equilibrium can occur is increased. A magnetic dip is also included in this model and the thermal equilibrium is studied. Finally, extensions of the present work is presented and some preliminary results are discussed.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/141931996-01-01T00:00:00ZMendoza Briceño, César AugustoThe thermal equilibrium and evolution of coronal structure is studied in this thesis. A symmetric and constant cross-sectional coronal loop is considered and, because of the strong magnetic field, the plasma is confined to move along the field lines, so that a one-dimensional problem can be assumed. We begin by giving a brief description of the Sun and corresponding phenomena. Then a discussion of the basic MHD equations is given. Here, it is assumed that the heating function is spatially dependent and the cooling function is due to an optically thin plasma. The thermal equilibrium of uniform-pressure coronal loop is investigated. The effects due to varying the values of the parameters involved in the governing equations are studied. It is found that there is a critical decay length of the heating below which a hot coronal loop does not exist. It is suggested that thermal non-equilibrium occurs, allowing the existence of catastrophic cooling. A study of the stability of the equilibrium up to the second order approximation is presented, and it is found that the response of the structure not only depends on the amplitude of the disturbance, but also on whether the disturbance increases or decreases the static temperature. The thermal evolution of the above structure is also investigated by assuming that the inertial terms are small. The previous results related to the critical heating decay length are verified. The numerical simulation shows that an initial hot plasma evolves to a new equilibrium which has a cool summit. This equilibrium is identified as a prominence-like solution. Further investigations are made in order to show how the structure can either evolve to a hot or a cool summit temperature depending on whether the initial conditions are above or below threshold values. The evolution of a disturbance increasing or decreasing an initial equilibrium temperature is followed numerically verifying the prediction made in the stability analysis. Furthermore, the effect of gravity is considered in the thermal equilibrium of loop. Similar results were found as studied for a constant-pressure loop. However, it was found that the critical values in which thermal non-equilibrium can occur is increased. A magnetic dip is also included in this model and the thermal equilibrium is studied. Finally, extensions of the present work is presented and some preliminary results are discussed.Alfvén waves in low-mass star-forming regions
https://hdl.handle.net/10023/14190
Low-mass star-forming regions have a lifetime which is greater than their dynamical time and must therefore be, in an average sense, in mechanical equilibrium. The work presented here proposes that an equilibrium exists between the self-gravity, gas pressure, and the magnetic field and the waves it supports. Specifically the equilibrium in the direction perpendicular to the ordered magnetic field is given by the Lorentz force, while that parallel to the field is given by an Alfvén wave pressure force. The work detailed in this thesis models a low-mass star-forming region as a one-dimensional gas slab with a magnetic field lying perpendicular to the layer. Analytical, self-consistent models are formulated to study the equilibrium parallel to the background magnetic field. It is found that both short-wavelength (modelled using the WKB approximation) and large-amplitude, long-wavelength Alfvén waves can provide the necessary support parallel to the magnetic field, generating model cloud thicknesses that are consistent with the observations. The effect of damping by the linear process of ion-neutral friction is considered. It is found that the damping of the waves is not a necessary condition for the support of the cloud although it is an advantage. The possible sources of these waves are discussed. The Alfvén waves are also found to make an important contribution to the heating of a low-mass star-forming region. By modelling the dominant heating and cooling mechanisms in a molecular cloud, it is discovered that a cloud supported against its self-gravity by short-wavelength Alfvén waves will be hotter at its outer edge than in the central regions. These models successfully describe a low-mass star-forming region in equilibrium between its self-gravity, the gas pressure and an Alfvén wave pressure force. The question of the stability of such an equilibrium is considered, specifically that of an isothermal gas slab supported by short-wavelength Alfvén waves. The initial results suggest that the presence of a magnetic field and its associated Alfvén waves have a stabilising effect on the layer, and encourage further consideration of the role of Alfvén waves in low-mass star-forming regions.
Fri, 01 Jan 1999 00:00:00 GMThttps://hdl.handle.net/10023/141901999-01-01T00:00:00ZMartin, Clare E.Low-mass star-forming regions have a lifetime which is greater than their dynamical time and must therefore be, in an average sense, in mechanical equilibrium. The work presented here proposes that an equilibrium exists between the self-gravity, gas pressure, and the magnetic field and the waves it supports. Specifically the equilibrium in the direction perpendicular to the ordered magnetic field is given by the Lorentz force, while that parallel to the field is given by an Alfvén wave pressure force. The work detailed in this thesis models a low-mass star-forming region as a one-dimensional gas slab with a magnetic field lying perpendicular to the layer. Analytical, self-consistent models are formulated to study the equilibrium parallel to the background magnetic field. It is found that both short-wavelength (modelled using the WKB approximation) and large-amplitude, long-wavelength Alfvén waves can provide the necessary support parallel to the magnetic field, generating model cloud thicknesses that are consistent with the observations. The effect of damping by the linear process of ion-neutral friction is considered. It is found that the damping of the waves is not a necessary condition for the support of the cloud although it is an advantage. The possible sources of these waves are discussed. The Alfvén waves are also found to make an important contribution to the heating of a low-mass star-forming region. By modelling the dominant heating and cooling mechanisms in a molecular cloud, it is discovered that a cloud supported against its self-gravity by short-wavelength Alfvén waves will be hotter at its outer edge than in the central regions. These models successfully describe a low-mass star-forming region in equilibrium between its self-gravity, the gas pressure and an Alfvén wave pressure force. The question of the stability of such an equilibrium is considered, specifically that of an isothermal gas slab supported by short-wavelength Alfvén waves. The initial results suggest that the presence of a magnetic field and its associated Alfvén waves have a stabilising effect on the layer, and encourage further consideration of the role of Alfvén waves in low-mass star-forming regions.Basic magnetic field configurations for solar filament channels and filaments
https://hdl.handle.net/10023/14188
The three-dimensional magnetic structure of solar filament channels and filaments is considered. A simple analytical potential model of a filament channel is setup with line sources representing the overlying arcades and point sources the flux of the filament. A possible explanation of the distinct upper and lower bounds of a filament is given. A more detailed numerical force-free model with discrete flux sources is then developed and the effect of magnetic shear on the separatrix surface explored. Dextral channels are shown to exist for a wider range of negative values of the force-free alpha and sinistral channels for positive values of alpha. Potential models of a variety of coronal structures are then considered. The bending of a filament is modelled and a method of determining the horizontal component of a filament's magnetic field is proposed. Next, the observed opposite skew of arcades lying above switchbacks of polarity inversion lines is shown to be produced by a local flux imbalance at the corner of the switchback. Then, the magnetic structure of a particular filament in a filament channel is modelled using observations from a photospheric magnetogram. It is shown that dips in the filaments magnetic field could result from opposite polarity fragments lying below the filament. Finally, the formation of a specific filament channel and filament is modelled. The formation of the channel is shown to be due to the emergence of new flux in a sheared state. It is shown that convergence and reconnections between the new flux and old remnant flux is required for the filament to form. The field lines that represent the filament form a thin vertical sheet of flux. The changing angle of inclination of the sheet gives the appearance of twist. The method of formation is then generalised to other cases and it is shown that a hemispheric pattern consistent with the results of Martin et al. (1995) can be obtained.
Wed, 01 Jan 1997 00:00:00 GMThttps://hdl.handle.net/10023/141881997-01-01T00:00:00ZMackay, Duncan HendryThe three-dimensional magnetic structure of solar filament channels and filaments is considered. A simple analytical potential model of a filament channel is setup with line sources representing the overlying arcades and point sources the flux of the filament. A possible explanation of the distinct upper and lower bounds of a filament is given. A more detailed numerical force-free model with discrete flux sources is then developed and the effect of magnetic shear on the separatrix surface explored. Dextral channels are shown to exist for a wider range of negative values of the force-free alpha and sinistral channels for positive values of alpha. Potential models of a variety of coronal structures are then considered. The bending of a filament is modelled and a method of determining the horizontal component of a filament's magnetic field is proposed. Next, the observed opposite skew of arcades lying above switchbacks of polarity inversion lines is shown to be produced by a local flux imbalance at the corner of the switchback. Then, the magnetic structure of a particular filament in a filament channel is modelled using observations from a photospheric magnetogram. It is shown that dips in the filaments magnetic field could result from opposite polarity fragments lying below the filament. Finally, the formation of a specific filament channel and filament is modelled. The formation of the channel is shown to be due to the emergence of new flux in a sheared state. It is shown that convergence and reconnections between the new flux and old remnant flux is required for the filament to form. The field lines that represent the filament form a thin vertical sheet of flux. The changing angle of inclination of the sheet gives the appearance of twist. The method of formation is then generalised to other cases and it is shown that a hemispheric pattern consistent with the results of Martin et al. (1995) can be obtained.Aspects of magnetic field theory in solar and laboratory plasmas
https://hdl.handle.net/10023/14183
Using the Magnetohydrodynamic model, two problems in the behaviour of magnetic field structures are investigated. Firstly, the stability of tokamak equilibria to coupled tearing modes is calculated. Secondly, the equilibrium structure of a solar coronal loop is examined. The flux co-ordinate method is used to construct toroidal equilibria of the type found in large aspect ratio tokamaks. In such a field configuration, the analysis of tearing modes is complicated by the coupling of different poloidal fourier modes. The effect of coupling through elliptic shaping of plasma surfaces is calculated. For certain current profiles, this effect may cause instability. The response of coronal loops to twisting at their photospheric footpoints is investigated. Long loops are shown to have an essentially 1-D nature. This observation is used to develop a 1-D, line-tied model for such loops. This model is used to conduct an extensive survey of the non-linear twist regime, including the effects of enhanced fluid pressure. The possibility of non-equilibrium, which would provide energy for coronal heating and compact flares, is examined. When the physical variable of footpoint displacement is specified, no loss of equilibrium is found by twisting. Loss of equilibrium is found for high pressures, which we do not, however, expect to find in the corona.
Mon, 01 Jan 1990 00:00:00 GMThttps://hdl.handle.net/10023/141831990-01-01T00:00:00ZLothian, Robert M.Using the Magnetohydrodynamic model, two problems in the behaviour of magnetic field structures are investigated. Firstly, the stability of tokamak equilibria to coupled tearing modes is calculated. Secondly, the equilibrium structure of a solar coronal loop is examined. The flux co-ordinate method is used to construct toroidal equilibria of the type found in large aspect ratio tokamaks. In such a field configuration, the analysis of tearing modes is complicated by the coupling of different poloidal fourier modes. The effect of coupling through elliptic shaping of plasma surfaces is calculated. For certain current profiles, this effect may cause instability. The response of coronal loops to twisting at their photospheric footpoints is investigated. Long loops are shown to have an essentially 1-D nature. This observation is used to develop a 1-D, line-tied model for such loops. This model is used to conduct an extensive survey of the non-linear twist regime, including the effects of enhanced fluid pressure. The possibility of non-equilibrium, which would provide energy for coronal heating and compact flares, is examined. When the physical variable of footpoint displacement is specified, no loss of equilibrium is found by twisting. Loss of equilibrium is found for high pressures, which we do not, however, expect to find in the corona.Exact solutions for axisymmetric and nonpolytropic astrophysical winds
https://hdl.handle.net/10023/14180
Astrophysical outflows are common in a large variety of objects with very different length-scales. They can be almost spherical, as in the case of the solar wind, or show a high degree of anisotropy as in pre-main sequence stars, star-forming regions or even extragalactic objects. This work is aimed at finding exact solutions of the axisymmetric wind equations in which all variables depend not only on the distance to the central object but on latitude as well. The geometry of the stream/field-lines is taken as helicoidal and this seems to be a good approximation in some examples of collimated flows. From a simple hydrodynamic approach, a straightforward technique based on separation of the variables yields the most general solution of the wind equations under the above assumptions. The way the different variables depend on latitude is controlled by three anisotropy parameters which are related to typical ratios at the base of the atmosphere. The density needs to be higher at the equator than at the pole for the outflow to be able to accelerate. In these circumstances, the radial velocity always increases from equator to pole. Contrary to Parker's model of the solar wind, the solution does not pass through any critical point, since no polytropic law is assumed. However, the general behaviour is similar, with a high acceleration at the base and the velocity rapidly attaining an almost constant asymptotic value. The heating rate that sustains this rapid increase is mostly concentrated near the surface of the central object. The inclusion of the magnetic field in the analysis introduces two critical points: the Alfvenic point and an extra X -type point filtering the solution that gives a vanishing pressure at infinity. If the density anisotropy is too low the wind is unable to accelerate to large asymptotic values. The dependence of the angular velocity of the roots of the fieldlines with latitude reproduces well the observed rotation profile of photospheric magnetic features. The mass loss rate can be substantially increased if the structure of the outflow is highly anisotropic. Some applications to the solar wind are also discussed. In particular, recent results from ULYSSES (pointing out that solar speed increases with latitude while the density decreases from equator to the pole) are in good agreement with the general behaviour of the solutions presented in this work.
Sun, 01 Jan 1995 00:00:00 GMThttps://hdl.handle.net/10023/141801995-01-01T00:00:00ZLima, João José de Faria Graça AfonsoAstrophysical outflows are common in a large variety of objects with very different length-scales. They can be almost spherical, as in the case of the solar wind, or show a high degree of anisotropy as in pre-main sequence stars, star-forming regions or even extragalactic objects. This work is aimed at finding exact solutions of the axisymmetric wind equations in which all variables depend not only on the distance to the central object but on latitude as well. The geometry of the stream/field-lines is taken as helicoidal and this seems to be a good approximation in some examples of collimated flows. From a simple hydrodynamic approach, a straightforward technique based on separation of the variables yields the most general solution of the wind equations under the above assumptions. The way the different variables depend on latitude is controlled by three anisotropy parameters which are related to typical ratios at the base of the atmosphere. The density needs to be higher at the equator than at the pole for the outflow to be able to accelerate. In these circumstances, the radial velocity always increases from equator to pole. Contrary to Parker's model of the solar wind, the solution does not pass through any critical point, since no polytropic law is assumed. However, the general behaviour is similar, with a high acceleration at the base and the velocity rapidly attaining an almost constant asymptotic value. The heating rate that sustains this rapid increase is mostly concentrated near the surface of the central object. The inclusion of the magnetic field in the analysis introduces two critical points: the Alfvenic point and an extra X -type point filtering the solution that gives a vanishing pressure at infinity. If the density anisotropy is too low the wind is unable to accelerate to large asymptotic values. The dependence of the angular velocity of the roots of the fieldlines with latitude reproduces well the observed rotation profile of photospheric magnetic features. The mass loss rate can be substantially increased if the structure of the outflow is highly anisotropic. Some applications to the solar wind are also discussed. In particular, recent results from ULYSSES (pointing out that solar speed increases with latitude while the density decreases from equator to the pole) are in good agreement with the general behaviour of the solutions presented in this work.Chromospheric effects on global solar oscillations
https://hdl.handle.net/10023/14173
A study has been made of the global solar oscillations known as p-modes. The Sun is represented by a plane-parallel stratified plasma. Solutions are found to the magnetohydrodynamic equations of motion in such a plasma, and normal mode frequencies are calculated by applying realistic boundary conditions to these solutions. The normal modes model solar p-modes. For a model consisting of an isothermal chromosphere with a uniform horizontal magnetic field, it is demonstrated that modes may form at all frequencies. Consideration is also given to the related problem of vertical propagation of fast magnetoacoustic waves in a uniform magnetic field. An investigation is carried out into the observed solar cycle variations in the frequencies of p-modes in the classical, low frequency range (1-5 mHz). A possible mechanism for the observed "turnover" effect is discussed. Through the use of a modified Bohr- Sommerfeld condition, the effect of a non-isothermal chromosphere is also considered, and a physical description of chromospheric effects on p-mode frequencies is given. The formation of modes above the acoustic cut-off frequency is investigated. The theoretically calcidated forms of frequency shift curves in this high frequency range agree well with observations. The special case of modes of degree zero is also briefly examined. A mathematical formulation for such modes is constructed, and frequency shifts are determined for a simple chromospheric model atmosphere.
Sat, 01 Jan 1994 00:00:00 GMThttps://hdl.handle.net/10023/141731994-01-01T00:00:00ZJohnston, AlanA study has been made of the global solar oscillations known as p-modes. The Sun is represented by a plane-parallel stratified plasma. Solutions are found to the magnetohydrodynamic equations of motion in such a plasma, and normal mode frequencies are calculated by applying realistic boundary conditions to these solutions. The normal modes model solar p-modes. For a model consisting of an isothermal chromosphere with a uniform horizontal magnetic field, it is demonstrated that modes may form at all frequencies. Consideration is also given to the related problem of vertical propagation of fast magnetoacoustic waves in a uniform magnetic field. An investigation is carried out into the observed solar cycle variations in the frequencies of p-modes in the classical, low frequency range (1-5 mHz). A possible mechanism for the observed "turnover" effect is discussed. Through the use of a modified Bohr- Sommerfeld condition, the effect of a non-isothermal chromosphere is also considered, and a physical description of chromospheric effects on p-mode frequencies is given. The formation of modes above the acoustic cut-off frequency is investigated. The theoretically calcidated forms of frequency shift curves in this high frequency range agree well with observations. The special case of modes of degree zero is also briefly examined. A mathematical formulation for such modes is constructed, and frequency shifts are determined for a simple chromospheric model atmosphere.Theoretical modelling of global oscillations in solar prominences
https://hdl.handle.net/10023/14169
This thesis aims to provide a basic theoretical explanation for the oscillatory motions observed in solar quiescent prominences. The prominence is treated as a simple plasma slab embedded in a hotter and rarer uniform coronal plasma. Both the slab and its environment are permeated by a uniform magnetic field. The field lines are anchored at rigid walls placed on either side of the plasma slab and representing the photospheric line-tying effect. The magnetohydrodynamic modes of oscillation of the plasma slab are then examined for different orientations of the magnetic field with respect to the long axis of the slab. Particularly interesting in this study is the appearance of the 'string MHD' modes that are analogous to the fundamental vibrations of a mass- loaded stretched elastic string. Such modes appear whenever the magnetic field vector is inclined to the long axis of the slab, thus producing a magnetic field component in the direction transverse to the axis of the slab. Observationally, this inclination of the field is generally small. For realistic values of the angle of inclination of the magnetic field lines, the 'string Alfven' mode and an 'internal slow' mode yield periods in the range 1/2-2 hr. These modes may correspond to the observed long period (40-90 minutes) oscillations in quiescent prominences. Intermediate periodicities, in the range 8-20 min, may be associated with an 'internal Alfven' mode and a 'fast string' mode of the prominence slab. The observed short periodicities, in the range 2-5 min, may correspond to an 'internal fast' mode in prominences. Having thus established a foundation for the theoretical modelling of prominence oscillations in terms of the magnetohydrodynamic modes of oscillation of a non-gravitating plasma slab, we discuss several factors, such as the effects of gravitational stratification, the curvature of the magnetic field lines, and the fine-structures in a prominence, that may complicate a description of its oscillatory modes. Some preliminary investigations of simple magnetohydrostatic equilibrium models suggest that gravity and the curvature of the magnetic field lines play only a secondary role in determining the periods of the oscillatory modes in prominences, the basic structure of the modes being similar to that present in simple slab models.
Sat, 01 Jan 1994 00:00:00 GMThttps://hdl.handle.net/10023/141691994-01-01T00:00:00ZJoarder, ParthasarathiThis thesis aims to provide a basic theoretical explanation for the oscillatory motions observed in solar quiescent prominences. The prominence is treated as a simple plasma slab embedded in a hotter and rarer uniform coronal plasma. Both the slab and its environment are permeated by a uniform magnetic field. The field lines are anchored at rigid walls placed on either side of the plasma slab and representing the photospheric line-tying effect. The magnetohydrodynamic modes of oscillation of the plasma slab are then examined for different orientations of the magnetic field with respect to the long axis of the slab. Particularly interesting in this study is the appearance of the 'string MHD' modes that are analogous to the fundamental vibrations of a mass- loaded stretched elastic string. Such modes appear whenever the magnetic field vector is inclined to the long axis of the slab, thus producing a magnetic field component in the direction transverse to the axis of the slab. Observationally, this inclination of the field is generally small. For realistic values of the angle of inclination of the magnetic field lines, the 'string Alfven' mode and an 'internal slow' mode yield periods in the range 1/2-2 hr. These modes may correspond to the observed long period (40-90 minutes) oscillations in quiescent prominences. Intermediate periodicities, in the range 8-20 min, may be associated with an 'internal Alfven' mode and a 'fast string' mode of the prominence slab. The observed short periodicities, in the range 2-5 min, may correspond to an 'internal fast' mode in prominences. Having thus established a foundation for the theoretical modelling of prominence oscillations in terms of the magnetohydrodynamic modes of oscillation of a non-gravitating plasma slab, we discuss several factors, such as the effects of gravitational stratification, the curvature of the magnetic field lines, and the fine-structures in a prominence, that may complicate a description of its oscillatory modes. Some preliminary investigations of simple magnetohydrostatic equilibrium models suggest that gravity and the curvature of the magnetic field lines play only a secondary role in determining the periods of the oscillatory modes in prominences, the basic structure of the modes being similar to that present in simple slab models.Magnetic surface effects on solar oscillations
https://hdl.handle.net/10023/14153
This thesis is concerned with the effects of magnetic atmospheres on solar oscillations. The behaviour of magnetohydrodynamic surface waves propagating on a single magnetic interface is discussed ignoring the effects of gravity. The effects of non-parallel propagation (where the wave vector is at an angle to the magnetic field direction) are considered. The effects of chromospheric magnetic fields on solar p- and f-modes in a stratified atmosphere are examined for three different models. In the first of these models, the chromosphere is assumed to be isothermal and permeated by a uniform and horizontal magnetic field. A dispersion relation for the p-modes trapped below such an atmosphere is derived. Asymptotic and numerical solutions for the p-modes are discussed in detail. An increase in chromospheric magnetic field strength leads to an increase in the frequency of the p-modes, whereas an increase in the chromospheric temperature leads to a decrease in the frequencies of these modes. Comparison with observational data suggests that both these effects may indeed take place. The second model is set up for magnetic fields which decrease with height in such a way that the Alfven speed remains constant. In addition to magnetic effects, the effects of non-parallel propagation on and f-modes are considered in the presence of such a non-uniform magnetic field. After deriving a very general dispersion relation, various asymptotic and numerical solutions have been obtained and the possible effects of magnetic fields and non-parallel propagation on these modes are examined. The presence of a horizontal non-uniform chromospheric field produces changes in the frequencies of the p- and f-modes, reducing the frequencies of p-modes and increasing the frequency of the f-mode. Besides depending upon magnetic field strength, frequencies also depend on both the mode's order n and its degree l. The effects of non-parallel propagation are found to be most significant for the f-mode and the low order p-modes. The magnetic structure of the chromosphere has been further generalised by combining the two models described above. In this three layer model, a dispersion relation is derived in a general manner and discussed in detail for the p-modes. The role of magnetoacoustic cut-off frequency is studied. Again, the results are qualitatively similar to those found from observation.
Fri, 01 Jan 1993 00:00:00 GMThttps://hdl.handle.net/10023/141531993-01-01T00:00:00ZJain, RekhaThis thesis is concerned with the effects of magnetic atmospheres on solar oscillations. The behaviour of magnetohydrodynamic surface waves propagating on a single magnetic interface is discussed ignoring the effects of gravity. The effects of non-parallel propagation (where the wave vector is at an angle to the magnetic field direction) are considered. The effects of chromospheric magnetic fields on solar p- and f-modes in a stratified atmosphere are examined for three different models. In the first of these models, the chromosphere is assumed to be isothermal and permeated by a uniform and horizontal magnetic field. A dispersion relation for the p-modes trapped below such an atmosphere is derived. Asymptotic and numerical solutions for the p-modes are discussed in detail. An increase in chromospheric magnetic field strength leads to an increase in the frequency of the p-modes, whereas an increase in the chromospheric temperature leads to a decrease in the frequencies of these modes. Comparison with observational data suggests that both these effects may indeed take place. The second model is set up for magnetic fields which decrease with height in such a way that the Alfven speed remains constant. In addition to magnetic effects, the effects of non-parallel propagation on and f-modes are considered in the presence of such a non-uniform magnetic field. After deriving a very general dispersion relation, various asymptotic and numerical solutions have been obtained and the possible effects of magnetic fields and non-parallel propagation on these modes are examined. The presence of a horizontal non-uniform chromospheric field produces changes in the frequencies of the p- and f-modes, reducing the frequencies of p-modes and increasing the frequency of the f-mode. Besides depending upon magnetic field strength, frequencies also depend on both the mode's order n and its degree l. The effects of non-parallel propagation are found to be most significant for the f-mode and the low order p-modes. The magnetic structure of the chromosphere has been further generalised by combining the two models described above. In this three layer model, a dispersion relation is derived in a general manner and discussed in detail for the p-modes. The role of magnetoacoustic cut-off frequency is studied. Again, the results are qualitatively similar to those found from observation.Thermal instabilities in the solar corona
https://hdl.handle.net/10023/14150
In this thesis, several problems relating to thermal instabilities in the solar corona are examined. Chapter 1 gives a brief description of the Sun and corresponding events with particular attention focused on prominences, their formation and eruption. Various problems concerning thermal instabilities are then tackled in the later Chapters. In Chapter 2, the basic MHD equations are introduced and a physical description of the thermal instability mechanism given. The MHD equations are linearised in a uniform, infinite medium and the basic instability criteria obtained. Chapter 3 investigates the normal mode spectrum for the linearised MHD equations for a cylindrical equilibrium. This spectrum is examined for zero perpendicular thermal conduction, with both zero and non-zero scalar resistivity. Particular attention is paid to the continuous branches of this spectrum, or continuous spectra. For zero resistivity there are three types of continuous spectra present, namely the Alfven, slow and thermal continua. It is shown that when dissipation due to resistivity is included, the slow and Alfven continua are removed and the thermal continuum is shifted to a different position (where the shift is independent of the exact value of resistivity). The 'old' location of the thermal continuum is covered by a dense set of nearly singular discrete modes called a quasi-continuum, for equilibria with the thermal time scale much smaller than the Alfven time scale. This quasi-continuum is investigated numerically and the eigenfunctions are shown to have rapid spatial oscillating behaviour. These oscillations are confined to the most unstable part of the equilibrium based on the Field criterion and may be the cause of fine structure in prominences. In Chapter 4, the normal mode spectrum for the linearised MHD equations is examined for a plasma in a cylindrical equilibrium. The equations describing these normal modes are solved numerically using a finite element code. In the ideal case the Hain-Lust equation is expanded and a WKB solution obtained for large axial wave numbers. This is compared to the numerical solutions. In the non-ideal case, the ballooning equations describing localised modes are manipulated in an arcade geometry and a dispersion relation derived. It is illustrated that as the axial wave number k is increased, the fundamental thermal and Alfven modes can coalesce to form overstable magnetothermal modes. The ratio between the magnetic and thermal terms is varied and the existence of the magnetothermal modes examined. The corresponding growth rates are predicted by a WKB solution to the ballooning equations. The interaction of thermal and magnetic instabilities and the existence of these magnetothermal modes may be significant in the eruption of prominences into solar flares. Chapter 5 extends the work presented in Chapter 4 to include the effects of line-tying in a coronal arcade. The ballooning equations which were introduced in Chapter 4 are manipulated to give a dispersion relation. This relation is a quadratic in the square of the azimuthal wave number m if parallel thermal conduction is neglected and a cubic in m2 if parallel conduction is included. Rigid wall boundary conditions are applied to this dispersion relation. This dispersion relation is then solved numerically subject to these boundary conditions and the solutions plotted. Unfortunately the expression for the thermal continuum in line-tied arcades is required since the thermal continuum must play an important role in the proceedings. This calculation is left for future work. From the results obtained, it can be seen that the thermal instability can play a major part in prominence formation and destruction. The thermal instability may help create the prominence. Resistivity and perpendicular thermal conduction can cause of the observed fine scale structure. Finally, a neighbouring thermal instability may trigger a magnetic instability that causes the prominence to erupt.
Sun, 01 Jan 1995 00:00:00 GMThttps://hdl.handle.net/10023/141501995-01-01T00:00:00ZIreland, Richard C.In this thesis, several problems relating to thermal instabilities in the solar corona are examined. Chapter 1 gives a brief description of the Sun and corresponding events with particular attention focused on prominences, their formation and eruption. Various problems concerning thermal instabilities are then tackled in the later Chapters. In Chapter 2, the basic MHD equations are introduced and a physical description of the thermal instability mechanism given. The MHD equations are linearised in a uniform, infinite medium and the basic instability criteria obtained. Chapter 3 investigates the normal mode spectrum for the linearised MHD equations for a cylindrical equilibrium. This spectrum is examined for zero perpendicular thermal conduction, with both zero and non-zero scalar resistivity. Particular attention is paid to the continuous branches of this spectrum, or continuous spectra. For zero resistivity there are three types of continuous spectra present, namely the Alfven, slow and thermal continua. It is shown that when dissipation due to resistivity is included, the slow and Alfven continua are removed and the thermal continuum is shifted to a different position (where the shift is independent of the exact value of resistivity). The 'old' location of the thermal continuum is covered by a dense set of nearly singular discrete modes called a quasi-continuum, for equilibria with the thermal time scale much smaller than the Alfven time scale. This quasi-continuum is investigated numerically and the eigenfunctions are shown to have rapid spatial oscillating behaviour. These oscillations are confined to the most unstable part of the equilibrium based on the Field criterion and may be the cause of fine structure in prominences. In Chapter 4, the normal mode spectrum for the linearised MHD equations is examined for a plasma in a cylindrical equilibrium. The equations describing these normal modes are solved numerically using a finite element code. In the ideal case the Hain-Lust equation is expanded and a WKB solution obtained for large axial wave numbers. This is compared to the numerical solutions. In the non-ideal case, the ballooning equations describing localised modes are manipulated in an arcade geometry and a dispersion relation derived. It is illustrated that as the axial wave number k is increased, the fundamental thermal and Alfven modes can coalesce to form overstable magnetothermal modes. The ratio between the magnetic and thermal terms is varied and the existence of the magnetothermal modes examined. The corresponding growth rates are predicted by a WKB solution to the ballooning equations. The interaction of thermal and magnetic instabilities and the existence of these magnetothermal modes may be significant in the eruption of prominences into solar flares. Chapter 5 extends the work presented in Chapter 4 to include the effects of line-tying in a coronal arcade. The ballooning equations which were introduced in Chapter 4 are manipulated to give a dispersion relation. This relation is a quadratic in the square of the azimuthal wave number m if parallel thermal conduction is neglected and a cubic in m2 if parallel conduction is included. Rigid wall boundary conditions are applied to this dispersion relation. This dispersion relation is then solved numerically subject to these boundary conditions and the solutions plotted. Unfortunately the expression for the thermal continuum in line-tied arcades is required since the thermal continuum must play an important role in the proceedings. This calculation is left for future work. From the results obtained, it can be seen that the thermal instability can play a major part in prominence formation and destruction. The thermal instability may help create the prominence. Resistivity and perpendicular thermal conduction can cause of the observed fine scale structure. Finally, a neighbouring thermal instability may trigger a magnetic instability that causes the prominence to erupt.Heating of turbulent solar and laboratory plasmas
https://hdl.handle.net/10023/14146
The model of Heyvaerts and Priest (1992) for steady-state heating of the turbulent medium within a sheared solar coronal arcade structure is here developed. The energy input into the corona is calculated at the large scales of the model. At the smaller scales the effects of coronal turbulence are modelled in the form of an enhanced turbulent viscosity and magnetic diffusivity, which are related to the injected power density in the steady state. Matching the expressions for the injected and dissipated power enables the calculation of a heating power consistent with both boundary motions and turbulent effects with a minimum of arbitrary parameters - the price to be paid is that the inertial range spectrum must be prescribed and imposed at all scales. While it is capable of reproducing the observed levels of coronal heating (300 Wm⁻² 3x10⁵ erg cm⁻² s⁻ⁱ for the quiet Sun, 800 Wm⁻² (8 x 10⁵ erg cm⁻² s⁻ⁱ) for a coronal hole and 10⁴ Wm ⁻² (10⁷ erg cm⁻² s⁻ⁱ) for an active region (Withbroe and Noyes, 1977)), there are some mathematical and physical difficulties present. These are eliminated as far as is possible and it is found that the final results for heating levels differ little from the original model although there is a much greater consistency between the imposed and predicted energy power spectra. The modified approach is applied to the problems of photospheric motions twisting a coronal flux tube and of rapid motions injecting Alfven waves into an arcade. In the former case comparable levels of heating are obtained. For a driven and damped standing wave, however, desired levels of heating are only obtained when a global resonance occurs. Attempts are also made to find similar steady-state equilibria possessing flow for fusion experiments in order to apply the above procedure to investigate turbulence in laboratory plasmas. This has been hampered by the difficulty in finding simple appropriate equilibria with many scales present.
Sun, 01 Jan 1995 00:00:00 GMThttps://hdl.handle.net/10023/141461995-01-01T00:00:00ZInverarity, Gordon W.The model of Heyvaerts and Priest (1992) for steady-state heating of the turbulent medium within a sheared solar coronal arcade structure is here developed. The energy input into the corona is calculated at the large scales of the model. At the smaller scales the effects of coronal turbulence are modelled in the form of an enhanced turbulent viscosity and magnetic diffusivity, which are related to the injected power density in the steady state. Matching the expressions for the injected and dissipated power enables the calculation of a heating power consistent with both boundary motions and turbulent effects with a minimum of arbitrary parameters - the price to be paid is that the inertial range spectrum must be prescribed and imposed at all scales. While it is capable of reproducing the observed levels of coronal heating (300 Wm⁻² 3x10⁵ erg cm⁻² s⁻ⁱ for the quiet Sun, 800 Wm⁻² (8 x 10⁵ erg cm⁻² s⁻ⁱ) for a coronal hole and 10⁴ Wm ⁻² (10⁷ erg cm⁻² s⁻ⁱ) for an active region (Withbroe and Noyes, 1977)), there are some mathematical and physical difficulties present. These are eliminated as far as is possible and it is found that the final results for heating levels differ little from the original model although there is a much greater consistency between the imposed and predicted energy power spectra. The modified approach is applied to the problems of photospheric motions twisting a coronal flux tube and of rapid motions injecting Alfven waves into an arcade. In the former case comparable levels of heating are obtained. For a driven and damped standing wave, however, desired levels of heating are only obtained when a global resonance occurs. Attempts are also made to find similar steady-state equilibria possessing flow for fusion experiments in order to apply the above procedure to investigate turbulence in laboratory plasmas. This has been hampered by the difficulty in finding simple appropriate equilibria with many scales present.WKB estimates to the critical length of twisted solar coronal loops
https://hdl.handle.net/10023/14092
The solar corona exhibits many different phenomena, observable from the Earth or space. Magnetohydrodynamic stability theory provides a method of investigating these phenomena by using it to test proposed mathematical models. WKB is a way of approximating the solutions of second order linear homogeneous differential equations with large parameters and so together with MHD stability theory, models for solar coronal loops can be investigated. In this thesis, the problem of a line tied twisted coronal loop is studied within the framework of ideal MHD using a WKB approximation to estimate the critical length at which the various magnetic fields become unstable. The problem will be split into two halves: (i) force-free and (ii) non force-free fields. Using a finite element/Fourier method, the full MHD equations will be solved numerically and the results compared with analytical solutions.
Sun, 01 Jan 1995 00:00:00 GMThttps://hdl.handle.net/10023/140921995-01-01T00:00:00ZHerbert, Simon I.The solar corona exhibits many different phenomena, observable from the Earth or space. Magnetohydrodynamic stability theory provides a method of investigating these phenomena by using it to test proposed mathematical models. WKB is a way of approximating the solutions of second order linear homogeneous differential equations with large parameters and so together with MHD stability theory, models for solar coronal loops can be investigated. In this thesis, the problem of a line tied twisted coronal loop is studied within the framework of ideal MHD using a WKB approximation to estimate the critical length at which the various magnetic fields become unstable. The problem will be split into two halves: (i) force-free and (ii) non force-free fields. Using a finite element/Fourier method, the full MHD equations will be solved numerically and the results compared with analytical solutions.Solar coronal stability problems
https://hdl.handle.net/10023/14090
Magnetohydrodynamic stability theory provides a powerful tool for understanding and testing hypothesized mathematical and physical models of observed phenomena on the surface of the Sun. In this thesis, the problem of applying the 'correct' boundary conditions at the photospheric/coronal interface used in modelling coronal arcades is tackled. Then some aspects of the stability of coronal loops and arcades are investigated using a Fourier truncated series approximation for the equation of motion. The problem involving the boundary conditions has been the subject of a controversy for the past decade with two principal conditions suggested, the 'rigid-wall' conditions where all perturbations vanish at the interface, and 'flow-through' conditions where flows parallel to the equilibrium magnetic field take place. By modelling the photosphere and corona as two different density regions and then varying the ratio of the densities of the two regions, growth rates and eigen-functions of both ideal and resistive modes are investigated in order to follow the evolution of the modes as the density ratio is increased. In order to simplify the analysis, the 2-D equations are reduced to 1-D equations by taking a WKB approximation for the spatial variations across the field to give a localized ballooning approach with ordinary differential equations along the fieldlines. Stability of coronal loops to kink modes transformed to localized modes by increasing the poloidal wavenumber, m, is investigated. Two fields generated numerically from the Grad-Shafranov equation and three analytic fields are investigated in detail and the effect of pressure on the marginal loop length is found, both for near force-free conditions such as is found in the solar corona, and away from force-free conditions. It was found that for near force-free conditions, kink modes are the most unstable with localized modes the most stable. As pressure and pressure gradients become important, there is a reversal in the most unstable modes with localized modes the most unstable.
Fri, 01 Jan 1993 00:00:00 GMThttps://hdl.handle.net/10023/140901993-01-01T00:00:00ZHardie, Ian S.Magnetohydrodynamic stability theory provides a powerful tool for understanding and testing hypothesized mathematical and physical models of observed phenomena on the surface of the Sun. In this thesis, the problem of applying the 'correct' boundary conditions at the photospheric/coronal interface used in modelling coronal arcades is tackled. Then some aspects of the stability of coronal loops and arcades are investigated using a Fourier truncated series approximation for the equation of motion. The problem involving the boundary conditions has been the subject of a controversy for the past decade with two principal conditions suggested, the 'rigid-wall' conditions where all perturbations vanish at the interface, and 'flow-through' conditions where flows parallel to the equilibrium magnetic field take place. By modelling the photosphere and corona as two different density regions and then varying the ratio of the densities of the two regions, growth rates and eigen-functions of both ideal and resistive modes are investigated in order to follow the evolution of the modes as the density ratio is increased. In order to simplify the analysis, the 2-D equations are reduced to 1-D equations by taking a WKB approximation for the spatial variations across the field to give a localized ballooning approach with ordinary differential equations along the fieldlines. Stability of coronal loops to kink modes transformed to localized modes by increasing the poloidal wavenumber, m, is investigated. Two fields generated numerically from the Grad-Shafranov equation and three analytic fields are investigated in detail and the effect of pressure on the marginal loop length is found, both for near force-free conditions such as is found in the solar corona, and away from force-free conditions. It was found that for near force-free conditions, kink modes are the most unstable with localized modes the most stable. As pressure and pressure gradients become important, there is a reversal in the most unstable modes with localized modes the most unstable.Instability and wave-growth within some oscillatory fluid flows
https://hdl.handle.net/10023/14087
Oscillatory fluid flows arise naturally in many systems. Whether or not these systems are stable is an important question and external periodic forcing of the flow may result in rich and complicated behaviours. Here three distinct oscillatory fluid flows are examined in detail, with the stability of each being established using a range of analytical and computational methods. The first system comprises standing surface capillary-gravity waves in second-harmonic resonance subject to Faraday excitation. Using the perturbation technique of multiple scales, the amplitude equations for the system are derived. At exact resonance, and with the absence of damping, the only fixed point of the equations is found to be the origin. A computational approach reveals that the amplitudes of the two waves remain either bounded or grow to infinity depending on initial data. With the introduction of detuning and damping into the system families of fixed points now exist and some special cases are considered. The second class of flows are unbounded time-periodic flows with fixed ellipsoidal stream surfaces, and having spatially uniform but time-periodic strain rates. Using a recently developed method based on theoretical study of the Schrodinger equation with quasi-periodic potential, a computational approach is adopted which determines the stability of the flow to three-dimensional plane wave disturbances. Results for the growth rate and winding number of the disturbance clearly reveal the regions of instability. It is found that almost all these flows are highly unstable. The third class is another set of three-dimensional time-periodic flows with spatially uniform strain rates. These flows are non-axisymmetric and have sinusoidally-fluctuating rates of strain directed along the fixed coordinate axes. The same computational method is employed and it is found that instability increases along with the non-axisymmetric nature of the flow.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/140871996-01-01T00:00:00ZForster, Graham KeithOscillatory fluid flows arise naturally in many systems. Whether or not these systems are stable is an important question and external periodic forcing of the flow may result in rich and complicated behaviours. Here three distinct oscillatory fluid flows are examined in detail, with the stability of each being established using a range of analytical and computational methods. The first system comprises standing surface capillary-gravity waves in second-harmonic resonance subject to Faraday excitation. Using the perturbation technique of multiple scales, the amplitude equations for the system are derived. At exact resonance, and with the absence of damping, the only fixed point of the equations is found to be the origin. A computational approach reveals that the amplitudes of the two waves remain either bounded or grow to infinity depending on initial data. With the introduction of detuning and damping into the system families of fixed points now exist and some special cases are considered. The second class of flows are unbounded time-periodic flows with fixed ellipsoidal stream surfaces, and having spatially uniform but time-periodic strain rates. Using a recently developed method based on theoretical study of the Schrodinger equation with quasi-periodic potential, a computational approach is adopted which determines the stability of the flow to three-dimensional plane wave disturbances. Results for the growth rate and winding number of the disturbance clearly reveal the regions of instability. It is found that almost all these flows are highly unstable. The third class is another set of three-dimensional time-periodic flows with spatially uniform strain rates. These flows are non-axisymmetric and have sinusoidally-fluctuating rates of strain directed along the fixed coordinate axes. The same computational method is employed and it is found that instability increases along with the non-axisymmetric nature of the flow.The effects of magnetic fields on oscillations in the solar atmosphere
https://hdl.handle.net/10023/14082
A study has been made of wave propagation in two regions of the solar atmosphere in which magnetic forces are significant. Sunspot observations indicate a rich variety of characteristic modes of oscillation roughly divided into three categories: three minute umbral oscillations, five minute umbral oscillations and penumbral waves. Outside of intense magnetic flux concentrations the oscillation spectrum is dominated by the five minute period. These waves are trapped in a cavity whose upper boundary may be affected by the magnetism of the chromosphere. A sunspot has been modelled by a uniform cylindrical flux tube. The allowable modes of oscillation are found to vary as the atmospheric parameters change with depth. Umbral three minute oscillations are interpreted as slow body modes. The umbral five minute oscillations arise through a complicated interaction with acoustic waves outside the sunspot. This drives fast body modes as well as waves simply passing through the flux tube. The former may propagate upwards and become fast surface waves. Fast and slow surface waves may explain some of the oscillations of the penumbra. The magnetic structure of the chromosphere has been modelled as an isothermal atmosphere permeated by a uniform and horizontal magnetic field. A dispersion relation for the trapped waves below such an atmosphere has been derived and both asymptotic and numerical solutions found. The effect of a uniform magnetic field is to increase the frequency of the trapped modes. A physical explanation for these changes in frequency has been put forward. Observational evidence may indicate that such effects are indeed seen. This model has been further generalised to take some account of the variation in canopy height which has been observed.
Mon, 01 Jan 1990 00:00:00 GMThttps://hdl.handle.net/10023/140821990-01-01T00:00:00ZEvans, David J.A study has been made of wave propagation in two regions of the solar atmosphere in which magnetic forces are significant. Sunspot observations indicate a rich variety of characteristic modes of oscillation roughly divided into three categories: three minute umbral oscillations, five minute umbral oscillations and penumbral waves. Outside of intense magnetic flux concentrations the oscillation spectrum is dominated by the five minute period. These waves are trapped in a cavity whose upper boundary may be affected by the magnetism of the chromosphere. A sunspot has been modelled by a uniform cylindrical flux tube. The allowable modes of oscillation are found to vary as the atmospheric parameters change with depth. Umbral three minute oscillations are interpreted as slow body modes. The umbral five minute oscillations arise through a complicated interaction with acoustic waves outside the sunspot. This drives fast body modes as well as waves simply passing through the flux tube. The former may propagate upwards and become fast surface waves. Fast and slow surface waves may explain some of the oscillations of the penumbra. The magnetic structure of the chromosphere has been modelled as an isothermal atmosphere permeated by a uniform and horizontal magnetic field. A dispersion relation for the trapped waves below such an atmosphere has been derived and both asymptotic and numerical solutions found. The effect of a uniform magnetic field is to increase the frequency of the trapped modes. A physical explanation for these changes in frequency has been put forward. Observational evidence may indicate that such effects are indeed seen. This model has been further generalised to take some account of the variation in canopy height which has been observed.Magnetic helicity and force-free equilibria in the solar corona and in laboratory devices
https://hdl.handle.net/10023/14080
Force-free equilibria are believed to be important in both an astrophysical and a laboratory context as minimum-energy configurations (see, for example, Woltjer, 1958; Taylor, 1974). Associated is the study of magnetic helicity and its invariance. In Chapter Two of this thesis we put forward a means of heating the corona by the rotation of the foot-points of a coronal "sunspot" magnetic field anchored in the photosphere. The method adopted is essentially that of Heyvaerts and Priest (1984), employing Taylor's Hypothesis (Taylor, 1974) and a magnetic helicity evolution equation. A characteristic of the Reversed-Field Pinch device is the appearance, at high enough values of the quantity "volt-seconds over toroidal flux", of a helical distortion to the basic axi-symmetric state. In Chapter Three we look for corresponding behaviour in the "sunspot equilibrium" of the previous chapter, with limited success. However, we go on to formulate a method of calculating general axi-symmetric fields above a sunspot given the normal field component at the photosphere. Chapters Four, Five and Six are concerned with equilibrium force-free fields in a sphere. The main aim here is the calculation minimum-energy configurations having magnetic flux crossing the boundary, and so we employ "relative helicity" (Berger and Field, 1984). In Chapter Four we consider the "P1(cos𝜃)" boundary radial field, finding that the minimum-energy state is always purely symmetric. In Chapter Five we treat the "P2(cos𝜃)" boundary condition. We find in this case that a "mixed state" is theoretically possible for high enough values of the helicity. In Chapter Six, we consider a general boundary field, which we use to model point sources of magnetic flux at the boundary of a spheromak, finding that in practice an axi-symmetric configuration is always the minimum-energy state. Finally, in Chapter Seven we present an extension to the theorem of Woltjer (1958), concerning the minimization of the magnetic energy of a magnetic structure, to include the case of a free boundary subjected to external pressure forces. To illustrate the theory, we have provided three applications, the first to a finite cylindrical flux and the remainder to possible spheromak configurations.
Fri, 01 Jan 1988 00:00:00 GMThttps://hdl.handle.net/10023/140801988-01-01T00:00:00ZDixon, Andrew MichaelForce-free equilibria are believed to be important in both an astrophysical and a laboratory context as minimum-energy configurations (see, for example, Woltjer, 1958; Taylor, 1974). Associated is the study of magnetic helicity and its invariance. In Chapter Two of this thesis we put forward a means of heating the corona by the rotation of the foot-points of a coronal "sunspot" magnetic field anchored in the photosphere. The method adopted is essentially that of Heyvaerts and Priest (1984), employing Taylor's Hypothesis (Taylor, 1974) and a magnetic helicity evolution equation. A characteristic of the Reversed-Field Pinch device is the appearance, at high enough values of the quantity "volt-seconds over toroidal flux", of a helical distortion to the basic axi-symmetric state. In Chapter Three we look for corresponding behaviour in the "sunspot equilibrium" of the previous chapter, with limited success. However, we go on to formulate a method of calculating general axi-symmetric fields above a sunspot given the normal field component at the photosphere. Chapters Four, Five and Six are concerned with equilibrium force-free fields in a sphere. The main aim here is the calculation minimum-energy configurations having magnetic flux crossing the boundary, and so we employ "relative helicity" (Berger and Field, 1984). In Chapter Four we consider the "P1(cos𝜃)" boundary radial field, finding that the minimum-energy state is always purely symmetric. In Chapter Five we treat the "P2(cos𝜃)" boundary condition. We find in this case that a "mixed state" is theoretically possible for high enough values of the helicity. In Chapter Six, we consider a general boundary field, which we use to model point sources of magnetic flux at the boundary of a spheromak, finding that in practice an axi-symmetric configuration is always the minimum-energy state. Finally, in Chapter Seven we present an extension to the theorem of Woltjer (1958), concerning the minimization of the magnetic energy of a magnetic structure, to include the case of a free boundary subjected to external pressure forces. To illustrate the theory, we have provided three applications, the first to a finite cylindrical flux and the remainder to possible spheromak configurations.MHD flows in the solar atmosphere
https://hdl.handle.net/10023/14075
In this thesis, different aspects of the physics of flows in the solar atmosphere are examined. These are described by means of the set of (ideal) magnetohydrodynamics (MHD) and throughout the thesis there is a progressive refinement in the mathematical methods to solve these equations. First, an analysis of symmetric MHD equilibria is presented and the difficulties that are found in solving the steady equations, both analytically and numerically, are discussed in detail. A novel method to find exact solutions in the incompressible case is presented and families of solutions are given in different geometries. Then, attention is turned to flows in coronal magnetic structures, namely quiescent prominences (closed fieldlines) and polar plumes (open fieldlines), and MHD models for these structures are developed by following two different methods: for the former a semi- analytic approach while for the latter a linearisation through a low 𝛽 assumption. In the prominence model, the effects of a subsonic flow along the fieldlines supporting the structure are studied and the results are compared both with a previous static model and with the observed flow speeds. For the plume model, flows are supposed to be transonic along the open fieldlines and their behaviour is studied for different distributions of temperature, density and magnetic flux. However, here the main goal is to demonstrate that coronal plumes are essentially magnetic features and some results of the model are compared with observations. Finally, a time dependent MHD code in spherical coordinates is presented. The aim is to study the interaction of the solar wind with the large scale coronal magnetic structures and the propagation of MHD waves. As a test in 1-D, simulations of the dynamic response of a spherically symmetric extended corona to changes at the outer pressure are studied, following a previous analytic work.
Wed, 01 Jan 1997 00:00:00 GMThttps://hdl.handle.net/10023/140751997-01-01T00:00:00ZDel Zanna, LucaIn this thesis, different aspects of the physics of flows in the solar atmosphere are examined. These are described by means of the set of (ideal) magnetohydrodynamics (MHD) and throughout the thesis there is a progressive refinement in the mathematical methods to solve these equations. First, an analysis of symmetric MHD equilibria is presented and the difficulties that are found in solving the steady equations, both analytically and numerically, are discussed in detail. A novel method to find exact solutions in the incompressible case is presented and families of solutions are given in different geometries. Then, attention is turned to flows in coronal magnetic structures, namely quiescent prominences (closed fieldlines) and polar plumes (open fieldlines), and MHD models for these structures are developed by following two different methods: for the former a semi- analytic approach while for the latter a linearisation through a low 𝛽 assumption. In the prominence model, the effects of a subsonic flow along the fieldlines supporting the structure are studied and the results are compared both with a previous static model and with the observed flow speeds. For the plume model, flows are supposed to be transonic along the open fieldlines and their behaviour is studied for different distributions of temperature, density and magnetic flux. However, here the main goal is to demonstrate that coronal plumes are essentially magnetic features and some results of the model are compared with observations. Finally, a time dependent MHD code in spherical coordinates is presented. The aim is to study the interaction of the solar wind with the large scale coronal magnetic structures and the propagation of MHD waves. As a test in 1-D, simulations of the dynamic response of a spherically symmetric extended corona to changes at the outer pressure are studied, following a previous analytic work.The nonuniform magnetohydrodynamic nature of the solar atmosphere
https://hdl.handle.net/10023/14073
The nonuniform structure observed in the solar atmosphere, and in particular the corona, is thought to arise due to the interaction between the magnetic field and plasma. Using a linear theory, the nature of these interactions is investigated, and it is shown how coronal structure may be modelled in a simple way by extended standing disturbances. The effect of inertial forces in considered in both a Cartesian and cylindrical geometries, and a first correction due to gravity is calculated. The restrictions of a linear theory may be overcome by finding exact solutions. Solutions are presented which may model plasma flows in closed, partially open and open magnetic field line structures. A new method for finding particular classes of exact steady solutions in a gravitationally stratified, isothermal atmosphere is presented, along with some examples of possible solutions.
Tue, 01 Jan 1991 00:00:00 GMThttps://hdl.handle.net/10023/140731991-01-01T00:00:00ZDe Ville, AndrewThe nonuniform structure observed in the solar atmosphere, and in particular the corona, is thought to arise due to the interaction between the magnetic field and plasma. Using a linear theory, the nature of these interactions is investigated, and it is shown how coronal structure may be modelled in a simple way by extended standing disturbances. The effect of inertial forces in considered in both a Cartesian and cylindrical geometries, and a first correction due to gravity is calculated. The restrictions of a linear theory may be overcome by finding exact solutions. Solutions are presented which may model plasma flows in closed, partially open and open magnetic field line structures. A new method for finding particular classes of exact steady solutions in a gravitationally stratified, isothermal atmosphere is presented, along with some examples of possible solutions.Aspects of solar coronal stability theory
https://hdl.handle.net/10023/14071
Solar coronal stability theory is a powerful tool for understanding the complex behaviour of the Sun's atmosphere. It enables one to discover the driving forces behind some intriguing phenomena and to gauge the soundness of theoretical models for observed structures. In this thesis, the linear stability analysis of line-tied symmetric magnetohydrostatic equilibria is studied within the framework of ideal MHD, aimed at its application to the solar corona. Firstly, a tractable stability procedure based on a variational method is devised. It provides a necessary condition for stability to disturbances localised about a particular flux surface, and a sufficient condition for stability to all accessible perturbations that vanish at the photosphere. The tests require the minimisation of a line integral along the magnetic field lines. For 1-D equilibria, this can be performed analytically, and simple stability criteria are obtained. The necessary condition then serves as an extended Suydam criterion, incorporating the stabilising effect of line-tying. For 2-D equilibria, the minimisation requires the integration of a system of ordinary differential equations along the field lines. This stability technique is applied to arcade, loop, and prominence models, yielding tight bounds on the equilibrium parameters. Secondly, global modes in 1-D coronal loops are investigated using a normal mode method, in order to clarify their link with localised interchange modes. For nearly force-free fields it is shown that instability to localised modes implies the existence of a fast growing global kink mode driven in the neighbourhood of the radius predicted by the local analysis. This confers a new significance on the study of localised interchange modes and the associated extended Suydam criterion.
Tue, 01 Jan 1991 00:00:00 GMThttps://hdl.handle.net/10023/140711991-01-01T00:00:00ZDe Bruyne, Peter J. J.Solar coronal stability theory is a powerful tool for understanding the complex behaviour of the Sun's atmosphere. It enables one to discover the driving forces behind some intriguing phenomena and to gauge the soundness of theoretical models for observed structures. In this thesis, the linear stability analysis of line-tied symmetric magnetohydrostatic equilibria is studied within the framework of ideal MHD, aimed at its application to the solar corona. Firstly, a tractable stability procedure based on a variational method is devised. It provides a necessary condition for stability to disturbances localised about a particular flux surface, and a sufficient condition for stability to all accessible perturbations that vanish at the photosphere. The tests require the minimisation of a line integral along the magnetic field lines. For 1-D equilibria, this can be performed analytically, and simple stability criteria are obtained. The necessary condition then serves as an extended Suydam criterion, incorporating the stabilising effect of line-tying. For 2-D equilibria, the minimisation requires the integration of a system of ordinary differential equations along the field lines. This stability technique is applied to arcade, loop, and prominence models, yielding tight bounds on the equilibrium parameters. Secondly, global modes in 1-D coronal loops are investigated using a normal mode method, in order to clarify their link with localised interchange modes. For nearly force-free fields it is shown that instability to localised modes implies the existence of a fast growing global kink mode driven in the neighbourhood of the radius predicted by the local analysis. This confers a new significance on the study of localised interchange modes and the associated extended Suydam criterion.Hysteresis and mode competition in Faraday waves
https://hdl.handle.net/10023/14054
Faraday waves arise on the surface of a liquid in a container that is undergoing vertical periodic oscillations. We investigate two-dimensional Faraday waves in a long rectangular container, both theoretically and experimentally. Hysteresis occurs when both finite amplitude solutions and the flat surface solution are available. We derive a nonlinear model of a standing wave, extending the Lagrangian method of Miles (1976). The model is used to investigate hysteresis. It is found necessary to retain cubic damping, cubic forcing and the fifth-order conservative term in order to achieve agreement with experiments. The fifth-order conservative term was omitted from all previous studies of Faraday waves. Stable limit cycles are found to arise from this single-mode equation. We examine the structure of this new solution in detail, both analytically and numerically. We describe local bifurcations using a multiple time scales analysis and global bifurcations using Melnikov's method. The coefficients of linear and cubic damping are derived for a standing wave in a rectangular container by considering energy dissipation in the main body of the fluid (due to potential flow and streaming) and in boundary layers at the sidewalls and at the surface. Surface contamination, due to the presence of a thin viscoelastic surface film, creates a boundary layer at the surface which causes enhanced dissipation comparable to, or greater than, that caused by the boundary layers at the walls of the container. Three-mode interaction equations are used to model intermittency and complex modulations which are found to arise from a sideband instability mechanism similar to that of Eckhaus (1963) and Benjamin & Feir (1967). The role of cubic and fifth-order nonlinear terms on this instability mechanism is examined. Theoretical results are found to compare quite favourably with experimental data.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/140541996-01-01T00:00:00ZDecent, Stephen PaulFaraday waves arise on the surface of a liquid in a container that is undergoing vertical periodic oscillations. We investigate two-dimensional Faraday waves in a long rectangular container, both theoretically and experimentally. Hysteresis occurs when both finite amplitude solutions and the flat surface solution are available. We derive a nonlinear model of a standing wave, extending the Lagrangian method of Miles (1976). The model is used to investigate hysteresis. It is found necessary to retain cubic damping, cubic forcing and the fifth-order conservative term in order to achieve agreement with experiments. The fifth-order conservative term was omitted from all previous studies of Faraday waves. Stable limit cycles are found to arise from this single-mode equation. We examine the structure of this new solution in detail, both analytically and numerically. We describe local bifurcations using a multiple time scales analysis and global bifurcations using Melnikov's method. The coefficients of linear and cubic damping are derived for a standing wave in a rectangular container by considering energy dissipation in the main body of the fluid (due to potential flow and streaming) and in boundary layers at the sidewalls and at the surface. Surface contamination, due to the presence of a thin viscoelastic surface film, creates a boundary layer at the surface which causes enhanced dissipation comparable to, or greater than, that caused by the boundary layers at the walls of the container. Three-mode interaction equations are used to model intermittency and complex modulations which are found to arise from a sideband instability mechanism similar to that of Eckhaus (1963) and Benjamin & Feir (1967). The role of cubic and fifth-order nonlinear terms on this instability mechanism is examined. Theoretical results are found to compare quite favourably with experimental data.The influence of thermal and magnetic layers on solar oscillation frequencies
https://hdl.handle.net/10023/14051
In this thesis, a study is made of the global solar oscillations known as p-modes, modelled by a plane-parallel stratified plasma, within which is embedded a horizontal layered magnetic field. A magnetohydrodynamic formalism is used to investigate the models. The main aim of the thesis is to model the turnover effect in the frequency shifts of the p-modes observed over the course of the solar cycle. Radial oscillations (modes of degree zero) of the Sun are studied for several atmospheric temperature and magnetic field profiles. It is found that the turnover in frequency shifts may be obtained by an increase in the strength of the atmospheric horizontal magnetic field (assumed to be uniform), coupled with a simultaneous increase in atmospheric temperature. The effect of a thin superadiabatic layer in the upper convection zone on p-mode frequencies is also considered. For this model we study modes of general degree, and find that the observed rise and subsequent downturn in the frequency shifts can be duplicated, in the absence of a magnetic field, by simultaneously steepening the temperature gradient of the superadiabatic layer and increasing the atmospheric temperature. In the presence of a magnetic field, where the atmosphere is permeated by a uniform horizontal magnetic field, turnover is reproduced by a combination of an increase in magnetic field strength, a steepening of the temperature gradient in the superadiabatic region, and an increase in atmospheric temperature. The unstable superadiabatic layer also gives rise to convective modes, which are considered briefly. Finally, a model incorporating a magnetic layer residing at the base of the convection zone is constructed and its influence on the frequencies of p-modes assessed. By simply changing the magnetic field strength of this layer, we are unable to reproduce the observed solar cycle variations in p-mode frequencies. The buried magnetic layer supports surface and body magnetoacoustic waves, and a brief study is made of their properties.
Thu, 01 Jan 1998 00:00:00 GMThttps://hdl.handle.net/10023/140511998-01-01T00:00:00ZDaniell, MarkIn this thesis, a study is made of the global solar oscillations known as p-modes, modelled by a plane-parallel stratified plasma, within which is embedded a horizontal layered magnetic field. A magnetohydrodynamic formalism is used to investigate the models. The main aim of the thesis is to model the turnover effect in the frequency shifts of the p-modes observed over the course of the solar cycle. Radial oscillations (modes of degree zero) of the Sun are studied for several atmospheric temperature and magnetic field profiles. It is found that the turnover in frequency shifts may be obtained by an increase in the strength of the atmospheric horizontal magnetic field (assumed to be uniform), coupled with a simultaneous increase in atmospheric temperature. The effect of a thin superadiabatic layer in the upper convection zone on p-mode frequencies is also considered. For this model we study modes of general degree, and find that the observed rise and subsequent downturn in the frequency shifts can be duplicated, in the absence of a magnetic field, by simultaneously steepening the temperature gradient of the superadiabatic layer and increasing the atmospheric temperature. In the presence of a magnetic field, where the atmosphere is permeated by a uniform horizontal magnetic field, turnover is reproduced by a combination of an increase in magnetic field strength, a steepening of the temperature gradient in the superadiabatic region, and an increase in atmospheric temperature. The unstable superadiabatic layer also gives rise to convective modes, which are considered briefly. Finally, a model incorporating a magnetic layer residing at the base of the convection zone is constructed and its influence on the frequencies of p-modes assessed. By simply changing the magnetic field strength of this layer, we are unable to reproduce the observed solar cycle variations in p-mode frequencies. The buried magnetic layer supports surface and body magnetoacoustic waves, and a brief study is made of their properties.Energy-balance models of the solar corona
https://hdl.handle.net/10023/14047
Solar coronal observations have shown that the corona has a highly complex structure which presumably owes its existence to the magnetic field. Models in thermal and hydrostatic equilibrium are here calculated in order to try and explain many of these observations. Coronal holes occur where open field lines reach out into space. The model of McWhirter, et al. (1975) for the inner corona in such a configuration is generalised to allow different types and magnitudes of heating as well as different area divergences and flows. It is found that hot, fast upflows cannot always exist in thermal equilibrium. The choice of boundary conditions can appreciably alter the results, and so different choices are compared. Most of the corona, especially in active regions, appears to consist of coronal loops. Subtle relations for energy balance models of such loops are found to exist between the physical parameters of a loop's length, base density, and heat input. No solution exists at coronal temperatures in certain cases, which may explain the observations of very cool loops. The effect of a loop's geometry and field line divergence on the structure is found. Results predicted from scaling laws are compared, and the uniqueness of the solution for a loop with a fixed mass is studied. The error in the predicted emission measure through assuming uniform pressure is shown to be considerable. The life-time of a loop can often be many days, suggesting the existence of a thermally stable state. A global stability analysis is performed, and it is found that a loop's stability may depend critically upon its length. Thermally isolated loops, which are the most unstable type, can be thermally stable, provided their pressure falls off sufficiently rapidly with height (due to hydrostatic equilibrium).
Fri, 01 Jan 1982 00:00:00 GMThttps://hdl.handle.net/10023/140471982-01-01T00:00:00ZWragg, M. A.Solar coronal observations have shown that the corona has a highly complex structure which presumably owes its existence to the magnetic field. Models in thermal and hydrostatic equilibrium are here calculated in order to try and explain many of these observations. Coronal holes occur where open field lines reach out into space. The model of McWhirter, et al. (1975) for the inner corona in such a configuration is generalised to allow different types and magnitudes of heating as well as different area divergences and flows. It is found that hot, fast upflows cannot always exist in thermal equilibrium. The choice of boundary conditions can appreciably alter the results, and so different choices are compared. Most of the corona, especially in active regions, appears to consist of coronal loops. Subtle relations for energy balance models of such loops are found to exist between the physical parameters of a loop's length, base density, and heat input. No solution exists at coronal temperatures in certain cases, which may explain the observations of very cool loops. The effect of a loop's geometry and field line divergence on the structure is found. Results predicted from scaling laws are compared, and the uniqueness of the solution for a loop with a fixed mass is studied. The error in the predicted emission measure through assuming uniform pressure is shown to be considerable. The life-time of a loop can often be many days, suggesting the existence of a thermally stable state. A global stability analysis is performed, and it is found that a loop's stability may depend critically upon its length. Thermally isolated loops, which are the most unstable type, can be thermally stable, provided their pressure falls off sufficiently rapidly with height (due to hydrostatic equilibrium).Three-dimensional topology of solar coronal magnetic fields
https://hdl.handle.net/10023/14036
This thesis investigates the topology of the magnetic field in the solar corona. It is important have an understanding of how the highly complex coronal magnetic field behaves in order to study many fundamental coronal phenomena, such as coronal heating events, solar flares and polar plumes. The magnetic fields due to three or four discrete sources are investigated and the corresponding topological states are found. The locations of these states in parameter space is calculated and the bifurcations between states are analysed. A complete analysis has been undertaken for the three-source case and a selective one for the four-source case in order to identify new non-generic behaviour. The thesis goes on to study the topological behaviour of a coronal bright point. Different phases during the lifetime of the bright point are identified and the responsible topological behaviour due to the movement of the magnetic fragments in the photosphere is discussed.
Fri, 01 Jan 1999 00:00:00 GMThttps://hdl.handle.net/10023/140361999-01-01T00:00:00ZBrown, Daniel StephenThis thesis investigates the topology of the magnetic field in the solar corona. It is important have an understanding of how the highly complex coronal magnetic field behaves in order to study many fundamental coronal phenomena, such as coronal heating events, solar flares and polar plumes. The magnetic fields due to three or four discrete sources are investigated and the corresponding topological states are found. The locations of these states in parameter space is calculated and the bifurcations between states are analysed. A complete analysis has been undertaken for the three-source case and a selective one for the four-source case in order to identify new non-generic behaviour. The thesis goes on to study the topological behaviour of a coronal bright point. Different phases during the lifetime of the bright point are identified and the responsible topological behaviour due to the movement of the magnetic fragments in the photosphere is discussed.External and internal magnetohydrostatic models of quiescent solar prominences
https://hdl.handle.net/10023/14029
Quiescent solar prominences are amongst the most interesting and yet least understood of the phenomena observed on the Sun and provide both the theorist and the observer with equally demanding challenges. The theoretical study of prominences is an important branch of solar physics as it contributes significantly to the overall understanding of the Sun and its atmosphere. One only needs to be presented with the illuminating fact that there is more mass contained in these bodies than in the remainder of the entire corona to be convinced of their importance. Although many of the physical mechanisms associated with prominence theory are important in their own right, they are also of much wider relevance for various other astrophysical phenomena. For example, radiative and magnetic instabilities are explored in detail in the context of solar prominences; yet clearly these are important processes that relate to many other branches of astrophysics. Prominences are intimately associated with solar flares which occur when a prominence loses equilibrium. Also, prominence eruptions are very important as they are closely connected with coronal mass ejections. These account for a large fraction of the total mass lost from the Sun and so are extremely important events, particularly when one considers the consequences as this plasma interacts with the Earth's environment. It is the period of global equilibrium of quiescent prominences, though, that is the focus of this thesis. Various models are proposed to help understand both the topology and supporting mechanisms of the external, coronal magnetic field, and also the internal prominence structure and the way in which the two regimes fit together. In Chapter 3 we extend a model for the equilibrium of a prominence sheet in a twisted magnetic flux-tube, given by Ridgway, Priest and Amari (1991), to incorporate a current sheet of finite height. This removes the discontinuity at the edge of the tube and provides a shear-free outer boundary which enables the tube to be matched onto a background potential field. In addition, internal prominence solutions are found by expanding the sheet to a finite width and matching suitable magnetic profiles across this region. Next we consider a global model for the magnetic field structure surrounding a polar-crown prominence. We examine potential configurations generated from typical distributions of photospheric flux, and select solutions for which there is a location of dipped magnetic field where prominence material may collect and form. Once such a configuration is available, it is necessary to construct the ensuing prominence solution. We achieve this in Chapter 4 by considering a simplified form for the photospheric field. We show that the equilibrium contains a weighted, curved prominence sheet supported in the location of dipped magnetic field. The equilibrium requires an enhanced magnetic pressure below the sheet to support the component of weight in the normal direction. The internal equilibrium of curved or inclined prominence material has not been considered previously and so we formulate, in Chapter 6, a simple one-dimensional isothermal solution for a cut across the prominence. This is developed to allow for variations along the sheet and in this way an internal solution for the curved prominence of Chapter 4 is given, which matches onto the external potential polar-crown field. Finally, in Chapter 7, we rewrite this solution in terms of its constituent internal and external components and show how the composite solution switches between the two in a region of overlap, or transition region. From this, the internal plasma properties are deduced and realistic profiles for the pressure, density and temperature are obtained.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/140291996-01-01T00:00:00ZCartledge, Nicholas P.Quiescent solar prominences are amongst the most interesting and yet least understood of the phenomena observed on the Sun and provide both the theorist and the observer with equally demanding challenges. The theoretical study of prominences is an important branch of solar physics as it contributes significantly to the overall understanding of the Sun and its atmosphere. One only needs to be presented with the illuminating fact that there is more mass contained in these bodies than in the remainder of the entire corona to be convinced of their importance. Although many of the physical mechanisms associated with prominence theory are important in their own right, they are also of much wider relevance for various other astrophysical phenomena. For example, radiative and magnetic instabilities are explored in detail in the context of solar prominences; yet clearly these are important processes that relate to many other branches of astrophysics. Prominences are intimately associated with solar flares which occur when a prominence loses equilibrium. Also, prominence eruptions are very important as they are closely connected with coronal mass ejections. These account for a large fraction of the total mass lost from the Sun and so are extremely important events, particularly when one considers the consequences as this plasma interacts with the Earth's environment. It is the period of global equilibrium of quiescent prominences, though, that is the focus of this thesis. Various models are proposed to help understand both the topology and supporting mechanisms of the external, coronal magnetic field, and also the internal prominence structure and the way in which the two regimes fit together. In Chapter 3 we extend a model for the equilibrium of a prominence sheet in a twisted magnetic flux-tube, given by Ridgway, Priest and Amari (1991), to incorporate a current sheet of finite height. This removes the discontinuity at the edge of the tube and provides a shear-free outer boundary which enables the tube to be matched onto a background potential field. In addition, internal prominence solutions are found by expanding the sheet to a finite width and matching suitable magnetic profiles across this region. Next we consider a global model for the magnetic field structure surrounding a polar-crown prominence. We examine potential configurations generated from typical distributions of photospheric flux, and select solutions for which there is a location of dipped magnetic field where prominence material may collect and form. Once such a configuration is available, it is necessary to construct the ensuing prominence solution. We achieve this in Chapter 4 by considering a simplified form for the photospheric field. We show that the equilibrium contains a weighted, curved prominence sheet supported in the location of dipped magnetic field. The equilibrium requires an enhanced magnetic pressure below the sheet to support the component of weight in the normal direction. The internal equilibrium of curved or inclined prominence material has not been considered previously and so we formulate, in Chapter 6, a simple one-dimensional isothermal solution for a cut across the prominence. This is developed to allow for variations along the sheet and in this way an internal solution for the curved prominence of Chapter 4 is given, which matches onto the external potential polar-crown field. Finally, in Chapter 7, we rewrite this solution in terms of its constituent internal and external components and show how the composite solution switches between the two in a region of overlap, or transition region. From this, the internal plasma properties are deduced and realistic profiles for the pressure, density and temperature are obtained.Dynamical processes in the solar atmosphere
https://hdl.handle.net/10023/14024
It has become clear that the closed-field regions of the solar atmosphere are not static (as was once thought) but that many types of steady and unsteady flows and other dynamical, processes such as flares are continually occurring, in them. This thesis investigates some theoretical aspects of these dynamical phenomena. Steady, one-dimensional flow along a coronal loop is investigated first of all. Such a flow may be driven by a pressure difference between the foot points, and a wide range of shocked and unshocked flows are found. The presence of steady flows removes the symmetry present in most static loop models, and these models are shown to form only one class of a much wider range of dynamic solutions to the equations of motion. Thermal non-equilibrium in hot coronal loops occurs if the pressure in a loop becomes too big. The non-linear evolution of this non-equilibrium state is followed, and the loop is found to cool from of order 10[super]6 K to below 10[super]5 K in a few hours. An upflow is driven, and non-equilibrium is suggested as a means of formation of either cool loop cores or prominences. Thermal non-equilibrium is also discussed as a possible mechanism for the simple-loop flare. It is suggested that a cool equilibrium at a temperature of a few times 10[super]4 K can flare to over. 10[super]7 K if the mechanical heating in the cool loop becomes too large. The evolution is followed and the loop is found to flare to over 10[super]7 K in approximately 5 minutes. Magnetohydrodynamic shock waves have long been regarded as a potentially efficient heating mechanism. Their behaviour is re-examined here, and it is found that certain types of shock can release very large amounts of energy. These results are then applied to the heating of "post"-flare loops, for which temperatures of 10[super]7 K at the loop summit may be obtained. Finally, some solutions of the magnetostatic equation are discussed, and it is pointed out that if the gas pressure is too big then magnetostatic equilibrium will break down. It is suggested that the subsequent evolution may give rise to a surge or other mass ejection.
Fri, 01 Jan 1982 00:00:00 GMThttps://hdl.handle.net/10023/140241982-01-01T00:00:00ZCargill, P. (Peter)It has become clear that the closed-field regions of the solar atmosphere are not static (as was once thought) but that many types of steady and unsteady flows and other dynamical, processes such as flares are continually occurring, in them. This thesis investigates some theoretical aspects of these dynamical phenomena. Steady, one-dimensional flow along a coronal loop is investigated first of all. Such a flow may be driven by a pressure difference between the foot points, and a wide range of shocked and unshocked flows are found. The presence of steady flows removes the symmetry present in most static loop models, and these models are shown to form only one class of a much wider range of dynamic solutions to the equations of motion. Thermal non-equilibrium in hot coronal loops occurs if the pressure in a loop becomes too big. The non-linear evolution of this non-equilibrium state is followed, and the loop is found to cool from of order 10[super]6 K to below 10[super]5 K in a few hours. An upflow is driven, and non-equilibrium is suggested as a means of formation of either cool loop cores or prominences. Thermal non-equilibrium is also discussed as a possible mechanism for the simple-loop flare. It is suggested that a cool equilibrium at a temperature of a few times 10[super]4 K can flare to over. 10[super]7 K if the mechanical heating in the cool loop becomes too large. The evolution is followed and the loop is found to flare to over 10[super]7 K in approximately 5 minutes. Magnetohydrodynamic shock waves have long been regarded as a potentially efficient heating mechanism. Their behaviour is re-examined here, and it is found that certain types of shock can release very large amounts of energy. These results are then applied to the heating of "post"-flare loops, for which temperatures of 10[super]7 K at the loop summit may be obtained. Finally, some solutions of the magnetostatic equation are discussed, and it is pointed out that if the gas pressure is too big then magnetostatic equilibrium will break down. It is suggested that the subsequent evolution may give rise to a surge or other mass ejection.Topological configurations of coronal magnetic fields and current sheets
https://hdl.handle.net/10023/14021
The question of topology in the coronal magnetic field is addressed in this thesis. Magnetic reconnection, which plays a major role in many of the fascinating phenomena seen in the solar atmosphere, is likely to occur at the boundaries between different topological regions of the magnetic field. By modelling the coronal field using discrete sources of flux, to represent the concentrations seen at the photospheric surface, we study the varying topological structures present in the field. We generate a criterion for determining the presence of null points above the photospheric surface and establish that any separatrix surfaces present in the field are due to the presence of either null points, or regions where the field tangentially grazes the surface. We follow the evolution of these separatrix surfaces and, in particular, determine the existence of a well-defined separator field line in the absence of coronal null points. Finally, we look locally at the configuration of the magnetic field in the region surrounding a straight current sheet. We derive an analytical expression to describe the topology of both potential and constant-current force-free fields in the neighbourhood of a sheet, and in so doing generalise the previously known expressions.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/140211996-01-01T00:00:00ZBungey, Timothy N.The question of topology in the coronal magnetic field is addressed in this thesis. Magnetic reconnection, which plays a major role in many of the fascinating phenomena seen in the solar atmosphere, is likely to occur at the boundaries between different topological regions of the magnetic field. By modelling the coronal field using discrete sources of flux, to represent the concentrations seen at the photospheric surface, we study the varying topological structures present in the field. We generate a criterion for determining the presence of null points above the photospheric surface and establish that any separatrix surfaces present in the field are due to the presence of either null points, or regions where the field tangentially grazes the surface. We follow the evolution of these separatrix surfaces and, in particular, determine the existence of a well-defined separator field line in the absence of coronal null points. Finally, we look locally at the configuration of the magnetic field in the region surrounding a straight current sheet. We derive an analytical expression to describe the topology of both potential and constant-current force-free fields in the neighbourhood of a sheet, and in so doing generalise the previously known expressions.Parametric instabilities in inhomogenous plasmas
https://hdl.handle.net/10023/14016
This thesis will deal with certain problems of parametric instabilities in the inhomogeneous plasma. A large amplitude, 'pump' wave can deposit some of its energy into the plasma through resonance with two lower frequency waves (which may be damped). This type of process is a parametric decay of the pump wave and has applications in many fields. We consider, predominantly, that of laser fusion, in which the pump wave is electromagnetic and incident on the plasma. The objective is to deposit as much energy as possible within the plasma. Instabilities reducing this energy input are therefore of importance and it is, mostly, to these that this thesis will turn. They are mostly scattering processes in which one of the decay modes is electromagnetic. We examine the stimulated Brillouin backscattering process (the other decay mode being an ion accoustic wave) from a reference frame in which the plasma is streaming outwards. It is found that, if this velocity is near the sound velocity, the ion acoustic wave has a frequency Doppler-shifted to zero, the electromagnetic waves then having equal frequencies. In such a situation, any reflection of the pump wave at the critical surface will enhance the initial level of the backscattered wave. We find that, allowing for this, there is considerable enhancement of backscatter from the plasma, with consequent energy loss to the pump. Since the effect is noticeably unaffected by 'off- resonance' situations, it is felt that this process could mount a barrier to possible applications. We next consider the stimulated Compton scattering process, where the pump is scattered off the 'bare' or thermal electrons in the plasma. It is found that this rather weak instability occurs predominantly only when electron plasma waves are heavily dampled. Substantial reflection only occurs for high pump powers. Whilst there is little loss to the pump energy, there is substantial perturbation to the background distribution function. However, at the high powers involved filamentation and modulation of the pump can occur with a resulting enhancement of the scattering. Finally, we consider the effect on the decay instability (photon → plasmon + phonon) of the presence of substantial filamentation of the critical surface. It is found that the growth rate is substantially reduced.
Thu, 01 Jan 1976 00:00:00 GMThttps://hdl.handle.net/10023/140161976-01-01T00:00:00ZBegg, Iain M.This thesis will deal with certain problems of parametric instabilities in the inhomogeneous plasma. A large amplitude, 'pump' wave can deposit some of its energy into the plasma through resonance with two lower frequency waves (which may be damped). This type of process is a parametric decay of the pump wave and has applications in many fields. We consider, predominantly, that of laser fusion, in which the pump wave is electromagnetic and incident on the plasma. The objective is to deposit as much energy as possible within the plasma. Instabilities reducing this energy input are therefore of importance and it is, mostly, to these that this thesis will turn. They are mostly scattering processes in which one of the decay modes is electromagnetic. We examine the stimulated Brillouin backscattering process (the other decay mode being an ion accoustic wave) from a reference frame in which the plasma is streaming outwards. It is found that, if this velocity is near the sound velocity, the ion acoustic wave has a frequency Doppler-shifted to zero, the electromagnetic waves then having equal frequencies. In such a situation, any reflection of the pump wave at the critical surface will enhance the initial level of the backscattered wave. We find that, allowing for this, there is considerable enhancement of backscatter from the plasma, with consequent energy loss to the pump. Since the effect is noticeably unaffected by 'off- resonance' situations, it is felt that this process could mount a barrier to possible applications. We next consider the stimulated Compton scattering process, where the pump is scattered off the 'bare' or thermal electrons in the plasma. It is found that this rather weak instability occurs predominantly only when electron plasma waves are heavily dampled. Substantial reflection only occurs for high pump powers. Whilst there is little loss to the pump energy, there is substantial perturbation to the background distribution function. However, at the high powers involved filamentation and modulation of the pump can occur with a resulting enhancement of the scattering. Finally, we consider the effect on the decay instability (photon → plasmon + phonon) of the presence of substantial filamentation of the critical surface. It is found that the growth rate is substantially reduced.Magnetic annihilation and reconnection
https://hdl.handle.net/10023/14014
This thesis presents several analytical models of magnetic annihilation and reconnection and studies their properties. The models investigated are 1. Steady-state magnetic annihilation. The assumption of straight field lines reduces the resistive, viscous MHD equations to two ordinary differential equations, one for the flow and one for the magnetic field. These equations can be solved exactly (for the case of a simple stagnation-point flow) and asymptotically (for a more general stagnation-point flow). In both cases the reconnection rates can be fast due to advection effects which create large magnetic gradients. 2. Time-dependent magnetic annihilation. The assumption of straight field lines whose strength can vary with time reduces the MHD equations to two partial differential equations, one for the flow and one for the magnetic field. The time-modulated simple stagnation-point flow is shown to be an exact solution and the equation for the magnetic field is then solved on infinite and finite intervals. For the infinite interval the reconnection rates are shown to be dependent on the nature of the advected initial field. Also examined are self-similar solutions and the effect of variation of diffusivity with time. 3. Annihilation in a compressible, inviscid plasma. Here, the assumption of straight field lines and an inviscid, compressible flow reduce the MHD equations to a pair of non-linear coupled partial differential equations. Further assuming that the density only varies in one direction and the flow is of a stagnation-point type allow these equations to be solved approximately analytically and exactly numerically. It is shown that the magnetic field and reconnection rates are the same in both the compressible and incompressible cases and that the density of the plasma is greatest within the current sheet. 4. Steady-state magnetic reconnection. For an incompressible flow the MHD equations can be reduced to two coupled non-linear partial differential equations. These two equations are studied by seeking asymptotic solutions around the annihilation solution and then looking for series solutions to the first-order equations. It is found that the magnetic field always has a magnetic cusp and never possesses an x-type neutral point. 5. Reconnection in a viscous plasma. Assuming that the viscous forces dominate, the induction equation and equation of motion decouple and become linear. The magnetic field is obtained for the case of a simple stagnation-point flow. It is shown that if the inflow magnetic field is taken to be straight then the magnetic field within the region tends towards the annihilation solution as the magnetic Reynolds number increases. 6. Magnetic flipping. A previous ideal model of magnetic flipping is refined so that it becomes an exact solution of the MHD equations. In the refined model the streamlines are straight rather than curved. Assuming straight streamlines, the MHD equations reduce to two linear ordinary differential equations, one for the flow and one for the magnetic field. These are then solved exactly analytically to find a flow containing a viscous boundary layer and a magnetic field that contains an x-type neutral point. The angle between the separatrices of the field is determined by the Reynolds and magnetic Reynolds numbers. It is shown that most of the ohmic heating occurs within the viscous boundary layer.
Sat, 01 Jan 1994 00:00:00 GMThttps://hdl.handle.net/10023/140141994-01-01T00:00:00ZAnderson, CraigThis thesis presents several analytical models of magnetic annihilation and reconnection and studies their properties. The models investigated are 1. Steady-state magnetic annihilation. The assumption of straight field lines reduces the resistive, viscous MHD equations to two ordinary differential equations, one for the flow and one for the magnetic field. These equations can be solved exactly (for the case of a simple stagnation-point flow) and asymptotically (for a more general stagnation-point flow). In both cases the reconnection rates can be fast due to advection effects which create large magnetic gradients. 2. Time-dependent magnetic annihilation. The assumption of straight field lines whose strength can vary with time reduces the MHD equations to two partial differential equations, one for the flow and one for the magnetic field. The time-modulated simple stagnation-point flow is shown to be an exact solution and the equation for the magnetic field is then solved on infinite and finite intervals. For the infinite interval the reconnection rates are shown to be dependent on the nature of the advected initial field. Also examined are self-similar solutions and the effect of variation of diffusivity with time. 3. Annihilation in a compressible, inviscid plasma. Here, the assumption of straight field lines and an inviscid, compressible flow reduce the MHD equations to a pair of non-linear coupled partial differential equations. Further assuming that the density only varies in one direction and the flow is of a stagnation-point type allow these equations to be solved approximately analytically and exactly numerically. It is shown that the magnetic field and reconnection rates are the same in both the compressible and incompressible cases and that the density of the plasma is greatest within the current sheet. 4. Steady-state magnetic reconnection. For an incompressible flow the MHD equations can be reduced to two coupled non-linear partial differential equations. These two equations are studied by seeking asymptotic solutions around the annihilation solution and then looking for series solutions to the first-order equations. It is found that the magnetic field always has a magnetic cusp and never possesses an x-type neutral point. 5. Reconnection in a viscous plasma. Assuming that the viscous forces dominate, the induction equation and equation of motion decouple and become linear. The magnetic field is obtained for the case of a simple stagnation-point flow. It is shown that if the inflow magnetic field is taken to be straight then the magnetic field within the region tends towards the annihilation solution as the magnetic Reynolds number increases. 6. Magnetic flipping. A previous ideal model of magnetic flipping is refined so that it becomes an exact solution of the MHD equations. In the refined model the streamlines are straight rather than curved. Assuming straight streamlines, the MHD equations reduce to two linear ordinary differential equations, one for the flow and one for the magnetic field. These are then solved exactly analytically to find a flow containing a viscous boundary layer and a magnetic field that contains an x-type neutral point. The angle between the separatrices of the field is determined by the Reynolds and magnetic Reynolds numbers. It is shown that most of the ohmic heating occurs within the viscous boundary layer.Plasma drift waves and instabilities
https://hdl.handle.net/10023/14011
The work of this thesis is concerned with the investigation of the propagation of waves in a magnetized plasma containing various parameter gradients, and with the stability of ion acoustic waves in a weakly collisional plasma with a strong temperature gradient. The thesis is divided into three sections. In the first section the intention is to derive in a compact and unambiguous tensor form the dispersion relation describing the propagation of waves in a magnetized plasma containing three-dimensional density and temperature gradients, an E̲⏜ B̲ drift, and differing temperatures parallel and perpendicular to the magnetic field. This is achieved by introducing and extending the polarized co-ordinate system first proposed by Buneman in 1961, and then carrying through the standard procedure of integration along unperturbed trajectories. The "local" approximation of Krall and Rosenbluth is used in order that an analytic result may be derived. The dispersion relation obtained includes certain moment tensors whose elements may be evaluated independently of the gradients involved in the problem. These elements may then be listed and the list referred to in order to obtain the elements required for a specific problem. The second section is concerned with the use of the theory and results of J.P. Dougherty to show that in the high-frequency regime the introduction of a small amount of collisions into a plasma is sufficient to disrupt the gyro-resonances which allow the existence of Bernstein waves at multiples of the gyro-frequencies perpendicular and near- perpendicular to the magnetic field. It is shown that a collision frequency v such that (k 𝜌) ⁻² ≲ v/Ω < (k 𝜌) ⁻¹ where k 𝜌 >> 1 is sufficient to do this; k is the wave-number, 𝜌 the Larmor radius, and the gyro-frequency. It is also shown that in this case the ion-acoustic dispersion relation is valid even for propagation perpendicular to the magnetic field. In the final section the result of the second section is used to derive a dispersion relation for high-frequency wave propagation in a weakly-collisional plasma containing an electron temperature gradient. The dispersion relation is solved numerically for various electron-ion temperature ratios and electron temperature gradient drift velocities. Earlier predictions, based on analytic calculations for small temperature ratios and drift velocities, are confirmed and some new results presented. In particular, it is shown that a temperature gradient is a more effective destabilizing agent then a simple drift between ions and electrons. Dispersion plots are given, along with analytic and physical explanations of their form; finally neutral stability curves are presented. The thesis concludes with a summary of the results obtained.
Tue, 01 Jan 1974 00:00:00 GMThttps://hdl.handle.net/10023/140111974-01-01T00:00:00ZAllan, WilliamThe work of this thesis is concerned with the investigation of the propagation of waves in a magnetized plasma containing various parameter gradients, and with the stability of ion acoustic waves in a weakly collisional plasma with a strong temperature gradient. The thesis is divided into three sections. In the first section the intention is to derive in a compact and unambiguous tensor form the dispersion relation describing the propagation of waves in a magnetized plasma containing three-dimensional density and temperature gradients, an E̲⏜ B̲ drift, and differing temperatures parallel and perpendicular to the magnetic field. This is achieved by introducing and extending the polarized co-ordinate system first proposed by Buneman in 1961, and then carrying through the standard procedure of integration along unperturbed trajectories. The "local" approximation of Krall and Rosenbluth is used in order that an analytic result may be derived. The dispersion relation obtained includes certain moment tensors whose elements may be evaluated independently of the gradients involved in the problem. These elements may then be listed and the list referred to in order to obtain the elements required for a specific problem. The second section is concerned with the use of the theory and results of J.P. Dougherty to show that in the high-frequency regime the introduction of a small amount of collisions into a plasma is sufficient to disrupt the gyro-resonances which allow the existence of Bernstein waves at multiples of the gyro-frequencies perpendicular and near- perpendicular to the magnetic field. It is shown that a collision frequency v such that (k 𝜌) ⁻² ≲ v/Ω < (k 𝜌) ⁻¹ where k 𝜌 >> 1 is sufficient to do this; k is the wave-number, 𝜌 the Larmor radius, and the gyro-frequency. It is also shown that in this case the ion-acoustic dispersion relation is valid even for propagation perpendicular to the magnetic field. In the final section the result of the second section is used to derive a dispersion relation for high-frequency wave propagation in a weakly-collisional plasma containing an electron temperature gradient. The dispersion relation is solved numerically for various electron-ion temperature ratios and electron temperature gradient drift velocities. Earlier predictions, based on analytic calculations for small temperature ratios and drift velocities, are confirmed and some new results presented. In particular, it is shown that a temperature gradient is a more effective destabilizing agent then a simple drift between ions and electrons. Dispersion plots are given, along with analytic and physical explanations of their form; finally neutral stability curves are presented. The thesis concludes with a summary of the results obtained.Aspects of current sheet theory
https://hdl.handle.net/10023/14000
Current sheets are widely believed to play an important role in astrophysics when regions of magnetic flux are in motion. Several models based on the formation of current sheets have been proposed to explain such phenomena as geomagnetic storms, solar flares and prominences. In this thesis three aspects of current sheet theory are studied with particular reference to the solar flare problem. Firstly the development of two-dimensional current sheets is investigated for several simple configurations. These include converging line current sources, converging and diverging line dipole sources and a dipole of increasing moment situated in either a uniform magnetic field or a constant dipole field. These last two may be thought of as modelling the emergence of bipolar flux from beneath the photosphere, a phenomena frequently observed prior to solar flares. The length, position and shape of the current sheet is determined from the requirement that the magnetic field be frozen-into the plasma. The sheet is found to be curved, except in the symmetrical case of converging line sources. In addition, the extra energy due to the presence of the current sheet is determined. Comparison with estimates of the energy dissipated during a flare indicate that the formation of current sheets may store an adequate amount of preflare magnetic energy, provided no reconnection occurs during the formation process. A three-dimensional axi-symmetric model for current sheet formation is then considered. Two equal and co-directional dipoles approach along the axis of symmetry to form an annular current sheet between them. The equations determining the magnetic field for this configuration are reduced to a single integral equation for the current density in the sheet as a function of radial distance from the axis. A numerical method is used to solve this integral equation. The inner and outer radii of the sheet are then determined from the conditions of flux conservation as for the two-dimensional case. Finally the energetics of a current sheet that forms between newly emerging flux and an ambient field are considered. As more and more flux emerges, so the sheet rises in the solar atmosphere. The various contributions to the thermal energy balance in the sheet are approximated and the resulting equation is solved for the internal temperature of the sheet. It is found that, for certain choices of the ambient magnetic field strength and velocity, the internal temperature increases until, when the sheet reaches some critical height, no neighbouring stable state exists. The temperature then increases rapidly seeking a hotter branch of the solution curve. During this dynamic heating the threshold temperature for the onset of microinstabilities may be attained. It is suggested that this may be a suitable trigger mechanism for the recently proposed "emerging flux" model of a solar flare.
Sat, 01 Jan 1977 00:00:00 GMThttps://hdl.handle.net/10023/140001977-01-01T00:00:00ZTur, T. J.Current sheets are widely believed to play an important role in astrophysics when regions of magnetic flux are in motion. Several models based on the formation of current sheets have been proposed to explain such phenomena as geomagnetic storms, solar flares and prominences. In this thesis three aspects of current sheet theory are studied with particular reference to the solar flare problem. Firstly the development of two-dimensional current sheets is investigated for several simple configurations. These include converging line current sources, converging and diverging line dipole sources and a dipole of increasing moment situated in either a uniform magnetic field or a constant dipole field. These last two may be thought of as modelling the emergence of bipolar flux from beneath the photosphere, a phenomena frequently observed prior to solar flares. The length, position and shape of the current sheet is determined from the requirement that the magnetic field be frozen-into the plasma. The sheet is found to be curved, except in the symmetrical case of converging line sources. In addition, the extra energy due to the presence of the current sheet is determined. Comparison with estimates of the energy dissipated during a flare indicate that the formation of current sheets may store an adequate amount of preflare magnetic energy, provided no reconnection occurs during the formation process. A three-dimensional axi-symmetric model for current sheet formation is then considered. Two equal and co-directional dipoles approach along the axis of symmetry to form an annular current sheet between them. The equations determining the magnetic field for this configuration are reduced to a single integral equation for the current density in the sheet as a function of radial distance from the axis. A numerical method is used to solve this integral equation. The inner and outer radii of the sheet are then determined from the conditions of flux conservation as for the two-dimensional case. Finally the energetics of a current sheet that forms between newly emerging flux and an ambient field are considered. As more and more flux emerges, so the sheet rises in the solar atmosphere. The various contributions to the thermal energy balance in the sheet are approximated and the resulting equation is solved for the internal temperature of the sheet. It is found that, for certain choices of the ambient magnetic field strength and velocity, the internal temperature increases until, when the sheet reaches some critical height, no neighbouring stable state exists. The temperature then increases rapidly seeking a hotter branch of the solution curve. During this dynamic heating the threshold temperature for the onset of microinstabilities may be attained. It is suggested that this may be a suitable trigger mechanism for the recently proposed "emerging flux" model of a solar flare.Thermal and resistive instabilities in the solar atmosphere
https://hdl.handle.net/10023/13998
The magnetic field greatly influences the plasma in the solar atmosphere and in this thesis we consider the effect of the field on the stability of the plasma. The many observations that have been made suggest that two types of field structure play a major role. Firstly a current sheet - this has field lines which change direction in a thin, current forming region, but are fairly uniform outside. We consider the case where the field strength is zero along the neutral line so that a gas pressure gradient is required across the sheet to balance the magnetic pressure gradient. Secondly a force-free field - here the magnetic force is zero, which requires the magnetic pressure to be much larger than the gas pressure. In the neutral current sheet we examine the thermal instability and the tearing-mode instability. While in the force-free magnetic arch system we look for a thermal instability which can occur when the foot points of the arch are sheared. When we investigated the thermal stability of the current sheet we found that as its length increases it passes through a series of stable equilibria until a value, L[sub]max, is reached when the sheet cools down to a max new stable equilibrium. For coronal conditions, values for L[sub]max and max cooling time are in fair agreement with the observed values for quiescent prominences. We calculate the growth rate of the tearing-mode instability in a neutral current sheet with no energy sources or sinks and find that the maximum growth rate can be significantly larger in the current sheet than in the sheared field of constant magnitude considered by others. Also the growth rate decreases when the ratio of gas to magnetic pressure is reduced. We find that the growth rate is significantly inhibited if the current sheet has a transverse magnetic field which is large enough. Lastly we examine the thermal balance in a sheared, force-free magnetic field and show that thermal instability can occur if the field is sheared enough. We assume thermal equilibrium between radiative loss and thermal conduction and we take gravity balanced by a pressure gradient. If, for example, the density at the base of the field is ten times larger than the normal coronal value, as it may be in coronal condensations, then there is instability if the shear angle is greater than 63 °. The presence of a large enough mechanical heating is found to prevent the instability occurring.
Sat, 01 Jan 1977 00:00:00 GMThttps://hdl.handle.net/10023/139981977-01-01T00:00:00ZSmith, E. A.The magnetic field greatly influences the plasma in the solar atmosphere and in this thesis we consider the effect of the field on the stability of the plasma. The many observations that have been made suggest that two types of field structure play a major role. Firstly a current sheet - this has field lines which change direction in a thin, current forming region, but are fairly uniform outside. We consider the case where the field strength is zero along the neutral line so that a gas pressure gradient is required across the sheet to balance the magnetic pressure gradient. Secondly a force-free field - here the magnetic force is zero, which requires the magnetic pressure to be much larger than the gas pressure. In the neutral current sheet we examine the thermal instability and the tearing-mode instability. While in the force-free magnetic arch system we look for a thermal instability which can occur when the foot points of the arch are sheared. When we investigated the thermal stability of the current sheet we found that as its length increases it passes through a series of stable equilibria until a value, L[sub]max, is reached when the sheet cools down to a max new stable equilibrium. For coronal conditions, values for L[sub]max and max cooling time are in fair agreement with the observed values for quiescent prominences. We calculate the growth rate of the tearing-mode instability in a neutral current sheet with no energy sources or sinks and find that the maximum growth rate can be significantly larger in the current sheet than in the sheared field of constant magnitude considered by others. Also the growth rate decreases when the ratio of gas to magnetic pressure is reduced. We find that the growth rate is significantly inhibited if the current sheet has a transverse magnetic field which is large enough. Lastly we examine the thermal balance in a sheared, force-free magnetic field and show that thermal instability can occur if the field is sheared enough. We assume thermal equilibrium between radiative loss and thermal conduction and we take gravity balanced by a pressure gradient. If, for example, the density at the base of the field is ten times larger than the normal coronal value, as it may be in coronal condensations, then there is instability if the shear angle is greater than 63 °. The presence of a large enough mechanical heating is found to prevent the instability occurring.Numerical studies of the Fokker-Planck equation
https://hdl.handle.net/10023/13995
Jorna and Wood recently developed a program that numerically solved the Fokker-Planck equation in spherical geometry. In this thesis, we describe how the original program has been redeveloped to produce a program that is an order of magnitude quicker and that has superior energy and density conservation. The revised version of the program has been used to extend the work of Jorna and Wood on thermal conduction in laser produced plasmas. It has been shown that the effect of curvature on heat flow can be described from a purely geometrical argument and that for aspect ratios similar to those found in targets, the heat flow is reduced by approximately 10%. Also, it has been shown, in contradiction with Jorna and Wood, that the inclusion of the anisotropic portion of the Rosenbluth potentials does not have a significant effect on the heat flow. Even for highly anisotropic plasmas, the inclusion of the anisotropic portion only increases the heat flow by 10%. In addition, the revised version of the program has been used to study the energy relaxation of model distributions It has been shown that the relaxation time of most non - thermal distributions depends on the detailed structure of the distribution and that the normal Spitzer collision time can under-estimate or over-estimate the time required for energy relaxation.
Wed, 01 Jan 1992 00:00:00 GMThttps://hdl.handle.net/10023/139951992-01-01T00:00:00ZMcGowan, Alastair DavidJorna and Wood recently developed a program that numerically solved the Fokker-Planck equation in spherical geometry. In this thesis, we describe how the original program has been redeveloped to produce a program that is an order of magnitude quicker and that has superior energy and density conservation. The revised version of the program has been used to extend the work of Jorna and Wood on thermal conduction in laser produced plasmas. It has been shown that the effect of curvature on heat flow can be described from a purely geometrical argument and that for aspect ratios similar to those found in targets, the heat flow is reduced by approximately 10%. Also, it has been shown, in contradiction with Jorna and Wood, that the inclusion of the anisotropic portion of the Rosenbluth potentials does not have a significant effect on the heat flow. Even for highly anisotropic plasmas, the inclusion of the anisotropic portion only increases the heat flow by 10%. In addition, the revised version of the program has been used to study the energy relaxation of model distributions It has been shown that the relaxation time of most non - thermal distributions depends on the detailed structure of the distribution and that the normal Spitzer collision time can under-estimate or over-estimate the time required for energy relaxation.Nonlinear plasma waves and their applications
https://hdl.handle.net/10023/13993
The possibility of beat wave current drive in tokamaks is considered in this thesis in steady state 2D geometry. The problem is considered by including in the analysis the 2D toroidal inhomogeneity effect and the effect of finite spatial width of the pump microwave pulses on the beat wave excitation. Both a Langmuir beat wave as well as an obliquely propagating upper-hybrid cyclotron beat wave are considered in this study. The three wave coupled system of equations in a magnetized plasma has been derived and solved numerically for this purpose. It has been found that Langmuir type beat wave excited by two almost antiparallel pump microwaves is more efficient for action transfer than a cyclotron beat wave. It has also been found that for the same input parameters, right hand polarized pumps are more efficient than left hand polarized pump microwaves for depositing power in the beat wave. The second part of the thesis considers the relativistic excitation mechanism of a large amplitude plasma wake field by a single ultra-short laser pulse. This type of large amplitude wake field has been proposed for particle acceleration to very high energies for future generation of accelerators. The problem has been modeled self consistently in ID geometry and the relevant coupled system of equations have been solved numerically. It has been found that the shape of the laser pulse profile and the ratio of the ambient plasma frequency to the incident laser frequency play an important role for the excitation of the wake-field and the stability of the laser pulse profile.
Fri, 01 Jan 1999 00:00:00 GMThttps://hdl.handle.net/10023/139931999-01-01T00:00:00ZAmin, Mohamed RuhulThe possibility of beat wave current drive in tokamaks is considered in this thesis in steady state 2D geometry. The problem is considered by including in the analysis the 2D toroidal inhomogeneity effect and the effect of finite spatial width of the pump microwave pulses on the beat wave excitation. Both a Langmuir beat wave as well as an obliquely propagating upper-hybrid cyclotron beat wave are considered in this study. The three wave coupled system of equations in a magnetized plasma has been derived and solved numerically for this purpose. It has been found that Langmuir type beat wave excited by two almost antiparallel pump microwaves is more efficient for action transfer than a cyclotron beat wave. It has also been found that for the same input parameters, right hand polarized pumps are more efficient than left hand polarized pump microwaves for depositing power in the beat wave. The second part of the thesis considers the relativistic excitation mechanism of a large amplitude plasma wake field by a single ultra-short laser pulse. This type of large amplitude wake field has been proposed for particle acceleration to very high energies for future generation of accelerators. The problem has been modeled self consistently in ID geometry and the relevant coupled system of equations have been solved numerically. It has been found that the shape of the laser pulse profile and the ratio of the ambient plasma frequency to the incident laser frequency play an important role for the excitation of the wake-field and the stability of the laser pulse profile.The effects of the Kelvin-Helmholtz instability of the magnetosphere
https://hdl.handle.net/10023/13990
In this thesis, the behaviour of Kelvin-Helmholtz unstable modes on the magnetospheric flanks and in the magnetotail are investigated. A model of a straight bounded magnetosphere connected to a semi-infinite field-free magnetosheath which is flowing with a uniform speed is used. First the magnetosphere is taken to be uniform with the magnetic field perpendicular to the flow in the magnetosheath and it is shown that the increase in Pc5 wave power observed for high solar wind flow speeds correlates well with the onset of instability of the fast body modes. A condition for the exact onset of instability of these modes is derived and the behaviour of fast surface and slow body and surface modes is also investigated. Using a non-uniform magnetosphere, it is shown that these unstable body modes may couple to field line resonances. The fastest growing modes are found to have a common azimuthal phase speed which depends only on the local conditions at the magnetopause and may be predicted using the theory of over-reflection. A finite width boundary layer is then added to the uniform magnetosphere model to investigate the space-time evolution of wave-packets on the magnetopause. Fast surface mode wave-packets are found to grow rapidly as they convect around the flanks so that non-linear effects will be important. Fast cavity mode wave-packets will remain relatively small on the flanks, explaining the robustness of the body of the magnetosphere here. Slow modes are found to grow very little in this region. Finally, a uniform magnetosphere with the magnetic field parallel to the flow in the magnetosheath is considered. Here, the fast modes are unlikely to be Kelvin-Helmholtz unstable for realistic flow speeds, and the magnetopause boundary may be reasonably assumed to be perfectly reflecting. The low value of the plasma pressure is this region suggests that slow modes will be unimportant.
Fri, 01 Jan 1999 00:00:00 GMThttps://hdl.handle.net/10023/139901999-01-01T00:00:00ZMills, Katharine J.In this thesis, the behaviour of Kelvin-Helmholtz unstable modes on the magnetospheric flanks and in the magnetotail are investigated. A model of a straight bounded magnetosphere connected to a semi-infinite field-free magnetosheath which is flowing with a uniform speed is used. First the magnetosphere is taken to be uniform with the magnetic field perpendicular to the flow in the magnetosheath and it is shown that the increase in Pc5 wave power observed for high solar wind flow speeds correlates well with the onset of instability of the fast body modes. A condition for the exact onset of instability of these modes is derived and the behaviour of fast surface and slow body and surface modes is also investigated. Using a non-uniform magnetosphere, it is shown that these unstable body modes may couple to field line resonances. The fastest growing modes are found to have a common azimuthal phase speed which depends only on the local conditions at the magnetopause and may be predicted using the theory of over-reflection. A finite width boundary layer is then added to the uniform magnetosphere model to investigate the space-time evolution of wave-packets on the magnetopause. Fast surface mode wave-packets are found to grow rapidly as they convect around the flanks so that non-linear effects will be important. Fast cavity mode wave-packets will remain relatively small on the flanks, explaining the robustness of the body of the magnetosphere here. Slow modes are found to grow very little in this region. Finally, a uniform magnetosphere with the magnetic field parallel to the flow in the magnetosheath is considered. Here, the fast modes are unlikely to be Kelvin-Helmholtz unstable for realistic flow speeds, and the magnetopause boundary may be reasonably assumed to be perfectly reflecting. The low value of the plasma pressure is this region suggests that slow modes will be unimportant.Study of solitary waves in space plasmas
https://hdl.handle.net/10023/13987
Theoretical investigations have been made of arbitrary amplitude electrostatic solitary waves in non-thermal plasmas, which may be of relevance to ionospheric and magnetospheric plasmas, and dusty plasmas, which are most common in earth's and cometary environments as well as in planetary rings, for understanding the nonlinear features of localised electrostatic disturbances in such space plasma systems. This thesis starts with an introductory chapter where a very brief historical review of solitary waves in plasmas has been presented. The study of arbitrary amplitude electrostatic solitary waves in non-thermal plasma has considered a plasma system consisting of warm adiabatic ions and non- thermal electrons. It is found that a non-thermal electron distribution may change the nature of ion-acoustic solitary waves. If the ions are assumed to respond as a fluid to perturbations in the potential, with no significant trapping in a potential well, then a thermal plasma only supports solitary waves with a density peak. However, with a suitable distribution of non-thermal electrons, solitary waves with both density peaks and density depressions may exist. This study has also included a numerical analysis showing how these electrostatic solitary structures evolve with time. The investigation has then been extended to magnetised plasmas to study the effects of magnetic field on obliquely propagating electrostatic solitary structures. This attempt first employed the reductive perturbation method and investigated the nonlinear properties of small but finite amplitude obliquely propagating solitary waves in this magnetised non-thermal plasma model. This study is then generalised to arbitrary amplitude solitary waves by the numerical solution of the full nonlinear system of equations. This numerical method has also been utilised to present a similar study in another popular plasma model, namely the two-electron-temperature plasma model. The study of arbitrary amplitude solitary waves in a dusty plasma has considered another plasma system which consists of an inertial dust fluid and ions with Maxwellian distribution and has investigated the nonlinear properties of dust- acoustic solitary waves. A numerical study has also been made to show how these dust-acoustic solitary waves evolve with time. The effects of non-thermal and vortex-like ion distributions are then incorporated into this study. The study of arbitrary amplitude electrostatic solitary waves in this thesis has finally been concluded with some brief discussion of our results and proposal for further studies, which are expected to generalise and develop our present work to some other extents, in this versatile area of research.
Wed, 01 Jan 1997 00:00:00 GMThttps://hdl.handle.net/10023/139871997-01-01T00:00:00ZMamun, A. A.Theoretical investigations have been made of arbitrary amplitude electrostatic solitary waves in non-thermal plasmas, which may be of relevance to ionospheric and magnetospheric plasmas, and dusty plasmas, which are most common in earth's and cometary environments as well as in planetary rings, for understanding the nonlinear features of localised electrostatic disturbances in such space plasma systems. This thesis starts with an introductory chapter where a very brief historical review of solitary waves in plasmas has been presented. The study of arbitrary amplitude electrostatic solitary waves in non-thermal plasma has considered a plasma system consisting of warm adiabatic ions and non- thermal electrons. It is found that a non-thermal electron distribution may change the nature of ion-acoustic solitary waves. If the ions are assumed to respond as a fluid to perturbations in the potential, with no significant trapping in a potential well, then a thermal plasma only supports solitary waves with a density peak. However, with a suitable distribution of non-thermal electrons, solitary waves with both density peaks and density depressions may exist. This study has also included a numerical analysis showing how these electrostatic solitary structures evolve with time. The investigation has then been extended to magnetised plasmas to study the effects of magnetic field on obliquely propagating electrostatic solitary structures. This attempt first employed the reductive perturbation method and investigated the nonlinear properties of small but finite amplitude obliquely propagating solitary waves in this magnetised non-thermal plasma model. This study is then generalised to arbitrary amplitude solitary waves by the numerical solution of the full nonlinear system of equations. This numerical method has also been utilised to present a similar study in another popular plasma model, namely the two-electron-temperature plasma model. The study of arbitrary amplitude solitary waves in a dusty plasma has considered another plasma system which consists of an inertial dust fluid and ions with Maxwellian distribution and has investigated the nonlinear properties of dust- acoustic solitary waves. A numerical study has also been made to show how these dust-acoustic solitary waves evolve with time. The effects of non-thermal and vortex-like ion distributions are then incorporated into this study. The study of arbitrary amplitude electrostatic solitary waves in this thesis has finally been concluded with some brief discussion of our results and proposal for further studies, which are expected to generalise and develop our present work to some other extents, in this versatile area of research.Steady models for magnetic reconnection
https://hdl.handle.net/10023/13985
Magnetic reconnection is a fundamental physical process by which stored magnetic energy may be released. It is already known that different reconnection regimes result from changes in the nature of the plasma inflow towards the reconnection site. In this thesis, we examine both how the outflow region responds to changes both in the inflow and outflow boundary conditions and also how introducing compressibility affects the results. We find that if the inflow is converging, the outflow velocity is least, the width of the outflow region is greatest and the ratio of outflowing thermal to kinetic energy is greatest. Also, there is one free outflow parameter which would naturally be specified by the velocity of plasma leaving the reconnection site. We suggest that reverse currents seen in numerical simulations may result from the specification of an extra boundary condition. In addition, we find that the main effects of including compressibility are: to enhance convergence or divergence of the inflow; to increase the maximum reconnection rate where the inflow is converging; to increase the flow speed near the reconnection site where the inflow is diverging; to give faster, narrower outflow jets; to increase variations between regimes in the energy conversion and to increase the ratio of thermal to kinetic energy in the outflow jet.
Sun, 01 Jan 1989 00:00:00 GMThttps://hdl.handle.net/10023/139851989-01-01T00:00:00ZJardine, MoiraMagnetic reconnection is a fundamental physical process by which stored magnetic energy may be released. It is already known that different reconnection regimes result from changes in the nature of the plasma inflow towards the reconnection site. In this thesis, we examine both how the outflow region responds to changes both in the inflow and outflow boundary conditions and also how introducing compressibility affects the results. We find that if the inflow is converging, the outflow velocity is least, the width of the outflow region is greatest and the ratio of outflowing thermal to kinetic energy is greatest. Also, there is one free outflow parameter which would naturally be specified by the velocity of plasma leaving the reconnection site. We suggest that reverse currents seen in numerical simulations may result from the specification of an extra boundary condition. In addition, we find that the main effects of including compressibility are: to enhance convergence or divergence of the inflow; to increase the maximum reconnection rate where the inflow is converging; to increase the flow speed near the reconnection site where the inflow is diverging; to give faster, narrower outflow jets; to increase variations between regimes in the energy conversion and to increase the ratio of thermal to kinetic energy in the outflow jet.Microinstabilities in high power electron cyclotron heating of plasmas
https://hdl.handle.net/10023/13977
Electron cyclotron resonance heating has been successfully used in a number of experiments, firstly to raise the plasma temperature and secondly to drive currents noninductively. Recently the microwaves in tokamak experiment (MTX) has been proposed at the Lawrence Livermore Laboratory, which will involve pulsed heating at powers much higher than have previously been possible, using a Free Electron Laser (PEL). The physics of such an experiment differs greatly from the physics of experiments using less powerful but continuous operation gyrotron sources. An analytical model of the interaction between a wave and an electron is presented on the assumption that the wave amplitude experienced along the electron guiding centre changes slowly with time as it passes through the beam. This model is tested numerically by integrating the equations of motion governing the electron's motion as it interacts with the wave. Finally this model is used to predict the possible growth of instabilities in a plasma heated by a FEL. The growth rates of these waves may be large enough to act on the plasma in time scales much shorter than typical electron collision times.
Tue, 01 Jan 1991 00:00:00 GMThttps://hdl.handle.net/10023/139771991-01-01T00:00:00ZMiller, Andrew GilbertElectron cyclotron resonance heating has been successfully used in a number of experiments, firstly to raise the plasma temperature and secondly to drive currents noninductively. Recently the microwaves in tokamak experiment (MTX) has been proposed at the Lawrence Livermore Laboratory, which will involve pulsed heating at powers much higher than have previously been possible, using a Free Electron Laser (PEL). The physics of such an experiment differs greatly from the physics of experiments using less powerful but continuous operation gyrotron sources. An analytical model of the interaction between a wave and an electron is presented on the assumption that the wave amplitude experienced along the electron guiding centre changes slowly with time as it passes through the beam. This model is tested numerically by integrating the equations of motion governing the electron's motion as it interacts with the wave. Finally this model is used to predict the possible growth of instabilities in a plasma heated by a FEL. The growth rates of these waves may be large enough to act on the plasma in time scales much shorter than typical electron collision times.A gyrokinetic analysis of electron plasma waves at resonance in magnetic field gradients
https://hdl.handle.net/10023/13975
To produce nuclear fusion in a Tokamak reactor requires the heating of a plasma to a temperature of the order of 10 keV. Electron cyclotron resonant heating (ECRH), in which the plasma is heated by radio waves in resonance with the Larmor frequency of the plasma's electrons, is one scheme under consideration for achieving this. A description of such a heating scheme requires a theory to explain the propagation and absorption of high frequency waves in a plasma in the presence of a magnetic field gradient. A WKB analysis can describe some of the processes involved but a complete explanation requires the use of full wave equations. In this thesis we shall develop a technique for deriving such equations which will be shown to be simpler and more general than calculations performed by earlier workers. The technique relies on including the effect of the magnetic gradient across the Larmor orbit of the electrons in the resonance condition of the wave, the so called Gyrokinetic correction, which has been ignored in calculations by previous workers. Once derived, the equations are solved numerically and the results applied to a number of experiments currently being performed on Tokamak fusion. In addition, we shall also look at the energy loss processes of runaway electrons, which have been shown experimentally to be shorter than would be expected.
Sun, 01 Jan 1995 00:00:00 GMThttps://hdl.handle.net/10023/139751995-01-01T00:00:00ZMcDonald, DarrenTo produce nuclear fusion in a Tokamak reactor requires the heating of a plasma to a temperature of the order of 10 keV. Electron cyclotron resonant heating (ECRH), in which the plasma is heated by radio waves in resonance with the Larmor frequency of the plasma's electrons, is one scheme under consideration for achieving this. A description of such a heating scheme requires a theory to explain the propagation and absorption of high frequency waves in a plasma in the presence of a magnetic field gradient. A WKB analysis can describe some of the processes involved but a complete explanation requires the use of full wave equations. In this thesis we shall develop a technique for deriving such equations which will be shown to be simpler and more general than calculations performed by earlier workers. The technique relies on including the effect of the magnetic gradient across the Larmor orbit of the electrons in the resonance condition of the wave, the so called Gyrokinetic correction, which has been ignored in calculations by previous workers. Once derived, the equations are solved numerically and the results applied to a number of experiments currently being performed on Tokamak fusion. In addition, we shall also look at the energy loss processes of runaway electrons, which have been shown experimentally to be shorter than would be expected.The theory of electron heating in collisonless plasma shock waves
https://hdl.handle.net/10023/13973
Equations are derived to describe the evolution of an electron distribution function under the action of electromagnetic instabilities in a non-uniform plasma using an extension of the quasilinear theory of Kennel and Engelmann. Variations in both the electron density and temperature and the background magnetic field are taken into account. These equations are simplified in the limit of small electron beta so that an electrostatic approximation is justified. Methods are then presented which allow the solution of these equations (or, in principle, the more complex electromagnetic equations). In particular, a method of solving the kinetic dispersion relation for an arbitrary background (first-order) distribution function with the minimum of additional assumptions and approximations is described in detail. The electrostatic equations are solved for a number of different cases in order to study the action of the modified two stream instability on the electron distribution function. Throughout, realistic values of the ratios of electron to ion mass and electron plasma to cyclotron frequency ratio are used. The applications to collisionless plasma shock waves are discussed, and it is found that the modified two stream instability can produce the (relatively small) amounts of electron heating observed at quasi-perpendicular terrestrial bow shocks, and the flat-topped electron distribution functions seen to evolve. Extensions to the model which would greatly improve its applicability and accuracy, as well as the amount of computational effort required, are discussed.
Fri, 01 Jan 1993 00:00:00 GMThttps://hdl.handle.net/10023/139731993-01-01T00:00:00ZBuckner, A. J. F.Equations are derived to describe the evolution of an electron distribution function under the action of electromagnetic instabilities in a non-uniform plasma using an extension of the quasilinear theory of Kennel and Engelmann. Variations in both the electron density and temperature and the background magnetic field are taken into account. These equations are simplified in the limit of small electron beta so that an electrostatic approximation is justified. Methods are then presented which allow the solution of these equations (or, in principle, the more complex electromagnetic equations). In particular, a method of solving the kinetic dispersion relation for an arbitrary background (first-order) distribution function with the minimum of additional assumptions and approximations is described in detail. The electrostatic equations are solved for a number of different cases in order to study the action of the modified two stream instability on the electron distribution function. Throughout, realistic values of the ratios of electron to ion mass and electron plasma to cyclotron frequency ratio are used. The applications to collisionless plasma shock waves are discussed, and it is found that the modified two stream instability can produce the (relatively small) amounts of electron heating observed at quasi-perpendicular terrestrial bow shocks, and the flat-topped electron distribution functions seen to evolve. Extensions to the model which would greatly improve its applicability and accuracy, as well as the amount of computational effort required, are discussed.Rotational flow in fluid dynamics
https://hdl.handle.net/10023/13967
The thesis is divided into four chapters. Chapter I gives a brief résumé of the state of rotational flow theory up to 1955. Chapter II contains a study of the constant shear flow past cylinders with various cross sections. Chapter III contains a method for obtaining the stream functions for cylinders in a variable shear flow when the latter approximates firstly to a linear vorticity distribution, and secondly to the rotational flow present in a boundary layer. Further, it illustrates the nature of the difficulties likely to be encountered in trying to obtain analytical solutions of problems where the rotation is of a more complicated nature. Finally, Chapter IV contains a relaxation solution to the two-dimensional isentropic compressible rotational flow of a gas through a channel containing a constriction, it also illustrates the complexity of the numerical work required in obtaining relaxation solutions of compressible flow problems with rotation.
Sat, 01 Jan 1955 00:00:00 GMThttps://hdl.handle.net/10023/139671955-01-01T00:00:00ZMurray, J. D. (James Dickson)The thesis is divided into four chapters. Chapter I gives a brief résumé of the state of rotational flow theory up to 1955. Chapter II contains a study of the constant shear flow past cylinders with various cross sections. Chapter III contains a method for obtaining the stream functions for cylinders in a variable shear flow when the latter approximates firstly to a linear vorticity distribution, and secondly to the rotational flow present in a boundary layer. Further, it illustrates the nature of the difficulties likely to be encountered in trying to obtain analytical solutions of problems where the rotation is of a more complicated nature. Finally, Chapter IV contains a relaxation solution to the two-dimensional isentropic compressible rotational flow of a gas through a channel containing a constriction, it also illustrates the complexity of the numerical work required in obtaining relaxation solutions of compressible flow problems with rotation.Mode conversion of plasma waves
https://hdl.handle.net/10023/13965
Linear mode conversion processes are much studied in plasma physics because they determine the efficiency of any radio frequency heating scheme. Mode coupling model equations, extracted with varying degrees of rigour from the Maxwell-linearized kinetic equations, are usually fourth or higher order O.D.E's. These are solved by complicated methods to obtain transmission, conversion, reflection and absorption coefficients. Recently, Fuchs et al and Cairns and Lashmore-Davies (C.L-D.) have postulated second order O.D.E's to describe pairwise coupling events. The second order theories have reproduced results previously obtained by much more sophisticated treatments. In this thesis, we firstly examine the hybrid resonances in a cold plasma and show that they have a mode conversion interpretation in the framework of the C.L-D. model. The Budden tunnelling coefficients are recovered for this case. Next, mode conversion between the fast and slow electromagnetic waves in the lower hybrid frequency range is considered. This phenomenon determines the accessibility of the lower hybrid resonance to the slow wave, and is also of theoretical interest because the mode coupling differs in certain aspects from cases previously investigated by C.L-D. A second order approximation to the dispersion relation is used in the mode conversion region leading to Weber's equation from which transmission coefficients are then obtained in various cases. Finally, we provide justification for the use of Weber's equation. The exact fourth order system of O.D.E's for the problem is set down, and a linear transformation, which is an extension of that given by Heading, reveals the second order nature of the coupling process. Numerical solutions of the fourth order system yield transmission coefficients in excellent agreement with the second order theory, and also demonstrate that the electric field variation across the mode conversion region is well approximated, via the above transformation, by our second order theory.
Thu, 01 Jan 1987 00:00:00 GMThttps://hdl.handle.net/10023/139651987-01-01T00:00:00ZWoods, Anna MariaLinear mode conversion processes are much studied in plasma physics because they determine the efficiency of any radio frequency heating scheme. Mode coupling model equations, extracted with varying degrees of rigour from the Maxwell-linearized kinetic equations, are usually fourth or higher order O.D.E's. These are solved by complicated methods to obtain transmission, conversion, reflection and absorption coefficients. Recently, Fuchs et al and Cairns and Lashmore-Davies (C.L-D.) have postulated second order O.D.E's to describe pairwise coupling events. The second order theories have reproduced results previously obtained by much more sophisticated treatments. In this thesis, we firstly examine the hybrid resonances in a cold plasma and show that they have a mode conversion interpretation in the framework of the C.L-D. model. The Budden tunnelling coefficients are recovered for this case. Next, mode conversion between the fast and slow electromagnetic waves in the lower hybrid frequency range is considered. This phenomenon determines the accessibility of the lower hybrid resonance to the slow wave, and is also of theoretical interest because the mode coupling differs in certain aspects from cases previously investigated by C.L-D. A second order approximation to the dispersion relation is used in the mode conversion region leading to Weber's equation from which transmission coefficients are then obtained in various cases. Finally, we provide justification for the use of Weber's equation. The exact fourth order system of O.D.E's for the problem is set down, and a linear transformation, which is an extension of that given by Heading, reveals the second order nature of the coupling process. Numerical solutions of the fourth order system yield transmission coefficients in excellent agreement with the second order theory, and also demonstrate that the electric field variation across the mode conversion region is well approximated, via the above transformation, by our second order theory.Some exact solutions in the one-dimensional unsteady motion of a gas
https://hdl.handle.net/10023/13964
In this thesis, we present certain exact solutions of the mathematical equations governing the one-dimensional unsteady flow of a compressible fluid. In Chapter 2 we introduce the well-known simplification of the equations (1.1.10), (1.1.11) and (1.1.12) which occurs when the entropy is assumed to be constant, and conditions for parching solutions of the equations along characteristics are obtained. These results are used to generalise a problem solved by Mackie. In chapter 3 we meet the concept of a shook, and exact solutions are obtained for two problems in which shocks occur in non-uniform flows. In chapter 4 the case of waves in shallow water which has differential equations similar to those of gas flow is discussed. The results of the previous section are applied to this case and a problem attacked which permits a comparison to be made of the results obtained by this theory and a simpler linearized theory. Finally in chapter 5 we examine a method introduced by Martin for dealing with certain non-isentropic flows. Some new exact solutions of non-isentropic flows are thus obtained.
Sun, 01 Jan 1961 00:00:00 GMThttps://hdl.handle.net/10023/139641961-01-01T00:00:00ZWeir, David GordonIn this thesis, we present certain exact solutions of the mathematical equations governing the one-dimensional unsteady flow of a compressible fluid. In Chapter 2 we introduce the well-known simplification of the equations (1.1.10), (1.1.11) and (1.1.12) which occurs when the entropy is assumed to be constant, and conditions for parching solutions of the equations along characteristics are obtained. These results are used to generalise a problem solved by Mackie. In chapter 3 we meet the concept of a shook, and exact solutions are obtained for two problems in which shocks occur in non-uniform flows. In chapter 4 the case of waves in shallow water which has differential equations similar to those of gas flow is discussed. The results of the previous section are applied to this case and a problem attacked which permits a comparison to be made of the results obtained by this theory and a simpler linearized theory. Finally in chapter 5 we examine a method introduced by Martin for dealing with certain non-isentropic flows. Some new exact solutions of non-isentropic flows are thus obtained.Stability of some free-surface flows
https://hdl.handle.net/10023/13960
The subject matter of this thesis is concerned with the stability of fluid flows; more particularly , with the stability of liquid films which have an interface with air. We will therefore begin by formulating the basic equations and ideas which pertain to this class of problems. Later in this chapter, a summary will be given of the topics dealt with in this dissertation.
Wed, 01 Jan 1969 00:00:00 GMThttps://hdl.handle.net/10023/139601969-01-01T00:00:00ZSmith, Frank Ian PittThe subject matter of this thesis is concerned with the stability of fluid flows; more particularly , with the stability of liquid films which have an interface with air. We will therefore begin by formulating the basic equations and ideas which pertain to this class of problems. Later in this chapter, a summary will be given of the topics dealt with in this dissertation.The unsteady expansion of a gas into a near vacuum
https://hdl.handle.net/10023/13956
This thesis is concerned with the unsteady expansion of an initially uniform, stationary gas into a low density, stationary atmosphere, studied from the viewpoint of inviscid gasdynamics. It is found that, there are two regions in the k-𝜎 parameter space having distinct forms for the large time solution, when the atmospheric density is initially proportional to r⁻[super]k, r being the spatial coordinate, k being constant and 𝜎, the geometry index, has its usual meaning. First of all a constant asymptotic shock velocity is assumed and matched expansions, for large r, are constructed. Inner expansions, valid near the shock, are matched to zeroth and first orders with the outer expansions which are valid near the contact front. Zeroth order matching, which, yields the constant asymptotic shock velocity, is possible only in a restricted region of the k-𝜎parameter space and this situation is clarified by appealing to the similarity solutions which are extended to cover cases which have not been dealt with previously.
In the other region of the k-𝜎 parameter space the asymptotic shock velocity is proportional to r[super]∈ where ∈, a positive constant, is found from the similarity solutions as a function of k, γ ,𝜎. An attempt is made at constructing matched asymptotic expansions for large r. The inner solution can be obtained, apart from the evaluation of certain constants, to zeroth and first orders but the outer solution is inaccessible and can only be determined from the full inviscid solution. However it is shown that there exists a solution to the outer equations which matches with the inner solution up to first order. In both cases matching of the first order inner terms to the outer solution produces an eigenvalue problem, the solution of which is not attempted here. Finally full numerical solutions of the inviscid equations, one for each case, were produced using the method of backward drawn characteristics, devised by Hartree, and it will be seen that they compare most favourably with the asymptotic analysis.
Wed, 01 Jan 1975 00:00:00 GMThttps://hdl.handle.net/10023/139561975-01-01T00:00:00ZMcLaughlin, RaymondThis thesis is concerned with the unsteady expansion of an initially uniform, stationary gas into a low density, stationary atmosphere, studied from the viewpoint of inviscid gasdynamics. It is found that, there are two regions in the k-𝜎 parameter space having distinct forms for the large time solution, when the atmospheric density is initially proportional to r⁻[super]k, r being the spatial coordinate, k being constant and 𝜎, the geometry index, has its usual meaning. First of all a constant asymptotic shock velocity is assumed and matched expansions, for large r, are constructed. Inner expansions, valid near the shock, are matched to zeroth and first orders with the outer expansions which are valid near the contact front. Zeroth order matching, which, yields the constant asymptotic shock velocity, is possible only in a restricted region of the k-𝜎parameter space and this situation is clarified by appealing to the similarity solutions which are extended to cover cases which have not been dealt with previously.
In the other region of the k-𝜎 parameter space the asymptotic shock velocity is proportional to r[super]∈ where ∈, a positive constant, is found from the similarity solutions as a function of k, γ ,𝜎. An attempt is made at constructing matched asymptotic expansions for large r. The inner solution can be obtained, apart from the evaluation of certain constants, to zeroth and first orders but the outer solution is inaccessible and can only be determined from the full inviscid solution. However it is shown that there exists a solution to the outer equations which matches with the inner solution up to first order. In both cases matching of the first order inner terms to the outer solution produces an eigenvalue problem, the solution of which is not attempted here. Finally full numerical solutions of the inviscid equations, one for each case, were produced using the method of backward drawn characteristics, devised by Hartree, and it will be seen that they compare most favourably with the asymptotic analysis.Hodograph methods applied to flow past finite wedges
https://hdl.handle.net/10023/13946
Thu, 01 Jan 1953 00:00:00 GMThttps://hdl.handle.net/10023/139461953-01-01T00:00:00ZMackie, A. G. (Andrew George)Two parameter integral methods in laminar boundary layer theory
https://hdl.handle.net/10023/13944
The work of this thesis is concerned, with the investigation and attempted improvement of an integral method for solving the two dimensional, incompressible laminar boundary layer equations of fluid dynamics. The method which is based on a theoretical two parameter representation of well-known boundary layer properties was first produced by Professor S. N. Curle. Its range of application, reliability and accuracy rely on four universal functions which have been derived from known exact solutions to the boundary layer equations, and are given tabulated in terms of a pressure gradient parameter 𝞴. This thesis seeks to improve these properties by making adjustments to the tabulated functions and also considers the extension of the method to certain compressible boundary layer problems. The first chapter contains the development of, and background to the method and gives a critical assessment of the existing functions. This analysis indicates that the method may be improved by supplying more data for certain ranges of 𝞴 from which the functions may be calculated; by improving the fitting process; and by the provision for small values of 𝞴 of an analytic form for a shape parameter H which the method involves.
To supply more data two new solutions for the flows u₁ = U₀ (1+𝜉) and u₁ = u₀ (𝜉+𝜉³) where 𝜉 is a non-dimensional co-ordinate in the direction of the flow, are investigated. The resulting work produces some interesting examples of the use of series expansions in boundary layer theory and these, and the results produced, are given in the second chapter. The fitting of the functions is carried out in chapter three. Polynomial models in terms of 𝞴 are fitted by least squares techniques to data from seven solutions and are adjusted to ensure an analytic form for H for small values of 𝞴. A comparison of results using new and old tables Indicates that an improvement has been made. The transformation relating certain compressible and incompressible flows is next examined and the extension of the method to such problems considered. An idea due to Stewartson for assessing the relative accuracies of methods under such circumstances indicates that the method should be highly accurate, a result confirmed by the calculation of the compressible flow u₁ = u₀ (1-𝜉) at a leading edge Mach number of four. The thesis is concluded with a review of the work carried out and the results obtained.
Fri, 01 Jan 1971 00:00:00 GMThttps://hdl.handle.net/10023/139441971-01-01T00:00:00ZLister, William MacraeThe work of this thesis is concerned, with the investigation and attempted improvement of an integral method for solving the two dimensional, incompressible laminar boundary layer equations of fluid dynamics. The method which is based on a theoretical two parameter representation of well-known boundary layer properties was first produced by Professor S. N. Curle. Its range of application, reliability and accuracy rely on four universal functions which have been derived from known exact solutions to the boundary layer equations, and are given tabulated in terms of a pressure gradient parameter 𝞴. This thesis seeks to improve these properties by making adjustments to the tabulated functions and also considers the extension of the method to certain compressible boundary layer problems. The first chapter contains the development of, and background to the method and gives a critical assessment of the existing functions. This analysis indicates that the method may be improved by supplying more data for certain ranges of 𝞴 from which the functions may be calculated; by improving the fitting process; and by the provision for small values of 𝞴 of an analytic form for a shape parameter H which the method involves.
To supply more data two new solutions for the flows u₁ = U₀ (1+𝜉) and u₁ = u₀ (𝜉+𝜉³) where 𝜉 is a non-dimensional co-ordinate in the direction of the flow, are investigated. The resulting work produces some interesting examples of the use of series expansions in boundary layer theory and these, and the results produced, are given in the second chapter. The fitting of the functions is carried out in chapter three. Polynomial models in terms of 𝞴 are fitted by least squares techniques to data from seven solutions and are adjusted to ensure an analytic form for H for small values of 𝞴. A comparison of results using new and old tables Indicates that an improvement has been made. The transformation relating certain compressible and incompressible flows is next examined and the extension of the method to such problems considered. An idea due to Stewartson for assessing the relative accuracies of methods under such circumstances indicates that the method should be highly accurate, a result confirmed by the calculation of the compressible flow u₁ = u₀ (1-𝜉) at a leading edge Mach number of four. The thesis is concluded with a review of the work carried out and the results obtained.The evaporation kinetics of liquid helium II
https://hdl.handle.net/10023/13941
This work is concerned with the evaporation and condensation processes occurring when liquid helium II is in equilibrium with its saturated vapour. We define the condensation coefficient a as the fraction of atoms incident on the liquid vapour interface which cross it to form part of the liquid. Experiments to measure are described, and the results are discussed in terms of microscopic condensation processes. The measurements are made by reflecting second sound pulses from the liquid vapour surface at normal incidence and measuring the reflection coefficient. An account is given of the phenomenological theories of Osborne (1962a) and Chernikova (1964), which describe the reflection of second sound from the surface and the associated effect, its transformation into first sound in the gas. Neither of these agree with the experimental results, and Osborne's theory is modified by taking account of the conditions in the gas a small fraction of a mean free path above the surface (rather than many mean free paths above the surface, as in Osborne's original theory). Thus modified, the theory is shown to be in agreement with the measurements of the reflection coefficient. Also described are measurements made in second sound pulses generated at the interface by first sound pulses, themselves generated at the interface by second sound, propagated up the tube, and reflected from its closed and back to the surface. From the time intervals between these pulses the velocity of first sound in the vapour is deduced, and found to be in agreement with previous work. Measurements of pulse amplitude corroborate the reflection coefficient measurements, and taking the two sets of measurements together wo have concluded that a is probably 1 and not less than 0.8 between 1.0°K and 2.14°K. The microscopic processes by which condensation can take place are considered. Experiments due to beaker (unpublished, see Osborne, 1962a) and Osborne (1962b) are described, which indicate that the vapour exchanges momentum with normal fluid only. We have therefore supposed that processes in which a gas atom condenses to form excitations must conserve energy and momentum. Processes involving both bulk excitations and surface excitations are considered, but effects due to the finite lifetime of the excitations and the linewidth of the excitations spectrum are neglected. No attempt has been made to calculate the matrix elements for condensation processes, but plausible estimates have been made of their relative magnitudes. In particular, only processes involving one gas atom and one or two excitations have been considered. Using the requirements of conservation of energy and momentum, it is shown that as the temperature decreases, a decreasing fraction of the incident atom have enough energy to form two excitations, and condensation must take place by the collision of an atom with an existing excitation. A rough estimate of the collision probability for such a process leads to the conclusion that at 1°K, a should be about 0.2. This disagreement with experiment has not been resolved. Finally, some remarks are made about the implications for other work on liquid helium II, and some suggestions for future work.
Mon, 01 Jan 1968 00:00:00 GMThttps://hdl.handle.net/10023/139411968-01-01T00:00:00ZHunter, George HuttonThis work is concerned with the evaporation and condensation processes occurring when liquid helium II is in equilibrium with its saturated vapour. We define the condensation coefficient a as the fraction of atoms incident on the liquid vapour interface which cross it to form part of the liquid. Experiments to measure are described, and the results are discussed in terms of microscopic condensation processes. The measurements are made by reflecting second sound pulses from the liquid vapour surface at normal incidence and measuring the reflection coefficient. An account is given of the phenomenological theories of Osborne (1962a) and Chernikova (1964), which describe the reflection of second sound from the surface and the associated effect, its transformation into first sound in the gas. Neither of these agree with the experimental results, and Osborne's theory is modified by taking account of the conditions in the gas a small fraction of a mean free path above the surface (rather than many mean free paths above the surface, as in Osborne's original theory). Thus modified, the theory is shown to be in agreement with the measurements of the reflection coefficient. Also described are measurements made in second sound pulses generated at the interface by first sound pulses, themselves generated at the interface by second sound, propagated up the tube, and reflected from its closed and back to the surface. From the time intervals between these pulses the velocity of first sound in the vapour is deduced, and found to be in agreement with previous work. Measurements of pulse amplitude corroborate the reflection coefficient measurements, and taking the two sets of measurements together wo have concluded that a is probably 1 and not less than 0.8 between 1.0°K and 2.14°K. The microscopic processes by which condensation can take place are considered. Experiments due to beaker (unpublished, see Osborne, 1962a) and Osborne (1962b) are described, which indicate that the vapour exchanges momentum with normal fluid only. We have therefore supposed that processes in which a gas atom condenses to form excitations must conserve energy and momentum. Processes involving both bulk excitations and surface excitations are considered, but effects due to the finite lifetime of the excitations and the linewidth of the excitations spectrum are neglected. No attempt has been made to calculate the matrix elements for condensation processes, but plausible estimates have been made of their relative magnitudes. In particular, only processes involving one gas atom and one or two excitations have been considered. Using the requirements of conservation of energy and momentum, it is shown that as the temperature decreases, a decreasing fraction of the incident atom have enough energy to form two excitations, and condensation must take place by the collision of an atom with an existing excitation. A rough estimate of the collision probability for such a process leads to the conclusion that at 1°K, a should be about 0.2. This disagreement with experiment has not been resolved. Finally, some remarks are made about the implications for other work on liquid helium II, and some suggestions for future work.Hydrodynamics of liquid helium II
https://hdl.handle.net/10023/13938
Observations have been made of the behaviour of a fine quartz fibre, weighted at its lower end and suspended inside a short, horizontal tunnel in which counterblow of the normal and superfluid components of liquid helium II can be produced by a heater. Section I of this thesis is an introduction to the hydrodynamic of liquid helium II. In section II the interaction with such a fibre of quantized vortex lines in the superfluid is discussed, and the effect of a short heat pulse on the fibre when it is carrying superfluid circulation in calculated approximately. The different responses of the fibre to turbulence in the normal fluid and in the superfluid are contrasted.
In section III, after a description of the apparatus and the experimental method, measurements, deduced from the response to heat pulses, of the circulation about the fibre from 1.3°K to2.1°K are reported. At all temperatures circulations of the expected order from magnitude are observed to grow and decay with time. At 1.3°K apparent circulations of up to about 1/5 quantum occur. In undisturbed helium the largest circulations are more stable than other values, persisting for up to five minutes. Measurement of the same circulation both by the heat-pulse method and by the deflection of the fibre in a steady heat current suggests that the large, persistent circulations may in fact be equal to one quantum. The sense of the observed circulations about the fibre at 1.3°K is strongly biased, this bias being probably associated with the heater geometry. In small heat current no change in the bins or persistence of circulation can be detected, but in currents above 11/2-3 mW/cm², depending on the heater, the circulation about the fibre is both more variable and of the opposite bias to that in undisturbed helium. This behaviour continues for 100 sec or more after the heat current has been switched off. At higher temperatures there are indications that the behaviour might be similar if it were possible for the helium to region its undisturbed condition after being stirred up by turbulent heat currents. In fact this seems other to be impossible, or to require many hundreds of seconds, and the situation is therefore rather confused.
In still higher heat currents measurement of superfluid circulation by heat pulses is impossible because the fibre is continuously agitated in a random way. From measurements of the rms deflections of the bob on the end of the fibre a critical heat current for the onset of such turbulence is found at 1.3°K. At higher temperatures the sensitivity is too low for the transition itself, if any, to be detected, but an upper limit to the critical heat current is given. At 1.3°K and 2.1°K the rms deflection increases monotonically with increasing heat currents, but at intermediate temperatures it is variable, because the bob is often hardly agitated for long periods during apparently supercritical heat currents. This is called quiescent behaviour.
When a supercritical heat current is a delay before agitation of the fibre begins. The delay time, which is often not very well defined, has been measured as a function of the heat current. When the current is switched off the agitation of the bob decoys in a few seconds, but at 1.3°K the circulation about the fibre is small and variable for 100 sec or more, until the persistence and bias characteristic of undisturbed helium regained. These results are discussed in section IV.
Wed, 01 Jan 1964 00:00:00 GMThttps://hdl.handle.net/10023/139381964-01-01T00:00:00ZGriffiths, D. J. (Derek John)Observations have been made of the behaviour of a fine quartz fibre, weighted at its lower end and suspended inside a short, horizontal tunnel in which counterblow of the normal and superfluid components of liquid helium II can be produced by a heater. Section I of this thesis is an introduction to the hydrodynamic of liquid helium II. In section II the interaction with such a fibre of quantized vortex lines in the superfluid is discussed, and the effect of a short heat pulse on the fibre when it is carrying superfluid circulation in calculated approximately. The different responses of the fibre to turbulence in the normal fluid and in the superfluid are contrasted.
In section III, after a description of the apparatus and the experimental method, measurements, deduced from the response to heat pulses, of the circulation about the fibre from 1.3°K to2.1°K are reported. At all temperatures circulations of the expected order from magnitude are observed to grow and decay with time. At 1.3°K apparent circulations of up to about 1/5 quantum occur. In undisturbed helium the largest circulations are more stable than other values, persisting for up to five minutes. Measurement of the same circulation both by the heat-pulse method and by the deflection of the fibre in a steady heat current suggests that the large, persistent circulations may in fact be equal to one quantum. The sense of the observed circulations about the fibre at 1.3°K is strongly biased, this bias being probably associated with the heater geometry. In small heat current no change in the bins or persistence of circulation can be detected, but in currents above 11/2-3 mW/cm², depending on the heater, the circulation about the fibre is both more variable and of the opposite bias to that in undisturbed helium. This behaviour continues for 100 sec or more after the heat current has been switched off. At higher temperatures there are indications that the behaviour might be similar if it were possible for the helium to region its undisturbed condition after being stirred up by turbulent heat currents. In fact this seems other to be impossible, or to require many hundreds of seconds, and the situation is therefore rather confused.
In still higher heat currents measurement of superfluid circulation by heat pulses is impossible because the fibre is continuously agitated in a random way. From measurements of the rms deflections of the bob on the end of the fibre a critical heat current for the onset of such turbulence is found at 1.3°K. At higher temperatures the sensitivity is too low for the transition itself, if any, to be detected, but an upper limit to the critical heat current is given. At 1.3°K and 2.1°K the rms deflection increases monotonically with increasing heat currents, but at intermediate temperatures it is variable, because the bob is often hardly agitated for long periods during apparently supercritical heat currents. This is called quiescent behaviour.
When a supercritical heat current is a delay before agitation of the fibre begins. The delay time, which is often not very well defined, has been measured as a function of the heat current. When the current is switched off the agitation of the bob decoys in a few seconds, but at 1.3°K the circulation about the fibre is small and variable for 100 sec or more, until the persistence and bias characteristic of undisturbed helium regained. These results are discussed in section IV.Approximate methods in high speed flow
https://hdl.handle.net/10023/13931
In many problems arising in the theory of compressible flow, the equations characterising the solution of the system are so intractable that recourse must be made to some approximate method which allows the essential features of the flow to be preserved, whilst to some degree, simplifying the mathematics. It is with certain methods of this type that this thesis is concerned.
In the subsequent work, we shall assume that the effects due to viscosity and heat conduction are so small as to be negligible. These assumptions may be shown to be largely valid except in those domains of the flow-field where the modified system of equations predicts regions in which the solution is in general multivalued. In the modified system, however, such ‘regions’ are avoided by the introduction of mathematical discontinuities and, assuming that the jump conditions across them can be determines, are sufficient to provide single-valued solutions valid everywhere, except at the discontinuity. The methods to be presented are formulated in the plane consisting of one space variable and one time variable.
Mon, 01 Jan 1962 00:00:00 GMThttps://hdl.handle.net/10023/139311962-01-01T00:00:00ZBurnside, Robert R.In many problems arising in the theory of compressible flow, the equations characterising the solution of the system are so intractable that recourse must be made to some approximate method which allows the essential features of the flow to be preserved, whilst to some degree, simplifying the mathematics. It is with certain methods of this type that this thesis is concerned.
In the subsequent work, we shall assume that the effects due to viscosity and heat conduction are so small as to be negligible. These assumptions may be shown to be largely valid except in those domains of the flow-field where the modified system of equations predicts regions in which the solution is in general multivalued. In the modified system, however, such ‘regions’ are avoided by the introduction of mathematical discontinuities and, assuming that the jump conditions across them can be determines, are sufficient to provide single-valued solutions valid everywhere, except at the discontinuity. The methods to be presented are formulated in the plane consisting of one space variable and one time variable.Aspects of natural convention and of non-linear hydridynamic stability
https://hdl.handle.net/10023/13922
In Part I of this thesis, steady and time-dependent, natural-convection similarity flows with mass transfer are discussed. Similarity flows for natural convection on families of two-dimensional bodies with closed lower ends are enumerated, when both a temperature distribution and a suction velocity distribution are prescribed at the body surface. For steady similarity flow on a heated vertical flat plate, with mass transfer at the surface, a numerical procedure is introduced for determining the velocity and temperature profiles. These results are presented in Figs. 2 and 3. Other similarity flows may be found by the same method.
A simplification, valid for “strong” suction, is discussed. An extension of Mangler’s transformation [1948] is given which reduces the equations governing axisymmetric flow to those for two-dimensional flow in steady natural convection.
In Part II non-linear resonant instability in parallel shear flows is discussed. A.D.D.Craik’s (see Usher and Craik [I]) modified version of Bateman’s [1956] variational formulation for viscous flows is employed to derive the second-order interaction equations governing the temporal evolution of a resonant wave triad in a sheer flow. (An extension of Craik’s variational formulation to free surface flows is presented but is not required in the subsequent analysis for the resonance problem). This problem was treated previously using a ‘direct’ approach (employing the Navier-Stokes equations) by Craik [1971]. The major advantage of the present method over the ‘direct’ method is the substantial reduction in algebraic complexity. Also, a justification of the validity of Craik’s previous analysis is given.
For this same resonance problem, third-order interaction equations are derived by the *direct* method since, to this order of approximation, little advantage is to be gained from the variational formulation. The resonance theory is thereby developed to the same order of approximation as the non-resonant third-order theory of Stuart [1960, 1962].
An asymptotic analysis for large Reynolds numbers reveals that the magnitudes of the third-order interaction coefficients – like certain of those at second-order – are remarkably large. Such results lead to a discussion of the regions of validity of the perturbation analysis. Also some light is shed on the roles played by resonance and three-dimensionality in the non-linear instability of shear flows.
Tue, 01 Jan 1974 00:00:00 GMThttps://hdl.handle.net/10023/139221974-01-01T00:00:00ZUsher, J. R.In Part I of this thesis, steady and time-dependent, natural-convection similarity flows with mass transfer are discussed. Similarity flows for natural convection on families of two-dimensional bodies with closed lower ends are enumerated, when both a temperature distribution and a suction velocity distribution are prescribed at the body surface. For steady similarity flow on a heated vertical flat plate, with mass transfer at the surface, a numerical procedure is introduced for determining the velocity and temperature profiles. These results are presented in Figs. 2 and 3. Other similarity flows may be found by the same method.
A simplification, valid for “strong” suction, is discussed. An extension of Mangler’s transformation [1948] is given which reduces the equations governing axisymmetric flow to those for two-dimensional flow in steady natural convection.
In Part II non-linear resonant instability in parallel shear flows is discussed. A.D.D.Craik’s (see Usher and Craik [I]) modified version of Bateman’s [1956] variational formulation for viscous flows is employed to derive the second-order interaction equations governing the temporal evolution of a resonant wave triad in a sheer flow. (An extension of Craik’s variational formulation to free surface flows is presented but is not required in the subsequent analysis for the resonance problem). This problem was treated previously using a ‘direct’ approach (employing the Navier-Stokes equations) by Craik [1971]. The major advantage of the present method over the ‘direct’ method is the substantial reduction in algebraic complexity. Also, a justification of the validity of Craik’s previous analysis is given.
For this same resonance problem, third-order interaction equations are derived by the *direct* method since, to this order of approximation, little advantage is to be gained from the variational formulation. The resonance theory is thereby developed to the same order of approximation as the non-resonant third-order theory of Stuart [1960, 1962].
An asymptotic analysis for large Reynolds numbers reveals that the magnitudes of the third-order interaction coefficients – like certain of those at second-order – are remarkably large. Such results lead to a discussion of the regions of validity of the perturbation analysis. Also some light is shed on the roles played by resonance and three-dimensionality in the non-linear instability of shear flows.Ion dynamics in collisionless shock waves
https://hdl.handle.net/10023/13917
In a laminar model of a collisionless magnetosonic shock wave, ion equations of motion are integrated through shock-like profiles. Conservation relations and Maxwell's equations allow a self-consistent determination of unknown downstream ion distribution functions fᵢ, ion temperature Tᵢ, and electric potential jump 𝛷. Favourable comparison of model Tᵢ, 𝛷.
Favourable comparison of model Tᵢ, 𝛷 , with experiment establishes (at low 𝛽 ≲ O.3, 𝛽=8 π N
[sub] l k (T[sub]e₂+Tᵢ[sub]l)/B₁²)
importance of laminar ion dynamics. Heating is due to distortion of Maxwellian distributions when entropy is conserved; in particular shock dynamics is dominated by a fast "tail" of reflected ions. The solutions for fᵢ are considered. The "stability" of the model to its assumptions (linear profiles, shock thickness (L[sub]s)) is shown. When reflections occur a self-consistent length emerges. The solutions Tᵢ, 𝛷 are extensively studied at various Mach numbers for different values of 𝛽. Laminar ion heating is very efficient and at high 𝛽 can exceed proper conservation levels due to ion reflections; at high 𝛽(≥ 𝛽 *) the electric potential is unable to slow the ions to conservation levels. The model predicts significant reflected ion currents in the plane of the shock. The boundary 𝛽 * is determined. Then laminar ion dynamics on the scale of the electron heating length (~10 C/w[sub]p ₑ) cannot occur for 𝛽 > 𝛽 *. Dependence on L[sub]s and T ₑ₁,/Tᵢ₁ is considered. The nature of non-laminar 𝛽 >𝛽* shocks is considered. Collisions are found to be important in laboratory shocks, and are efficient in slowing the reflected ions. In the absence of collisions, ion instabilities must be considered. It is shown that turbulent slowing of the fast ions cannot take place in L[sub]s alone. Further it is shown possible to construct a shock so that non-laminar mechanisms cannot occur significantly. Then the laminar model is re-instated. A decoupling of ion and electron heating lengths is proposed. Reflection heating in the Earth's Bow Shock (𝛽>𝛽*) is modelled, and is comparable with experiment.
Thu, 01 Jan 1976 00:00:00 GMThttps://hdl.handle.net/10023/139171976-01-01T00:00:00ZSherwell, DavidIn a laminar model of a collisionless magnetosonic shock wave, ion equations of motion are integrated through shock-like profiles. Conservation relations and Maxwell's equations allow a self-consistent determination of unknown downstream ion distribution functions fᵢ, ion temperature Tᵢ, and electric potential jump 𝛷. Favourable comparison of model Tᵢ, 𝛷.
Favourable comparison of model Tᵢ, 𝛷 , with experiment establishes (at low 𝛽 ≲ O.3, 𝛽=8 π N
[sub] l k (T[sub]e₂+Tᵢ[sub]l)/B₁²)
importance of laminar ion dynamics. Heating is due to distortion of Maxwellian distributions when entropy is conserved; in particular shock dynamics is dominated by a fast "tail" of reflected ions. The solutions for fᵢ are considered. The "stability" of the model to its assumptions (linear profiles, shock thickness (L[sub]s)) is shown. When reflections occur a self-consistent length emerges. The solutions Tᵢ, 𝛷 are extensively studied at various Mach numbers for different values of 𝛽. Laminar ion heating is very efficient and at high 𝛽 can exceed proper conservation levels due to ion reflections; at high 𝛽(≥ 𝛽 *) the electric potential is unable to slow the ions to conservation levels. The model predicts significant reflected ion currents in the plane of the shock. The boundary 𝛽 * is determined. Then laminar ion dynamics on the scale of the electron heating length (~10 C/w[sub]p ₑ) cannot occur for 𝛽 > 𝛽 *. Dependence on L[sub]s and T ₑ₁,/Tᵢ₁ is considered. The nature of non-laminar 𝛽 >𝛽* shocks is considered. Collisions are found to be important in laboratory shocks, and are efficient in slowing the reflected ions. In the absence of collisions, ion instabilities must be considered. It is shown that turbulent slowing of the fast ions cannot take place in L[sub]s alone. Further it is shown possible to construct a shock so that non-laminar mechanisms cannot occur significantly. Then the laminar model is re-instated. A decoupling of ion and electron heating lengths is proposed. Reflection heating in the Earth's Bow Shock (𝛽>𝛽*) is modelled, and is comparable with experiment.Investigations on classical symmetries theory of quantization
https://hdl.handle.net/10023/13913
The thesis divides naturally into two parts. Part I raises, and in some cases answers, questions concerning symmetry in classical mechanics. The main result (Theorem 6.4) shows that the assumption of the existence of a realization puts an upper limit on the rank of the algebra.
The heart of the thesis (covering three-quarters of the volume) is section II on the quantization of classical systems. §1 lists axioms desirable in any quantization rule for the 'functions of the q's'. The momentum observables are introduced in §2 prior to their quantization in §4. §5 essentially shows how conventional quantum mechanics fits into this scheme of things. By progressive specialization from a general manifold to a vector space, from a general quantization scheme to one which is linear on the linear momentum functions, and finally to an entirely well-behaved (admissible) quantization rule, into which conventional quantum mechanics fits nicely, we obtain in §7-§9 results which become progressively more and more powerful. The final theorem (Theorem 9.2) is perhaps the most significant of all. This result states that there exists a class of functions, which contains all functions of the q's and functions of the p's and all momentum observables and which is closed with respect to any linear canonical transformation L; a rule A assigning a unique self-adjoint operator to each such function f; a unitary operator WL corresponding to L and an equation
𝛬(𝑓 ∘ 𝐿) = 𝑊[sub]𝐿⁻ 𝛬 𝑓 𝑊[sub]𝐿
Sat, 01 Jan 1972 00:00:00 GMThttps://hdl.handle.net/10023/139131972-01-01T00:00:00ZGuest, P. B.The thesis divides naturally into two parts. Part I raises, and in some cases answers, questions concerning symmetry in classical mechanics. The main result (Theorem 6.4) shows that the assumption of the existence of a realization puts an upper limit on the rank of the algebra.
The heart of the thesis (covering three-quarters of the volume) is section II on the quantization of classical systems. §1 lists axioms desirable in any quantization rule for the 'functions of the q's'. The momentum observables are introduced in §2 prior to their quantization in §4. §5 essentially shows how conventional quantum mechanics fits into this scheme of things. By progressive specialization from a general manifold to a vector space, from a general quantization scheme to one which is linear on the linear momentum functions, and finally to an entirely well-behaved (admissible) quantization rule, into which conventional quantum mechanics fits nicely, we obtain in §7-§9 results which become progressively more and more powerful. The final theorem (Theorem 9.2) is perhaps the most significant of all. This result states that there exists a class of functions, which contains all functions of the q's and functions of the p's and all momentum observables and which is closed with respect to any linear canonical transformation L; a rule A assigning a unique self-adjoint operator to each such function f; a unitary operator WL corresponding to L and an equation
𝛬(𝑓 ∘ 𝐿) = 𝑊[sub]𝐿⁻ 𝛬 𝑓 𝑊[sub]𝐿An algebraic formulation of asmptotically separable quantum mechanics
https://hdl.handle.net/10023/13909
This thesis explores the possibility of an algebraic formulation of non-relativistic quantum theory in which certain paradoxes associated with non-locality may be resolved. It is shown that the localisation of a free quantum mechanical wave function at large time coincides approximately with the localisation of an ensemble of classical particles having the same momentum range. This result is used to give a formal definition of spatially separating states and spatially separating particles. We then study certain C*-algebras on which expectation values converge in an infinite time limit. By considering such algebras which contain local observables it is possible to introduce states at infinity as limits of states described by wave functions. In such a state at infinity there is zero probability of a position measurement finding the system in any bounded region in configuration space. It is shown that a C*-algebra exists on which any coherent superposition of spatially separating states will converge in an infinite time limit to a mixture of disjoint states. This allows us to obtain an asymptotic resolution of de Broglie's paradox and the Einstein, Podolsy and Rosen paradox. These results are obtained for the simplest types of quantum systems i.e. a one particle system without spin having configuration space IRⁿ and a system consisting of two such particles which may be distinguished from each other.
Sun, 01 Jan 1984 00:00:00 GMThttps://hdl.handle.net/10023/139091984-01-01T00:00:00ZMcLean, R. G. DerekThis thesis explores the possibility of an algebraic formulation of non-relativistic quantum theory in which certain paradoxes associated with non-locality may be resolved. It is shown that the localisation of a free quantum mechanical wave function at large time coincides approximately with the localisation of an ensemble of classical particles having the same momentum range. This result is used to give a formal definition of spatially separating states and spatially separating particles. We then study certain C*-algebras on which expectation values converge in an infinite time limit. By considering such algebras which contain local observables it is possible to introduce states at infinity as limits of states described by wave functions. In such a state at infinity there is zero probability of a position measurement finding the system in any bounded region in configuration space. It is shown that a C*-algebra exists on which any coherent superposition of spatially separating states will converge in an infinite time limit to a mixture of disjoint states. This allows us to obtain an asymptotic resolution of de Broglie's paradox and the Einstein, Podolsy and Rosen paradox. These results are obtained for the simplest types of quantum systems i.e. a one particle system without spin having configuration space IRⁿ and a system consisting of two such particles which may be distinguished from each other.Geometrical and topological properties of fractal percolation
https://hdl.handle.net/10023/13907
The basic 'fractal percolation' process was first proposed by Mandelbrot in 1974 and takes the following form. Let M ≥2 and P ∈ [0,1]; we start with the unit square C₀ = [0,1]²; Divide C₀ into M² equal closed squares, each of side-length M⁻¹ , in the natural way and retain each of these squares with probability p, or else remove it with probability 1 - p. We let C₁ be the union of those squares retained. The process is now repeated within each square of C₁ to give a new set C₂⊆C₁, consisting of squares of side-length M⁻². Iterating the construction in the obvious way, we obtain a decreasing sequence of sets C₀⊇ C₁ ⊇ C₂ ⊇ … with limit C[sub]∞ = ∩[sub]n≥₁C[sub]n.
The set C[sub]∞ is an example of a random Cantor set, and is typically highly intricate in nature. It may be empty, dust-like or highly connected, depending on the value of p; percolation is said to occur if C[sub]∞ contains large connected components linking opposite sides of the unit square.
In this thesis we shall investigate some of the geometrical and topological properties of C[sub]∞ that hold either almost surely (with probability 1) or with non-zero probability. In particular, the following results are established. We obtain (almost sure) lower and upper bounds on the box-counting dimension of the 'straightest' crossings in C[sub]∞ whenever percolation occurs; we also look at the distribution of the sizes of the connected components and the probability of percolation. In the three-dimensional version of the process, we establish the existence of two distinct phases of percolation, corresponding to the occurrence of paths and surfaces (or 'sheets') in the limit set, and study the limiting behaviour of the phase transition to sheet percolation as M → ∞. We also consider the results of some computer simulations of fractal percolation and present a number of generalisations of the basic process and other closely related constructions.
Thu, 01 Jan 1998 00:00:00 GMThttps://hdl.handle.net/10023/139071998-01-01T00:00:00ZOrzechowski, Mark E.The basic 'fractal percolation' process was first proposed by Mandelbrot in 1974 and takes the following form. Let M ≥2 and P ∈ [0,1]; we start with the unit square C₀ = [0,1]²; Divide C₀ into M² equal closed squares, each of side-length M⁻¹ , in the natural way and retain each of these squares with probability p, or else remove it with probability 1 - p. We let C₁ be the union of those squares retained. The process is now repeated within each square of C₁ to give a new set C₂⊆C₁, consisting of squares of side-length M⁻². Iterating the construction in the obvious way, we obtain a decreasing sequence of sets C₀⊇ C₁ ⊇ C₂ ⊇ … with limit C[sub]∞ = ∩[sub]n≥₁C[sub]n.
The set C[sub]∞ is an example of a random Cantor set, and is typically highly intricate in nature. It may be empty, dust-like or highly connected, depending on the value of p; percolation is said to occur if C[sub]∞ contains large connected components linking opposite sides of the unit square.
In this thesis we shall investigate some of the geometrical and topological properties of C[sub]∞ that hold either almost surely (with probability 1) or with non-zero probability. In particular, the following results are established. We obtain (almost sure) lower and upper bounds on the box-counting dimension of the 'straightest' crossings in C[sub]∞ whenever percolation occurs; we also look at the distribution of the sizes of the connected components and the probability of percolation. In the three-dimensional version of the process, we establish the existence of two distinct phases of percolation, corresponding to the occurrence of paths and surfaces (or 'sheets') in the limit set, and study the limiting behaviour of the phase transition to sheet percolation as M → ∞. We also consider the results of some computer simulations of fractal percolation and present a number of generalisations of the basic process and other closely related constructions.Graph directed self-conformal multifractals
https://hdl.handle.net/10023/13903
In this thesis we study the multifractal structure of graph directed self-conformal measures. We begin by introducing a number of notions from geometric measure theory. In particular, several notions of dimension, graph directed iterated function schemes, and the thermodynamic formalism. We then give an historical introduction to multifractal analysis. Finally, we develop our own contribution to multifractal analysis. Our own contribution to multifractal analysis can be broken into three parts; the proof of two multifractal density theorems, the calculation of the multifractal spectrum of self-conformal measures coded by graph directed iterated function schemes, and the introduction of a relative multifractal formalism together with an investigation of the relative multifractal structure of one graph directed self-conformal measure with respect to another. Specifically, in Chapter 5 we show that by interpreting the multifractal Hausdorff and packing measures Olsen introduced in [0195] as Henstock-Thomson variation measures we are able to obtain two stronger density theorems than those obtained by Olsen. In Chapter 6 we give full details of the calculation of the multifractal spectrum of graph directed self-conformal measures satisfying the strong open set condition and show that the multifractal Hausdorff and packing measures introduced by Olsen in [0195] take positive and finite values at the critical dimension provided that the self-conformal measures satisfy the strong separation condition. In Chapter 7 we formalise the idea of performing multifractal analysis with respect to an arbitrary reference measure by developing a formalism for the multifractal analysis of one measure with respect to another. This formalism is based on the ideas of the 'multifractal formalism' as first introduced by Halsey et. al. [HJKPS86] and closely parallels Olsen's formal treatment of this formalism in [0195]. In Chapter 8 we illustrate our relative multifractal formalism by investigating the relative multifractal structure of one graph directed self-conformal measure with respect to another where the two measures are based on the same graph directed self-conformal iterated function scheme which satisfies the strong open set condition.
Fri, 01 Jan 1999 00:00:00 GMThttps://hdl.handle.net/10023/139031999-01-01T00:00:00ZCole, JulianIn this thesis we study the multifractal structure of graph directed self-conformal measures. We begin by introducing a number of notions from geometric measure theory. In particular, several notions of dimension, graph directed iterated function schemes, and the thermodynamic formalism. We then give an historical introduction to multifractal analysis. Finally, we develop our own contribution to multifractal analysis. Our own contribution to multifractal analysis can be broken into three parts; the proof of two multifractal density theorems, the calculation of the multifractal spectrum of self-conformal measures coded by graph directed iterated function schemes, and the introduction of a relative multifractal formalism together with an investigation of the relative multifractal structure of one graph directed self-conformal measure with respect to another. Specifically, in Chapter 5 we show that by interpreting the multifractal Hausdorff and packing measures Olsen introduced in [0195] as Henstock-Thomson variation measures we are able to obtain two stronger density theorems than those obtained by Olsen. In Chapter 6 we give full details of the calculation of the multifractal spectrum of graph directed self-conformal measures satisfying the strong open set condition and show that the multifractal Hausdorff and packing measures introduced by Olsen in [0195] take positive and finite values at the critical dimension provided that the self-conformal measures satisfy the strong separation condition. In Chapter 7 we formalise the idea of performing multifractal analysis with respect to an arbitrary reference measure by developing a formalism for the multifractal analysis of one measure with respect to another. This formalism is based on the ideas of the 'multifractal formalism' as first introduced by Halsey et. al. [HJKPS86] and closely parallels Olsen's formal treatment of this formalism in [0195]. In Chapter 8 we illustrate our relative multifractal formalism by investigating the relative multifractal structure of one graph directed self-conformal measure with respect to another where the two measures are based on the same graph directed self-conformal iterated function scheme which satisfies the strong open set condition.Parametric models of surfaces
https://hdl.handle.net/10023/13897
Tue, 01 Jan 1957 00:00:00 GMThttps://hdl.handle.net/10023/138971957-01-01T00:00:00ZRobertson, Stewart A. (Stewart Alexander)Finite difference techniques of improved accuracy
https://hdl.handle.net/10023/13888
It is the major purpose of this thesis to propose finite difference techniques of improved accuracy for the numerical solution of ordinary differential equations, and for the numerical evaluation of definite integrals, the former problem being discussed in Chapter II, and the latter in Chapter IV. In Chapter III the stability of the formulae evolved in Chapter II is studied.
Tue, 01 Jan 1963 00:00:00 GMThttps://hdl.handle.net/10023/138881963-01-01T00:00:00ZLambert, J. D.It is the major purpose of this thesis to propose finite difference techniques of improved accuracy for the numerical solution of ordinary differential equations, and for the numerical evaluation of definite integrals, the former problem being discussed in Chapter II, and the latter in Chapter IV. In Chapter III the stability of the formulae evolved in Chapter II is studied.Polynomial interpolation on a triangular region
https://hdl.handle.net/10023/13887
It is well known that given f there is a unique polynomial of degree at most n which interpolates f on the standard triangle with uniform nodes (i, j), i, j ≥ 0, i + j ≤n. This leads us to the study of polynomial interpolation on a "triangular" domain with the nodes,
S = {([i], [j]): i, j ≥ 0, i + j ≤n}, [k] = [k][sub]q = (1-qᵏ)/(1-q), q > 0, which includes the standard triangle as a special case. In Chapter 2 of this thesis we derive a forward difference formula (of degree at most n) in the x and y directions for the interpolating polynomial P[sub]n on S. We also construct a Lagrange form of an interpolating polynomial which uses hyperbolas (although its coefficients are of degree up to 2n) and discuss a Neville-Aitken algorithm. In Chapter 3 we derive the Newton formula for the interpolating polynomial P[sub]n on the set of distinct points {(xᵢ, y[sub]j): i, j ≥ 0, i + j ≤n}. In particular if xᵢ = [i][sub]p and y[sub]j = [j]q, we show that Newton's form of P[sub]n reduces to a forward difference formula. We show further that we can express the interpolating polynomial on S itself in a Lagrange form and although its coefficients Ln/ij are not as simple as those of the first Lagrange form, they all have degree n. Moreover, Ln/ij can all be expressed in terms of Lm/0,0, 0 ≤ m ≤ n. In Chapter 4 we show that P[sub]n has a limit when both p, q → 0. We then verify that the interpolation properties of the limit form depend on the appropriate partial derivatives of f(x, y). In Chapter 5 we study integration rules I[sub]n of interpolatory type on the triangle S[sub] = {(x, y): 0 ≤ x ≤y ≤ [n]). For 1 ≤ n ≤5, we calculate the weights wn/ij for I[sub]n in terms of the parameter q and study certain general properties which govern wn/ij on S[sub]n. Finally, Chapter 6 deals with the behaviour of the Lebesgue functions 𝜆[sub]n(x, y; q) and the corresponding Lebesgue constant. We prove a property concerning where 𝜆[sub]n takes the value 1 at points other than the interpolation nodes. We also analyse the discontinuity of the directional derivative of 𝜆[sub]n on S[sub]n.
Sat, 01 Jan 1994 00:00:00 GMThttps://hdl.handle.net/10023/138871994-01-01T00:00:00ZYahaya, DaudIt is well known that given f there is a unique polynomial of degree at most n which interpolates f on the standard triangle with uniform nodes (i, j), i, j ≥ 0, i + j ≤n. This leads us to the study of polynomial interpolation on a "triangular" domain with the nodes,
S = {([i], [j]): i, j ≥ 0, i + j ≤n}, [k] = [k][sub]q = (1-qᵏ)/(1-q), q > 0, which includes the standard triangle as a special case. In Chapter 2 of this thesis we derive a forward difference formula (of degree at most n) in the x and y directions for the interpolating polynomial P[sub]n on S. We also construct a Lagrange form of an interpolating polynomial which uses hyperbolas (although its coefficients are of degree up to 2n) and discuss a Neville-Aitken algorithm. In Chapter 3 we derive the Newton formula for the interpolating polynomial P[sub]n on the set of distinct points {(xᵢ, y[sub]j): i, j ≥ 0, i + j ≤n}. In particular if xᵢ = [i][sub]p and y[sub]j = [j]q, we show that Newton's form of P[sub]n reduces to a forward difference formula. We show further that we can express the interpolating polynomial on S itself in a Lagrange form and although its coefficients Ln/ij are not as simple as those of the first Lagrange form, they all have degree n. Moreover, Ln/ij can all be expressed in terms of Lm/0,0, 0 ≤ m ≤ n. In Chapter 4 we show that P[sub]n has a limit when both p, q → 0. We then verify that the interpolation properties of the limit form depend on the appropriate partial derivatives of f(x, y). In Chapter 5 we study integration rules I[sub]n of interpolatory type on the triangle S[sub] = {(x, y): 0 ≤ x ≤y ≤ [n]). For 1 ≤ n ≤5, we calculate the weights wn/ij for I[sub]n in terms of the parameter q and study certain general properties which govern wn/ij on S[sub]n. Finally, Chapter 6 deals with the behaviour of the Lebesgue functions 𝜆[sub]n(x, y; q) and the corresponding Lebesgue constant. We prove a property concerning where 𝜆[sub]n takes the value 1 at points other than the interpolation nodes. We also analyse the discontinuity of the directional derivative of 𝜆[sub]n on S[sub]n.Some contributions to the theory and application of polynomial approximation
https://hdl.handle.net/10023/13883
The fundamental theorem, as far as this work is concerned, is Weierstrass' theorem (1885) on the approximability of continuous functions by polynomials. Since the time of Weierstrass (1815-97) and his equally important contemporary Chebyshev (1821-94), the topic of approximation has grown enormously into a subject of considerable interest to both pure and applied mathematicians. The subject matter of this thesis, being exclusively concerned with polynomial approximations to a single-valued, function of one real variable, is on the side of 'applied' side of approximation theory. The first chapter lists the definitions and theorems required subsequently. Chapter is devoted to estimates for the maximum error in minimax polynomial approximations. Extensions of this are used to obtain crude error estimates for cubic spline approximations. The following chapter extends the minimax results to deal also with best L[sub]p polynomial approximations, which include beat least squares (L₂) and best modulus of integral (L₁) approximations as special cases. Chapter 4 is different in character. It is on the practical problem of approximating to convex or nearly convex data.
Wed, 01 Jan 1969 00:00:00 GMThttps://hdl.handle.net/10023/138831969-01-01T00:00:00ZPhillips, G. M. (George McArtney)The fundamental theorem, as far as this work is concerned, is Weierstrass' theorem (1885) on the approximability of continuous functions by polynomials. Since the time of Weierstrass (1815-97) and his equally important contemporary Chebyshev (1821-94), the topic of approximation has grown enormously into a subject of considerable interest to both pure and applied mathematicians. The subject matter of this thesis, being exclusively concerned with polynomial approximations to a single-valued, function of one real variable, is on the side of 'applied' side of approximation theory. The first chapter lists the definitions and theorems required subsequently. Chapter is devoted to estimates for the maximum error in minimax polynomial approximations. Extensions of this are used to obtain crude error estimates for cubic spline approximations. The following chapter extends the minimax results to deal also with best L[sub]p polynomial approximations, which include beat least squares (L₂) and best modulus of integral (L₁) approximations as special cases. Chapter 4 is different in character. It is on the practical problem of approximating to convex or nearly convex data.Some consequences of symmetry in strong Stieltjes distributions
https://hdl.handle.net/10023/13881
The main purpose of this work is to study a class of strong Stieltjes distributions 𝜓(t), defined on an interval (a, b) ⊆ (0, ∞), where 0 < 𝛽 < b ≤ ∞ and a = 𝛽²/b which satisfy the symmetric property
(dψ(t))/t[super]ω=-(dψ(β^2/t))/((β^2/t)[super]ω), tε (a,b), 2ωε𝓩
We investigate the consequences of this symmetric property on the orthogonal L-polynomials related to distributions ψ(t)and which are the denominators of the two-point Pade approximants for the power series that arise in the moment problem. We examine relations involving the coefficients of the continued fractions that correspond to these power series. We also study the consequences of the symmetry on the associated quadrature formulae.
Thu, 01 Jan 1998 00:00:00 GMThttps://hdl.handle.net/10023/138811998-01-01T00:00:00ZBracciali, Cleonice Fátima BraccialiThe main purpose of this work is to study a class of strong Stieltjes distributions 𝜓(t), defined on an interval (a, b) ⊆ (0, ∞), where 0 < 𝛽 < b ≤ ∞ and a = 𝛽²/b which satisfy the symmetric property
(dψ(t))/t[super]ω=-(dψ(β^2/t))/((β^2/t)[super]ω), tε (a,b), 2ωε𝓩
We investigate the consequences of this symmetric property on the orthogonal L-polynomials related to distributions ψ(t)and which are the denominators of the two-point Pade approximants for the power series that arise in the moment problem. We examine relations involving the coefficients of the continued fractions that correspond to these power series. We also study the consequences of the symmetry on the associated quadrature formulae.Some aspects of the Jacobian conjecture: (the geometry of automorphisms of ℂ²)
https://hdl.handle.net/10023/13878
We consider the affine varieties which arise by considering invertible polynomial maps from ℂ² to itself of less than or equal to a given-degree. These varieties arise naturally in the investigation of the long-standing Jacobian Conjecture. We start with some calculations in the lower degree cases. These calculations provide a proof of the Jacobian conjecture in these cases and suggest how the investigation in the higher degree cases should proceed. We then show how invertible polynomial maps can be decomposed as products of what we call triangular maps and we are able to prove a uniqueness result which gives a stronger version of Jung's theorem [j] which is one of the most important results in this area. Our proof also gives a new derivation of Jung's theorem from Segre's lemma. We give a different decomposition of an invertible polynomial map as a composition of "irreducible maps" and we are able to write down standard forms for these irreducibles. We use these standard forms to give a description of the structure of the varieties of invertible maps. We consider some interesting group actions on our varieties and show how these fit in with the structure we describe. Finally, we look at the problem of identifying polynomial maps of finite order. Our description of the structure of the above varieties allows us to solve this problem completely and we are able to show that the only elements of finite order are those which arise from conjugating linear elements of finite order.
Thu, 01 Jan 1987 00:00:00 GMThttps://hdl.handle.net/10023/138781987-01-01T00:00:00ZAli, A. Hamid A. HussainWe consider the affine varieties which arise by considering invertible polynomial maps from ℂ² to itself of less than or equal to a given-degree. These varieties arise naturally in the investigation of the long-standing Jacobian Conjecture. We start with some calculations in the lower degree cases. These calculations provide a proof of the Jacobian conjecture in these cases and suggest how the investigation in the higher degree cases should proceed. We then show how invertible polynomial maps can be decomposed as products of what we call triangular maps and we are able to prove a uniqueness result which gives a stronger version of Jung's theorem [j] which is one of the most important results in this area. Our proof also gives a new derivation of Jung's theorem from Segre's lemma. We give a different decomposition of an invertible polynomial map as a composition of "irreducible maps" and we are able to write down standard forms for these irreducibles. We use these standard forms to give a description of the structure of the varieties of invertible maps. We consider some interesting group actions on our varieties and show how these fit in with the structure we describe. Finally, we look at the problem of identifying polynomial maps of finite order. Our description of the structure of the above varieties allows us to solve this problem completely and we are able to show that the only elements of finite order are those which arise from conjugating linear elements of finite order.On Riesz summability
https://hdl.handle.net/10023/13825
The thesis is divided into four chapters. The first contains notation and fundamental results. The others contain a number of theorems on Riesz summability, ordinary in the second, absolute in the third and strong in the fourth. The substance of chapter II has appeared in the Proceedings of the Glasgow Mathermatical Association [2].
Tue, 01 Jan 1963 00:00:00 GMThttps://hdl.handle.net/10023/138251963-01-01T00:00:00ZShawyer, BruceThe thesis is divided into four chapters. The first contains notation and fundamental results. The others contain a number of theorems on Riesz summability, ordinary in the second, absolute in the third and strong in the fourth. The substance of chapter II has appeared in the Proceedings of the Glasgow Mathermatical Association [2].Global optimization using interval arithmetic
https://hdl.handle.net/10023/13824
This thesis contains a description of algorithm, MW, for bounding the global minimizers and globally minimum value of a twice continuously differentiable function f :Rⁿ → R¹ R1 in a compact sub-interval of Rⁿ. The algorithm MW is similar to the algorithm of Hansen (Han-80a] in that interval arithmetic is used together with certain of Hansen's ideas, but is different from Hansen's algorithm in that MW bounds the Kuhn Tucker points corresponding to the global minimizers of f in the given sab-interval. The Kuhn Tucker points are bounded with prescribed precision by using either of the algorithms KMSW [SheW-85c] or MAP [SheW-85b]. Numerical results which are obtained from Triplex [BaCM-82a] [MorC-83a] implementations of H and MW axe presented.
Thu, 01 Jan 1987 00:00:00 GMThttps://hdl.handle.net/10023/138241987-01-01T00:00:00ZMohd, Ismail BinThis thesis contains a description of algorithm, MW, for bounding the global minimizers and globally minimum value of a twice continuously differentiable function f :Rⁿ → R¹ R1 in a compact sub-interval of Rⁿ. The algorithm MW is similar to the algorithm of Hansen (Han-80a] in that interval arithmetic is used together with certain of Hansen's ideas, but is different from Hansen's algorithm in that MW bounds the Kuhn Tucker points corresponding to the global minimizers of f in the given sab-interval. The Kuhn Tucker points are bounded with prescribed precision by using either of the algorithms KMSW [SheW-85c] or MAP [SheW-85b]. Numerical results which are obtained from Triplex [BaCM-82a] [MorC-83a] implementations of H and MW axe presented.Modifications of some algorithms for unconstrained optimization
https://hdl.handle.net/10023/13822
This thesis contains an account of several modifications to two algorithms for unconstrained optimization, both of which are due to Gill and Murray. Chapter One contains a brief survey of unconstrained optimization and contains also some results which are used subsequently. Chapter Two contains an account of some work on iterative procedures for the solution of operator equations in Banach spaces due to Wolfe (1978a) in which it is suggested that it may be possible, in certain circumstances, to use high-order iterative procedures rather than Newton's method, thereby obtaining computational advantages. In Chapter Three the Newton-type algorithm of Gill and Murray (1974) is described and the ideas contained in Chapter Two are used to construct some modifications of this algorithm. Chapter Four contains some algorithms for the numerical estimation of both full and b and-type Hessian matrices. These algorithms may be used in conjunction with the optimization algorithms which are described in Chapters Three and Five. In Chapter Five the least-squares algorithm of Gill and Murray (1976) is described and the ideas contained in Chapter Two are used to construct some modifications of this algorithm. Chapter Six contains the computational results which were obtained by using the algorithms which are described in Chapters Three, Four and Five to solve the test problems which are listed in Appendices One and Two.
Mon, 01 Jan 1979 00:00:00 GMThttps://hdl.handle.net/10023/138221979-01-01T00:00:00ZMirnia-Harikandi, KThis thesis contains an account of several modifications to two algorithms for unconstrained optimization, both of which are due to Gill and Murray. Chapter One contains a brief survey of unconstrained optimization and contains also some results which are used subsequently. Chapter Two contains an account of some work on iterative procedures for the solution of operator equations in Banach spaces due to Wolfe (1978a) in which it is suggested that it may be possible, in certain circumstances, to use high-order iterative procedures rather than Newton's method, thereby obtaining computational advantages. In Chapter Three the Newton-type algorithm of Gill and Murray (1974) is described and the ideas contained in Chapter Two are used to construct some modifications of this algorithm. Chapter Four contains some algorithms for the numerical estimation of both full and b and-type Hessian matrices. These algorithms may be used in conjunction with the optimization algorithms which are described in Chapters Three and Five. In Chapter Five the least-squares algorithm of Gill and Murray (1976) is described and the ideas contained in Chapter Two are used to construct some modifications of this algorithm. Chapter Six contains the computational results which were obtained by using the algorithms which are described in Chapters Three, Four and Five to solve the test problems which are listed in Appendices One and Two.Formal methods for deriving Green-type transitional and uniform asymptotic expansions from differential equations
https://hdl.handle.net/10023/13821
In the present work, we develop and illustrate powerful, but straightforward, formal methods for deriving asymptotic expansions from differential equations. In the second chapter, the ‘inverse Frobenius method’ for deriving Stokes expansions is exemplified. The main body of this thesis, however, consists of the development of the new Green-Liouville-Melin transform method, and its detailed application to modified Bessel functions, parabolic cylinder functions, Whittaker functions, Poiseuille functions, confluent hypergeometric functions, and also to periodic Mathieu functions and oblate spheroidal wave functions, all with at least one parameter large⁺. The wide scope of the method is evinced by the fact that treatment of the essentially eigenvalue problem posed by the two last-named cases requires no additional techniques. This method, as will be explained in detail in chapter 3, yields Green-type, transitional and uniform expansions.
The transitional expansions found in this way are usually of a simpler form than those derived by alternative processes (e.g. perturbation theory). To state an example, the asymptotic expansions for the periodic Mathieu functions ce(z,h) and se(z,h) valid near |z| = 1/2π that have been obtained in earlier work contain the complicated parabolic cylinder functions (c.f. Meixner 1948, Sips 1949, Dingle and Müller 1962). By contrast, our methods yield expansions of comparable applicability, but involving only elementary functions. To demonstrate their usefulness, we have fed these expansions into a digital computer and obtained extensive tables for ce(z,h) and se(z,h) in the range 50°≤ z ≤90° . Extracts from these tables and comparisons with correct results are given in §8.71.
Following the chapters on the introduction and applications of the Mellin transform technique, there is some preliminary work on a new method for determining the general term in Green-type expansions. The method is illustrated by detailed calculations for modified Bessel and parabolic cylinder functions.
In the final chapter, we present certain suggestions for further work.
Fri, 01 Jan 1965 00:00:00 GMThttps://hdl.handle.net/10023/138211965-01-01T00:00:00ZJorna, SiebeIn the present work, we develop and illustrate powerful, but straightforward, formal methods for deriving asymptotic expansions from differential equations. In the second chapter, the ‘inverse Frobenius method’ for deriving Stokes expansions is exemplified. The main body of this thesis, however, consists of the development of the new Green-Liouville-Melin transform method, and its detailed application to modified Bessel functions, parabolic cylinder functions, Whittaker functions, Poiseuille functions, confluent hypergeometric functions, and also to periodic Mathieu functions and oblate spheroidal wave functions, all with at least one parameter large⁺. The wide scope of the method is evinced by the fact that treatment of the essentially eigenvalue problem posed by the two last-named cases requires no additional techniques. This method, as will be explained in detail in chapter 3, yields Green-type, transitional and uniform expansions.
The transitional expansions found in this way are usually of a simpler form than those derived by alternative processes (e.g. perturbation theory). To state an example, the asymptotic expansions for the periodic Mathieu functions ce(z,h) and se(z,h) valid near |z| = 1/2π that have been obtained in earlier work contain the complicated parabolic cylinder functions (c.f. Meixner 1948, Sips 1949, Dingle and Müller 1962). By contrast, our methods yield expansions of comparable applicability, but involving only elementary functions. To demonstrate their usefulness, we have fed these expansions into a digital computer and obtained extensive tables for ce(z,h) and se(z,h) in the range 50°≤ z ≤90° . Extracts from these tables and comparisons with correct results are given in §8.71.
Following the chapters on the introduction and applications of the Mellin transform technique, there is some preliminary work on a new method for determining the general term in Green-type expansions. The method is illustrated by detailed calculations for modified Bessel and parabolic cylinder functions.
In the final chapter, we present certain suggestions for further work.Presentations of linear groups
https://hdl.handle.net/10023/13814
If d(M) denotes the rank of the Schur multiplicator of a finite group G, then a group is efficient if -def G = d(M). Efficient presentations of the simple groups PSL(2,p), p an odd prime > 3, were obtained by J.G. Sunday.This raised the question of whether or not all finite simple groups are efficient.
In this thesis, we investigate the deficiency of the groups PSL(2,pⁿ). J.A. Todd gave presentations for PSL(2,pⁿ) which use large numbers of generators and relations. Starting with these, we obtain, at best, deficiency -1 presentations for PSL(2,2ⁿ) (= SL(2,2ⁿ)) and deficiency -6 presentations for PSL(2,pⁿ), p an odd prime. If pⁿ = -1(mod 4), the latter can be reduced to a deficiency -4 presentation. Efficient presentations for PSL(2,25), PSL(2,27) and PSL(2,49) are obtained.
The Behr-Mennicke presentation for PSL(2,p) is one of the most fundamental in the sense that it forms the basis for others, such as those given by Sunday, Zassenhaus and Sidki. Behr and Mennicke derived their presentation indirectly, and it would be desirable to have a more direct proof. The groups G[sub]p(a) are defined as
< U, R, S | U³ = (UR)² = (US)² = Sᵖ = Rᵗ = (SaRU)³= 1, Sᵃ²R = RS >
where a ε GF(p)* and a²ᵗ = 1 (mod p) . We show that G[sub]p (2) is isomorphic with the Behr-Mennicke presentation for PSL(2,p), p > 3. Conditions are found to discover when Gp (a) is isomorphic with PSL(2,p) and, under these conditions, this provides a direct proof of the Behr-Mennicke presentations. For any odd positive integer m, we show that the groups SL(2,ℤ (m)) and PSL(2,ℤ(m)) are efficient.
Sat, 01 Jan 1983 00:00:00 GMThttps://hdl.handle.net/10023/138141983-01-01T00:00:00ZWilliams, Peter D.If d(M) denotes the rank of the Schur multiplicator of a finite group G, then a group is efficient if -def G = d(M). Efficient presentations of the simple groups PSL(2,p), p an odd prime > 3, were obtained by J.G. Sunday.This raised the question of whether or not all finite simple groups are efficient.
In this thesis, we investigate the deficiency of the groups PSL(2,pⁿ). J.A. Todd gave presentations for PSL(2,pⁿ) which use large numbers of generators and relations. Starting with these, we obtain, at best, deficiency -1 presentations for PSL(2,2ⁿ) (= SL(2,2ⁿ)) and deficiency -6 presentations for PSL(2,pⁿ), p an odd prime. If pⁿ = -1(mod 4), the latter can be reduced to a deficiency -4 presentation. Efficient presentations for PSL(2,25), PSL(2,27) and PSL(2,49) are obtained.
The Behr-Mennicke presentation for PSL(2,p) is one of the most fundamental in the sense that it forms the basis for others, such as those given by Sunday, Zassenhaus and Sidki. Behr and Mennicke derived their presentation indirectly, and it would be desirable to have a more direct proof. The groups G[sub]p(a) are defined as
< U, R, S | U³ = (UR)² = (US)² = Sᵖ = Rᵗ = (SaRU)³= 1, Sᵃ²R = RS >
where a ε GF(p)* and a²ᵗ = 1 (mod p) . We show that G[sub]p (2) is isomorphic with the Behr-Mennicke presentation for PSL(2,p), p > 3. Conditions are found to discover when Gp (a) is isomorphic with PSL(2,p) and, under these conditions, this provides a direct proof of the Behr-Mennicke presentations. For any odd positive integer m, we show that the groups SL(2,ℤ (m)) and PSL(2,ℤ(m)) are efficient.A study of the infinite dimensional linear and symplectic groups
https://hdl.handle.net/10023/13810
By a linear group we shall mean essentially a group of invertible matrices over a ring. Thus, we include in our class of linear groups the 'classical' geometric groups. These are the general linear group, GL[sub]n(F), the orthogonal groups, 0[sub]n (F) and the syraplectic groups Sp[sub]n(F). The normal and subnormal subgroup structure of these groups is well known and has been the subject of much investigation since the turn of the century. We study here the normal and subnormal structure of some of their infinite dimensional counterparts, namely, the infinite dimensional linear group GL(Ω,R), for arbitrary rings R, and the infinite dimensional syraplectic group Sp(Ω,R), for commutative rings R with identity. We shall see that a key role in the classification of the normal and subnormal subgroups of GL(Ω,R) and Sp(Ω,R) is played by the 'elementary' normal subgroups E(Ω,R) and ESp(Ω,R). We shall also see that, in the case of the infinite dimensional linear group, the normal subgroup structure depends very much upon the way in which R is generated as a right R-module. We shall also give a presentation for the 'elementary' subgroup E(Ω,R) when R is a division ring.
Mon, 01 Jan 1979 00:00:00 GMThttps://hdl.handle.net/10023/138101979-01-01T00:00:00ZArrell, David G.By a linear group we shall mean essentially a group of invertible matrices over a ring. Thus, we include in our class of linear groups the 'classical' geometric groups. These are the general linear group, GL[sub]n(F), the orthogonal groups, 0[sub]n (F) and the syraplectic groups Sp[sub]n(F). The normal and subnormal subgroup structure of these groups is well known and has been the subject of much investigation since the turn of the century. We study here the normal and subnormal structure of some of their infinite dimensional counterparts, namely, the infinite dimensional linear group GL(Ω,R), for arbitrary rings R, and the infinite dimensional syraplectic group Sp(Ω,R), for commutative rings R with identity. We shall see that a key role in the classification of the normal and subnormal subgroups of GL(Ω,R) and Sp(Ω,R) is played by the 'elementary' normal subgroups E(Ω,R) and ESp(Ω,R). We shall also see that, in the case of the infinite dimensional linear group, the normal subgroup structure depends very much upon the way in which R is generated as a right R-module. We shall also give a presentation for the 'elementary' subgroup E(Ω,R) when R is a division ring.A variable input boundary problem in contaminant transport
https://hdl.handle.net/10023/13801
This thesis considers the large-time behaviour of the equation
(∂(u+uᵖ) )/( ∂t ) + Q(t) ∂u/( ∂x) = ∂²u/∂x² p>0, r≥ -1
With 0 ≤ 𝓍 < ∞, t ≥ 0 and Q (t) ~ tʳ, t ∞. This equation models, after suitable scalings are introduced, the one-dimensional flow of a solute through a porous medium with the solute undergoing adsorption by the solid matrix. We consider two models for the contaminant input at 𝓍= 0, the first being continuous input and the second being an initial pulse of contaminant which terminates after a finite time. Thus the total mass of the solute both adsorbed and in solution is considered to be dependent on time. It is found that the asymptotic solution depends crucially on both p and r. In finding the asymptotic solution, a similarity variable is introduced which for p ≥ 1 may involve spatial translation. We also have that when p < 1 interfaces appear and hence we have bounded support, whilst for p≥1 we do not. The principal role of r is to determine the balance between diffusion and convection effects. In the continuous input case this balance is independent of p, whilst in the pulse problem p is also involved in determining the balance.
Wed, 01 Jan 1997 00:00:00 GMThttps://hdl.handle.net/10023/138011997-01-01T00:00:00ZWarner, G. C.This thesis considers the large-time behaviour of the equation
(∂(u+uᵖ) )/( ∂t ) + Q(t) ∂u/( ∂x) = ∂²u/∂x² p>0, r≥ -1
With 0 ≤ 𝓍 < ∞, t ≥ 0 and Q (t) ~ tʳ, t ∞. This equation models, after suitable scalings are introduced, the one-dimensional flow of a solute through a porous medium with the solute undergoing adsorption by the solid matrix. We consider two models for the contaminant input at 𝓍= 0, the first being continuous input and the second being an initial pulse of contaminant which terminates after a finite time. Thus the total mass of the solute both adsorbed and in solution is considered to be dependent on time. It is found that the asymptotic solution depends crucially on both p and r. In finding the asymptotic solution, a similarity variable is introduced which for p ≥ 1 may involve spatial translation. We also have that when p < 1 interfaces appear and hence we have bounded support, whilst for p≥1 we do not. The principal role of r is to determine the balance between diffusion and convection effects. In the continuous input case this balance is independent of p, whilst in the pulse problem p is also involved in determining the balance.The prediction of thermal phase-change boundaries and associated temperature distributions
https://hdl.handle.net/10023/13797
The past three decades have seen a fast expanding interest in thermal problems exhibiting a change of phase, more commonly known as Stefan problems. With the rapid advance in computer technology the use and expansion of numerical simulation schemes has been responsible for large advances in this field. The increasing size of computers has led to more sophisticated and complex numerical solutions becoming feasible from a computational point of view. On the other hand, part of this interest has developed from industrial quarters where a knowledge of the location of a melting/freezing boundary may be of critical importance for certain processes. Much experimental work has been completed in this field. However, it is still useful to be able to obtain quick, accurate numerical solutions to such problems and it is with this in mind that this thesis is presented. Ideas from both of the above areas of interest are treated. In the first case a simple to program and computationally efficient numerical scheme is proposed for solving one dimensional Stefan problems and its merits are discussed in relation to several of the more common existing solution schemes. This scheme is then modified to cater for a two dimensional problem which crudely imitates a possible heating configuration in some industrial processes. The problem, with its attendant difficulties, is first approximated by a 'test' problem which is constructed so as to admit an analytic solution. This allows assessment of the numerical procedure in two dimensions. In the course of this work a pseudo-analytic solution was obtained for the original two dimensional problem. Finally, in collaboration with the British Gas Corporation, a complex industrial freezing problem is discussed concerning the flow of liquid through an enclosed channel. Some simplifying assumptions are proposed to reduce the problem to a form for which a relatively simple numerical scheme may be adopted. Several simulations are completed to examine the effect of varying physical parameters on the solution and in particular to test for situations of blockage or steady-state.
Sun, 01 Jan 1984 00:00:00 GMThttps://hdl.handle.net/10023/137971984-01-01T00:00:00ZWood, A. S.The past three decades have seen a fast expanding interest in thermal problems exhibiting a change of phase, more commonly known as Stefan problems. With the rapid advance in computer technology the use and expansion of numerical simulation schemes has been responsible for large advances in this field. The increasing size of computers has led to more sophisticated and complex numerical solutions becoming feasible from a computational point of view. On the other hand, part of this interest has developed from industrial quarters where a knowledge of the location of a melting/freezing boundary may be of critical importance for certain processes. Much experimental work has been completed in this field. However, it is still useful to be able to obtain quick, accurate numerical solutions to such problems and it is with this in mind that this thesis is presented. Ideas from both of the above areas of interest are treated. In the first case a simple to program and computationally efficient numerical scheme is proposed for solving one dimensional Stefan problems and its merits are discussed in relation to several of the more common existing solution schemes. This scheme is then modified to cater for a two dimensional problem which crudely imitates a possible heating configuration in some industrial processes. The problem, with its attendant difficulties, is first approximated by a 'test' problem which is constructed so as to admit an analytic solution. This allows assessment of the numerical procedure in two dimensions. In the course of this work a pseudo-analytic solution was obtained for the original two dimensional problem. Finally, in collaboration with the British Gas Corporation, a complex industrial freezing problem is discussed concerning the flow of liquid through an enclosed channel. Some simplifying assumptions are proposed to reduce the problem to a form for which a relatively simple numerical scheme may be adopted. Several simulations are completed to examine the effect of varying physical parameters on the solution and in particular to test for situations of blockage or steady-state.Compressible boundary layers with sharp pressure gradients
https://hdl.handle.net/10023/13795
The work of this thesis was undertaken as a C.A.S.E. award project in collaboration with Rolls-Royce to examine compressible laminar boundary layers with sharp adverse pressure-gradients. Much of the work is devoted to the solution of two important particular problems. The first flow considered is that along a semi-infinite flat plate with uniform pressure when X < X₀ and with the pressure for X > X₀ being so chosen that the boundary layer is just on the point of separation for all X > X₀. Immediately downstream of X₀ there is a sharp pressure rise to which the flow reacts mainly in a thin inner sublayer; so inner and outer asymptotic expansions are derived and matched for the stream function and a function of the temperature. Throughout the thesis the ratio of the viscosity to the absolute temperature is taken to be a function of x, the distance along the wall, alone, and the Illingworth-Stewartson transformation is applied. The Prandtl number, σ, is taken to be of order unity and detailed results are presented for σ= 1 and 0.72. The second flow considered is that along a finite flat plate where the transformed external velocity U₁(X) is chosen such that
U₁(X) = u₀(-X/L)[super]ε, where O< ε <<1,
is the transformed length of the plate and X represents transformed distance downstream from the trailing edge. The skin friction, position of separation and heat transfer right up to separation are determined. On the basis of these two solutions, another solution which is not presented in detail, and a solution (due to Curie) to a fourth sharp pressure gradient problem, a general Stratford-type method for computing compressible boundary layers is derived, which may be used to predict the position of separation, skin friction, heat transfer, displacement and momentum thicknesses for a compressible boundary layer with an unfavourable pressure gradient. In all this work techniques of series analysis are used to good effect. This led us to look at another boundary-layer problem in which such techniques could be used, one in which two parallel infinite disks are initially rotating with angular velocity Ω about a common axis in incompressible fluid, the appropriate Reynolds number being very large. Suddenly the angular velocity of one of the disks is reversed. A new examination of this problem is presented in the appendix to the thesis.
Thu, 01 Jan 1981 00:00:00 GMThttps://hdl.handle.net/10023/137951981-01-01T00:00:00ZReader-Harris, Michael JohnThe work of this thesis was undertaken as a C.A.S.E. award project in collaboration with Rolls-Royce to examine compressible laminar boundary layers with sharp adverse pressure-gradients. Much of the work is devoted to the solution of two important particular problems. The first flow considered is that along a semi-infinite flat plate with uniform pressure when X < X₀ and with the pressure for X > X₀ being so chosen that the boundary layer is just on the point of separation for all X > X₀. Immediately downstream of X₀ there is a sharp pressure rise to which the flow reacts mainly in a thin inner sublayer; so inner and outer asymptotic expansions are derived and matched for the stream function and a function of the temperature. Throughout the thesis the ratio of the viscosity to the absolute temperature is taken to be a function of x, the distance along the wall, alone, and the Illingworth-Stewartson transformation is applied. The Prandtl number, σ, is taken to be of order unity and detailed results are presented for σ= 1 and 0.72. The second flow considered is that along a finite flat plate where the transformed external velocity U₁(X) is chosen such that
U₁(X) = u₀(-X/L)[super]ε, where O< ε <<1,
is the transformed length of the plate and X represents transformed distance downstream from the trailing edge. The skin friction, position of separation and heat transfer right up to separation are determined. On the basis of these two solutions, another solution which is not presented in detail, and a solution (due to Curie) to a fourth sharp pressure gradient problem, a general Stratford-type method for computing compressible boundary layers is derived, which may be used to predict the position of separation, skin friction, heat transfer, displacement and momentum thicknesses for a compressible boundary layer with an unfavourable pressure gradient. In all this work techniques of series analysis are used to good effect. This led us to look at another boundary-layer problem in which such techniques could be used, one in which two parallel infinite disks are initially rotating with angular velocity Ω about a common axis in incompressible fluid, the appropriate Reynolds number being very large. Suddenly the angular velocity of one of the disks is reversed. A new examination of this problem is presented in the appendix to the thesis.The numerical solution of boundary value problems in partial differential equations
https://hdl.handle.net/10023/13793
Sun, 01 Jan 1967 00:00:00 GMThttps://hdl.handle.net/10023/137931967-01-01T00:00:00ZKeast, PatrickOn the fast and accurate computer solution of partial differential systems
https://hdl.handle.net/10023/13791
Two methods are presented for use on an electronic computer for the solution of partial differential systems. The first is concerned with accurate solutions of differential equations. It is equally applicable to ordinary differential equations and partial differential equations, and can be used for parabolic, hyperbolic or elliptic systems, and also for non-linear and mixed systems. It can be used in conjunction with existing schemes. Conversely, the method can be used as a very fast method of obtaining a rough solution of the system. It has an additional advantage over traditional higher order methods in that it does not require extra boundary conditions. The second method is concerned with the acceleration of the convergence rate in the solution of hyperbolic systems. The number of iterations has been reduced from tens of thousands with the traditional Lax-Wendroff methods to the order of twenty iterations. Analyses for both the differential and the difference systems are presented. Again the method is easily added to existing programs. The two methods may be used together to give one fast and accurate method.
Tue, 01 Jan 1974 00:00:00 GMThttps://hdl.handle.net/10023/137911974-01-01T00:00:00ZHill, Michael T.Two methods are presented for use on an electronic computer for the solution of partial differential systems. The first is concerned with accurate solutions of differential equations. It is equally applicable to ordinary differential equations and partial differential equations, and can be used for parabolic, hyperbolic or elliptic systems, and also for non-linear and mixed systems. It can be used in conjunction with existing schemes. Conversely, the method can be used as a very fast method of obtaining a rough solution of the system. It has an additional advantage over traditional higher order methods in that it does not require extra boundary conditions. The second method is concerned with the acceleration of the convergence rate in the solution of hyperbolic systems. The number of iterations has been reduced from tens of thousands with the traditional Lax-Wendroff methods to the order of twenty iterations. Analyses for both the differential and the difference systems are presented. Again the method is easily added to existing programs. The two methods may be used together to give one fast and accurate method.Alternating direction methods for hyperbolic systems
https://hdl.handle.net/10023/13788
Sat, 01 Jan 1966 00:00:00 GMThttps://hdl.handle.net/10023/137881966-01-01T00:00:00ZGourlay, A. R.Alternating direction implicit methods for partial differential equations
https://hdl.handle.net/10023/13784
Sat, 01 Jan 1966 00:00:00 GMThttps://hdl.handle.net/10023/137841966-01-01T00:00:00ZFairweather, GraemeThe use of non-polynomial interpolants in the numerical solution of ordinary differential equations
https://hdl.handle.net/10023/13783
Sat, 01 Jan 1966 00:00:00 GMThttps://hdl.handle.net/10023/137831966-01-01T00:00:00ZShaw, BrianFinite difference methods for non-linear hyperbolic systems
https://hdl.handle.net/10023/13782
Mon, 01 Jan 1968 00:00:00 GMThttps://hdl.handle.net/10023/137821968-01-01T00:00:00ZMorris, John Ll.Inner product quadrature formulas
https://hdl.handle.net/10023/13780
We investigate an approach to approximating the integral
(0.1) ⨍[sub]R w(x)f(x)g(x)dx ≡ I (f;g),
where R is a region in one-dimensional Euclidean space, and w a weight function. Since (0.1) may be regarded as a continuous bi-linear functional in f and g we approximate it by a discrete bi-linear functional, which we term an Inner Product Quadrature Formula (I.P.Q.F.).
(0.2) Q(f;g) ≡ f̲ᵀAg̲,
Where f̲ᵀ = (Sₒ(f), . . . , s[sub]m(f))ᵀ
g̲ᵀ = (Tₒ(g), . . . , T[sub]n(g)) ᵀ
A = (aᵢ[sub]j)ᵐi=o,ⁿj=0,
And a[sub]i[sub]j are real numbers, 𝛴 ᵐi=0 𝛴ⁿj =0 |aᵢ [sub]j | > 0
The so-called elementary functionals {Sᵢ}ᵐi=0 and {T[sub]j}ⁿj=0 are two sets of linearly independent linear functionals, acting f and g respectively, defined over a certain subspace of functions to which f and g belong. The simplest example of these functionals is function evaluation at a given point.
The matrix A is determined by requiring (0.2) to be exact for certain classes of functions f and g, say
F𝜀𝛷𝛾 ≡ {𝛷₀, . . . , 𝛷ᵧ}, 𝛾≥0
G𝜀𝛹𝛿 ≡ {𝛹₀, . . . , 𝛹[sub] 𝛿} 𝛿≥0
In Chapter 1 we introduce the concept of I.P.Q.F. in more detail and make some general comments about approaches available when examining numerical integration. After explaining in some detail why we feel I.P.Q.F. are a useful tool in §2.1, we proceed in the remainder of Chapter 2 to investigate various conditions which may be placed on 𝛷ᵞ, 𝛹[super] 𝛿 {Sᵢ}ᵐi=0 and {T[sub]j}ⁿj=0 in order to guarantee the existence of I.P.Q.F. exact when F𝜀𝛷𝛾 and G𝜀𝛹𝛿.
In particular we investigate the question of maximizing 𝛾+ 𝛿. In the case where 𝛷ᵢ and 𝛹[sub]j are the standard monomials of degree i and j respectively, some results have already been published in B.I.T. (1977) p. 392-408. We investigate various choices of 𝛷ᵢ and 𝛹[sub]j :
(a) {𝛷ᵢ}ᵐ⁺¹ I = 0 (i.e. 𝛾 = m+1) and {𝛹[sub]j}ᵐ[sub]j = 0 (i.e. 𝛿 = m) being Tchebychev sets (§2.7),
(b) {𝛷ᵢ}²ᵐ⁺¹ I = 0 (i.e. 𝛾 = 2m+1) being a Tchebychev set and 𝛹[super]𝛿 contains only one function (i.e. 𝛿 = 0) (§2.6)
(c) 𝛷ᵢ ≡ (𝛷[sub]l)ⁱ, i=0,1, . . . and 𝛷ᵢ = 𝛹ᵢ, i= 0, 1, … (§2.8).
In Chapter 3 we consider the question of compounding I.P.Q.F. both in the classical sense, and, briefly, by examining spline functions, regarding them as providing a link between an I.P.Q.F on one hand and a compounded I.P.Q.F. on the other. Various methods of theoretically estimating the errors involved are considered in Chapter M-. In the fifth Chapter we examine various ways in which the concept of I.P.Q.F. might (or might not) be extended. Finally, we make some brief comments about the possible applications of I.P.Q.F., and give a few examples.
Mon, 01 Jan 1979 00:00:00 GMThttps://hdl.handle.net/10023/137801979-01-01T00:00:00ZGribble, Julian de GruchyWe investigate an approach to approximating the integral
(0.1) ⨍[sub]R w(x)f(x)g(x)dx ≡ I (f;g),
where R is a region in one-dimensional Euclidean space, and w a weight function. Since (0.1) may be regarded as a continuous bi-linear functional in f and g we approximate it by a discrete bi-linear functional, which we term an Inner Product Quadrature Formula (I.P.Q.F.).
(0.2) Q(f;g) ≡ f̲ᵀAg̲,
Where f̲ᵀ = (Sₒ(f), . . . , s[sub]m(f))ᵀ
g̲ᵀ = (Tₒ(g), . . . , T[sub]n(g)) ᵀ
A = (aᵢ[sub]j)ᵐi=o,ⁿj=0,
And a[sub]i[sub]j are real numbers, 𝛴 ᵐi=0 𝛴ⁿj =0 |aᵢ [sub]j | > 0
The so-called elementary functionals {Sᵢ}ᵐi=0 and {T[sub]j}ⁿj=0 are two sets of linearly independent linear functionals, acting f and g respectively, defined over a certain subspace of functions to which f and g belong. The simplest example of these functionals is function evaluation at a given point.
The matrix A is determined by requiring (0.2) to be exact for certain classes of functions f and g, say
F𝜀𝛷𝛾 ≡ {𝛷₀, . . . , 𝛷ᵧ}, 𝛾≥0
G𝜀𝛹𝛿 ≡ {𝛹₀, . . . , 𝛹[sub] 𝛿} 𝛿≥0
In Chapter 1 we introduce the concept of I.P.Q.F. in more detail and make some general comments about approaches available when examining numerical integration. After explaining in some detail why we feel I.P.Q.F. are a useful tool in §2.1, we proceed in the remainder of Chapter 2 to investigate various conditions which may be placed on 𝛷ᵞ, 𝛹[super] 𝛿 {Sᵢ}ᵐi=0 and {T[sub]j}ⁿj=0 in order to guarantee the existence of I.P.Q.F. exact when F𝜀𝛷𝛾 and G𝜀𝛹𝛿.
In particular we investigate the question of maximizing 𝛾+ 𝛿. In the case where 𝛷ᵢ and 𝛹[sub]j are the standard monomials of degree i and j respectively, some results have already been published in B.I.T. (1977) p. 392-408. We investigate various choices of 𝛷ᵢ and 𝛹[sub]j :
(a) {𝛷ᵢ}ᵐ⁺¹ I = 0 (i.e. 𝛾 = m+1) and {𝛹[sub]j}ᵐ[sub]j = 0 (i.e. 𝛿 = m) being Tchebychev sets (§2.7),
(b) {𝛷ᵢ}²ᵐ⁺¹ I = 0 (i.e. 𝛾 = 2m+1) being a Tchebychev set and 𝛹[super]𝛿 contains only one function (i.e. 𝛿 = 0) (§2.6)
(c) 𝛷ᵢ ≡ (𝛷[sub]l)ⁱ, i=0,1, . . . and 𝛷ᵢ = 𝛹ᵢ, i= 0, 1, … (§2.8).
In Chapter 3 we consider the question of compounding I.P.Q.F. both in the classical sense, and, briefly, by examining spline functions, regarding them as providing a link between an I.P.Q.F on one hand and a compounded I.P.Q.F. on the other. Various methods of theoretically estimating the errors involved are considered in Chapter M-. In the fifth Chapter we examine various ways in which the concept of I.P.Q.F. might (or might not) be extended. Finally, we make some brief comments about the possible applications of I.P.Q.F., and give a few examples.Interval methods for non-linear systems
https://hdl.handle.net/10023/13779
In numerical mathematics, there is a need for methods which provide a user with the solution to his problem without requiring him to understand the mathematics underlying the method of solution. Such a method involves computable tests to determine whether or not a solution exists in a given region, and whether, if it exists, such a solution may be found by using the given method. Two valuable tools for the implementation of such methods are interval mathematics and symbolic computation. In. practice all computers have memories of finite size and cannot perform exact arithmetic. Therefore, in addition to the error which is inherent in a given numerical method, namely truncation error, there is also the error due to rounding. Using interval arithmetic, computable tests which guarantee the existence of a solution to a given problem in a given region, and the convergence of a particular iterative method to this solution, become practically realizable. This is not possible using real arithmetic due to the accumulation of rounding error on a computer. The advent of packages which allow symbolic computations to be carried out on a given computer is an important advance for computational numerical mathematics. In particular, the ability to compute derivatives automatically removes the need for a user to supply them, thus eliminating a major source of error in the use of methods requiring first or higher derivatives. In this thesis some methods which use interval arithmetic and symbolic computation for the solution of systems of nonlinear algebraic equations are presented. Some algorithms based on the symmetric single-step algorithm are described. These methods however do not possess computable existence, uniqueness, and convergence tests. Algorithms which do possess such tests, based on the Krawczyk-Moore algorithm are also presented. A simple package which allows symbolic computations to be carried out is described. Several applications for such a package are given. In particular, an interval form of Brown's method is presented.
Wed, 01 Jan 1986 00:00:00 GMThttps://hdl.handle.net/10023/137791986-01-01T00:00:00ZShearer, J. M.In numerical mathematics, there is a need for methods which provide a user with the solution to his problem without requiring him to understand the mathematics underlying the method of solution. Such a method involves computable tests to determine whether or not a solution exists in a given region, and whether, if it exists, such a solution may be found by using the given method. Two valuable tools for the implementation of such methods are interval mathematics and symbolic computation. In. practice all computers have memories of finite size and cannot perform exact arithmetic. Therefore, in addition to the error which is inherent in a given numerical method, namely truncation error, there is also the error due to rounding. Using interval arithmetic, computable tests which guarantee the existence of a solution to a given problem in a given region, and the convergence of a particular iterative method to this solution, become practically realizable. This is not possible using real arithmetic due to the accumulation of rounding error on a computer. The advent of packages which allow symbolic computations to be carried out on a given computer is an important advance for computational numerical mathematics. In particular, the ability to compute derivatives automatically removes the need for a user to supply them, thus eliminating a major source of error in the use of methods requiring first or higher derivatives. In this thesis some methods which use interval arithmetic and symbolic computation for the solution of systems of nonlinear algebraic equations are presented. Some algorithms based on the symmetric single-step algorithm are described. These methods however do not possess computable existence, uniqueness, and convergence tests. Algorithms which do possess such tests, based on the Krawczyk-Moore algorithm are also presented. A simple package which allows symbolic computations to be carried out is described. Several applications for such a package are given. In particular, an interval form of Brown's method is presented.Statistical problems in measuring surface ozone and modelling its patterns
https://hdl.handle.net/10023/13773
The thesis examines ground level air pollution data supplied by ITE Bush, Penicuik, Midlothian, Scotland. There is a brief examination of sulphur dioxide concentration data, but the Thesis is primarily concerned with ozone. The diurnal behaviour of ozone is the major topic, and a new methodology of classification of 'ozone days' is introduced and discussed. In chapter 2, the inverse Gaussian distribution is considered and rejected as a possible alternative to the standard approach of using the lognormal as a model for the frequency distribution of observed sulphur dioxide concentrations. In chapter 3, the behaviour of digital gas pollution analysers is investigated by making use of data obtained from two such machines operating side by side. A time series model of the differences between the readings obtained from the two machines is considered, and possible effects on modelling discussed. In chapter 4, the changes in the diurnal behaviour of ozone over a year are examined. A new approach involving a distortion of the time axis is shown to give diurnal ozone curves more homogeneous properties and have beneficial effects for modelling purposes. Chapter 5 extends the analysis of the diurnal behaviour of ozone begun in chapter 4 by considering individual 'ozone days' and attempting to classify them as one of several typical 'types' of day. The time distortion method introduced in chapter 4 is used, and a new classification methodology is introduced for considering data of this type. The statistical properties of this method are discussed in chapter 6.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/137731996-01-01T00:00:00ZHutchison, Paul StewartThe thesis examines ground level air pollution data supplied by ITE Bush, Penicuik, Midlothian, Scotland. There is a brief examination of sulphur dioxide concentration data, but the Thesis is primarily concerned with ozone. The diurnal behaviour of ozone is the major topic, and a new methodology of classification of 'ozone days' is introduced and discussed. In chapter 2, the inverse Gaussian distribution is considered and rejected as a possible alternative to the standard approach of using the lognormal as a model for the frequency distribution of observed sulphur dioxide concentrations. In chapter 3, the behaviour of digital gas pollution analysers is investigated by making use of data obtained from two such machines operating side by side. A time series model of the differences between the readings obtained from the two machines is considered, and possible effects on modelling discussed. In chapter 4, the changes in the diurnal behaviour of ozone over a year are examined. A new approach involving a distortion of the time axis is shown to give diurnal ozone curves more homogeneous properties and have beneficial effects for modelling purposes. Chapter 5 extends the analysis of the diurnal behaviour of ozone begun in chapter 4 by considering individual 'ozone days' and attempting to classify them as one of several typical 'types' of day. The time distortion method introduced in chapter 4 is used, and a new classification methodology is introduced for considering data of this type. The statistical properties of this method are discussed in chapter 6.A study of character recognition using geometric moments under conditions of simple and non-simple loss
https://hdl.handle.net/10023/13768
The theory of Loss Functions Is a fundamental part of Statistical Decision Theory and of Pattern Recognition. However It is a subject which few have studied In detail. This thesis is an attempt to develop a simple character recognition process In which losses may be Implemented when and where necessary. After a brief account of the history of Loss Functions and an Introduction to elementary Decision Theory, some examples have been constructed to demonstrate how various decision boundaries approximate to the optimal boundary and what Increase In loss would be associated with these sub-optimal boundaries. The results show that the Euclidean and Hamming distance discriminants can be sufficiently close approximations that the decision process may be legitimately simplified by the use of these linear boundaries. Geometric moments were adopted for the computer simulation of the recognition process because each moment is closely related to the symmetry and structure of a character, unlike many other features. The theory of Moments is discussed, in particular their geometrical properties. A brief description of the programs used in the simulation follows. Two different data sets were investigated, the first being hand-drawn capitals and the second machine-scanned lower case type script. This latter set was in the form of a message, which presented interesting programming problems in itself. The results from the application of different discriminants to these sets under conditions of simple loss are analysed and the recognition efficiencies are found to vary between about 30% and. 99% depending on the number of moments being used and the type of discriminant. Next certain theoretical problems are studied. The relations between the rejection rate, the error rate and the rejection threshold are discussed both theoretically and practically. Also an attempt is made to predict theoretically the variation of efficiency with the number of moments used in the discrimination. This hypothesis is then tested on the data already calculated and shown to be true within reasonable limits. A discussion of moment ordering by defining their re-solving powers is undertaken and it seems likely that the moments normally used unordered are among the most satisfactory. Finally, some time is devoted towards methods of improving recognition efficiency. Information content is discussed along with the possibilities inherent in the use of digraph and trigraph probabilities. A breakdown of the errors in the recognition system adopted here is presented along with suggestions to improve the technique. The execution time of the different decision mechanisms is then inspected and a refined 2-Stage method is produced. Lastly the various methods by which a decision mechanism might be improved are united under a common loss matrix, formed by a product of matrices each of which represents a particular facet of the recognition problem.
Tue, 01 Jan 1974 00:00:00 GMThttps://hdl.handle.net/10023/137681974-01-01T00:00:00ZTucker, N. D.The theory of Loss Functions Is a fundamental part of Statistical Decision Theory and of Pattern Recognition. However It is a subject which few have studied In detail. This thesis is an attempt to develop a simple character recognition process In which losses may be Implemented when and where necessary. After a brief account of the history of Loss Functions and an Introduction to elementary Decision Theory, some examples have been constructed to demonstrate how various decision boundaries approximate to the optimal boundary and what Increase In loss would be associated with these sub-optimal boundaries. The results show that the Euclidean and Hamming distance discriminants can be sufficiently close approximations that the decision process may be legitimately simplified by the use of these linear boundaries. Geometric moments were adopted for the computer simulation of the recognition process because each moment is closely related to the symmetry and structure of a character, unlike many other features. The theory of Moments is discussed, in particular their geometrical properties. A brief description of the programs used in the simulation follows. Two different data sets were investigated, the first being hand-drawn capitals and the second machine-scanned lower case type script. This latter set was in the form of a message, which presented interesting programming problems in itself. The results from the application of different discriminants to these sets under conditions of simple loss are analysed and the recognition efficiencies are found to vary between about 30% and. 99% depending on the number of moments being used and the type of discriminant. Next certain theoretical problems are studied. The relations between the rejection rate, the error rate and the rejection threshold are discussed both theoretically and practically. Also an attempt is made to predict theoretically the variation of efficiency with the number of moments used in the discrimination. This hypothesis is then tested on the data already calculated and shown to be true within reasonable limits. A discussion of moment ordering by defining their re-solving powers is undertaken and it seems likely that the moments normally used unordered are among the most satisfactory. Finally, some time is devoted towards methods of improving recognition efficiency. Information content is discussed along with the possibilities inherent in the use of digraph and trigraph probabilities. A breakdown of the errors in the recognition system adopted here is presented along with suggestions to improve the technique. The execution time of the different decision mechanisms is then inspected and a refined 2-Stage method is produced. Lastly the various methods by which a decision mechanism might be improved are united under a common loss matrix, formed by a product of matrices each of which represents a particular facet of the recognition problem.A study of the work and methods of Henry Briggs, with special reference to the early history of interpolation
https://hdl.handle.net/10023/13760
Wed, 01 Jan 1941 00:00:00 GMThttps://hdl.handle.net/10023/137601941-01-01T00:00:00ZWaterson, AndrewTransformations in regression, estimation, testing and modelling
https://hdl.handle.net/10023/13759
Transformation is a powerful tool for model building. In regression the response variable is transformed in order to achieve the usual assumptions of normality, constant variance and additivity of effects. Here the normality assumption is replaced by the Laplace distributional assumption, appropriate when more large errors occur than would be expected if the errors were normally distributed. The parametric model is enlarged to include a transformation parameter and a likelihood procedure is adopted for estimating this parameter simultaneously with other parameters of interest. Diagnostic methods are described for assessing the influence of individual observations on the choice of transformation. Examples are presented. In distribution methodology the independent responses are transformed in order that a distributional assumption is satisfied for the transformed data. Here the interest is in the family of distributions which are not dependent on an unknown shape parameter. The gamma distribution (known order), with special case the exponential distribution, is a member of this family. An information number approach is proposed for transforming a known distribution to the gamma distribution (known order). The approach provides an insight into the large-sample behaviour of the likelihood procedure considered by Draper and Guttman (1968) for investigating transformations of data which allow the transformed observations to follow a gamma distribution. The information number approach is illustrated for three examples end the improvement towards the gamma distribution introduced by transformation is measured numerically and graphically. A graphical procedure is proposed for the general case of investigating transformations of data which allow the transformed observations to follow a distribution dependent on unknown threshold and scale parameters. The procedure is extended to include model testing and estimation for any distribution which with the aid of a power transformation can be put in the simple form of a distribution that is not dependent on an unknown shape parameter. The procedure is based on a ratio, R(y), which is constructed from the power transformation. Also described is a ratio-based technique for estimating the threshold parameter in important parametric models, including the three-parameter Weibull and lognormal distributions. Ratio estimation for the weibull distribution is assessed and compared with the modified maximum likelihood estimation of Cohen and Whitten (1982) in terms of bias and root mean squared error, by means of a simulation study. The methods are illustrated with several examples and extend naturally to singly Type 1 and Type 2 censored data.
Fri, 01 Jan 1988 00:00:00 GMThttps://hdl.handle.net/10023/137591988-01-01T00:00:00ZParker, ImeldaTransformation is a powerful tool for model building. In regression the response variable is transformed in order to achieve the usual assumptions of normality, constant variance and additivity of effects. Here the normality assumption is replaced by the Laplace distributional assumption, appropriate when more large errors occur than would be expected if the errors were normally distributed. The parametric model is enlarged to include a transformation parameter and a likelihood procedure is adopted for estimating this parameter simultaneously with other parameters of interest. Diagnostic methods are described for assessing the influence of individual observations on the choice of transformation. Examples are presented. In distribution methodology the independent responses are transformed in order that a distributional assumption is satisfied for the transformed data. Here the interest is in the family of distributions which are not dependent on an unknown shape parameter. The gamma distribution (known order), with special case the exponential distribution, is a member of this family. An information number approach is proposed for transforming a known distribution to the gamma distribution (known order). The approach provides an insight into the large-sample behaviour of the likelihood procedure considered by Draper and Guttman (1968) for investigating transformations of data which allow the transformed observations to follow a gamma distribution. The information number approach is illustrated for three examples end the improvement towards the gamma distribution introduced by transformation is measured numerically and graphically. A graphical procedure is proposed for the general case of investigating transformations of data which allow the transformed observations to follow a distribution dependent on unknown threshold and scale parameters. The procedure is extended to include model testing and estimation for any distribution which with the aid of a power transformation can be put in the simple form of a distribution that is not dependent on an unknown shape parameter. The procedure is based on a ratio, R(y), which is constructed from the power transformation. Also described is a ratio-based technique for estimating the threshold parameter in important parametric models, including the three-parameter Weibull and lognormal distributions. Ratio estimation for the weibull distribution is assessed and compared with the modified maximum likelihood estimation of Cohen and Whitten (1982) in terms of bias and root mean squared error, by means of a simulation study. The methods are illustrated with several examples and extend naturally to singly Type 1 and Type 2 censored data.Parameterisation-invariant versions of Wald tests
https://hdl.handle.net/10023/13750
Although Wald tests form one of the major classes of hypothesis tests, they suffer from the well-known major drawback that they are not invariant under reparameterisation. This thesis uses the differential-geometric concept of a yoke to introduce one-parameter families of geometric Wald statistics, which are parameterisation-invariant statistics in the spirit of the traditional Wald statistics. Both the geometric Wald statistics based on the expected likelihood yokes and those based on the observed likelihood yokes are investigated. Bartlett-type adjustments of the geometric Wald statistics are obtained, in order to improve the accuracy of the chi-squared approximations to their distributions under the null hypothesis.
Fri, 01 Jan 1999 00:00:00 GMThttps://hdl.handle.net/10023/137501999-01-01T00:00:00ZLarsen, Pia VeldtAlthough Wald tests form one of the major classes of hypothesis tests, they suffer from the well-known major drawback that they are not invariant under reparameterisation. This thesis uses the differential-geometric concept of a yoke to introduce one-parameter families of geometric Wald statistics, which are parameterisation-invariant statistics in the spirit of the traditional Wald statistics. Both the geometric Wald statistics based on the expected likelihood yokes and those based on the observed likelihood yokes are investigated. Bartlett-type adjustments of the geometric Wald statistics are obtained, in order to improve the accuracy of the chi-squared approximations to their distributions under the null hypothesis.Estimating the parameters in mixtures of circular and spherical distributions
https://hdl.handle.net/10023/13748
In this thesis we compare various methods for estimating the unknown parameters in mixtures of circular and spherical distributions. We study the von Mises distribution on the circle and the Fisher distribution on the sphere. We propose a new method of estimation based on the characteristic function and compare it with the classical methods based on maximum likelihood and moments. Thus far these methods have only been successfully applied to distributions on the line. Here we show that the extension to circular and spherical distributions is reasonably straightforward and convergence to the final estimates is fairly rapid. We apply these methods to various simulated and real data sets and show that the results obtained for the mixture of two von Mises distributions are satisfactory but generally depend on the sample size and method of estimation used. However, results obtained for the mixture of two Fisher distributions show that maximum likelihood performs best overall.
Mon, 01 Jan 1990 00:00:00 GMThttps://hdl.handle.net/10023/137481990-01-01T00:00:00ZKoutbeiy, Majdi AmineIn this thesis we compare various methods for estimating the unknown parameters in mixtures of circular and spherical distributions. We study the von Mises distribution on the circle and the Fisher distribution on the sphere. We propose a new method of estimation based on the characteristic function and compare it with the classical methods based on maximum likelihood and moments. Thus far these methods have only been successfully applied to distributions on the line. Here we show that the extension to circular and spherical distributions is reasonably straightforward and convergence to the final estimates is fairly rapid. We apply these methods to various simulated and real data sets and show that the results obtained for the mixture of two von Mises distributions are satisfactory but generally depend on the sample size and method of estimation used. However, results obtained for the mixture of two Fisher distributions show that maximum likelihood performs best overall.Statistical aspects of the population regulation of migrating brown trout "Salmo trutta" in a Lake District stream
https://hdl.handle.net/10023/13746
Statistical aspects of the population regulation of a migratory brown trout population are investigated. The life cycle of the trout, the study area and the sampling routine are described in Chapter 1. Models of numerical changes in fish populations are reviewed in Chapter 2. Measures that assess the nonlinear behaviour of nonlinear regression models are described in Chapter 3. The additive error Ricker model describes the relationship between the number of 0+ parr in May/June and the number of eggs. The nonlinear behaviour of the model is investigated in Chapter 4. The parameter effects nonlinearity of the model is reduced by a reparameterisation. Chapter 5 investigates the effect of errors in the egg variable on the distributions of the least squares estimators of the additive error and the multiplicative error Ricker models. The errors-in-variables considerably increase the variances of the least squares estimators. Models of the relationships between the numbers of 0+ parr in August/September, the number of 1+ parr, the egg production of a year class and the number of eggs are developed in Chapter 6. These models account for the effect of summer drought on survival. Survival is density dependent during the first summer of the life cycle and density independent thereafter. Standard measures of nonlinearity can seriously underestimate the nonlinear behaviour of piecewise linear change-point models. New measures of nonlinearity appropriate for piecewise linear change-point models are developed in Chapter 7. Chapter 8 develops a model of the growth of brown trout fed on maximum rations as a function of time, body weight and water temperature. Chapter 9 develops a model that relates the survival rate of 0+ parr between May/June and August/September to the length distribution of the trout in May/June. The results of the Thesis are discussed in Chapter 10.
Mon, 01 Jan 1990 00:00:00 GMThttps://hdl.handle.net/10023/137461990-01-01T00:00:00ZFryer, Robert JohnStatistical aspects of the population regulation of a migratory brown trout population are investigated. The life cycle of the trout, the study area and the sampling routine are described in Chapter 1. Models of numerical changes in fish populations are reviewed in Chapter 2. Measures that assess the nonlinear behaviour of nonlinear regression models are described in Chapter 3. The additive error Ricker model describes the relationship between the number of 0+ parr in May/June and the number of eggs. The nonlinear behaviour of the model is investigated in Chapter 4. The parameter effects nonlinearity of the model is reduced by a reparameterisation. Chapter 5 investigates the effect of errors in the egg variable on the distributions of the least squares estimators of the additive error and the multiplicative error Ricker models. The errors-in-variables considerably increase the variances of the least squares estimators. Models of the relationships between the numbers of 0+ parr in August/September, the number of 1+ parr, the egg production of a year class and the number of eggs are developed in Chapter 6. These models account for the effect of summer drought on survival. Survival is density dependent during the first summer of the life cycle and density independent thereafter. Standard measures of nonlinearity can seriously underestimate the nonlinear behaviour of piecewise linear change-point models. New measures of nonlinearity appropriate for piecewise linear change-point models are developed in Chapter 7. Chapter 8 develops a model of the growth of brown trout fed on maximum rations as a function of time, body weight and water temperature. Chapter 9 develops a model that relates the survival rate of 0+ parr between May/June and August/September to the length distribution of the trout in May/June. The results of the Thesis are discussed in Chapter 10.Reliability theory in operational research
https://hdl.handle.net/10023/13745
This thesis is concerned principally with the problem of estimating the parameters of the Weibull and Beta distributions using several different techniques. These distributions are used in the area of reliability testing and it is important to achieve the best estimates possible of the parameters involved. After considering several accepted methods of estimating the relevant parameters, it is considered that the best method depends on the aim of the analysis, and on the value of the shape parameter 𝛽. For estimating the two-parameter Weibull distribution, it is recommended that Generalized Least Squares (GLS) is the best method to use for values of 𝛽 between 0.5 and 30. However, Maximum Likelihood Estimator (MLE) is a good method for estimating quantiles.
On this basis, the three-parameter Weibull distribution is investigated. The traditional parametrization is compared with a new parametrization developed in this work. By considering parameter effects and intrinsic curvature it is shown that the new parametrization results in a linear effect of the shape parameter. Also it has advantages in quantile estimation because of its ability to provide estimates for a wider range of data sets.
A less frequently used distribution in the field of reliability is the Beta distribution. The lack of frequency of its use is partly due to the difficulty in estimating its parameters. A simple, applicable method is developed here of estimating these parameters. This 'group method' involves estimating the two ends of the distribution. It is shown that this procedure can be used, together with other methods of estimating the two- parameter Beta distribution successfully to estimate the four-parameter Beta distribution.
Tue, 01 Jan 1991 00:00:00 GMThttps://hdl.handle.net/10023/137451991-01-01T00:00:00ZAl-Baidhani, Fadil AjabThis thesis is concerned principally with the problem of estimating the parameters of the Weibull and Beta distributions using several different techniques. These distributions are used in the area of reliability testing and it is important to achieve the best estimates possible of the parameters involved. After considering several accepted methods of estimating the relevant parameters, it is considered that the best method depends on the aim of the analysis, and on the value of the shape parameter 𝛽. For estimating the two-parameter Weibull distribution, it is recommended that Generalized Least Squares (GLS) is the best method to use for values of 𝛽 between 0.5 and 30. However, Maximum Likelihood Estimator (MLE) is a good method for estimating quantiles.
On this basis, the three-parameter Weibull distribution is investigated. The traditional parametrization is compared with a new parametrization developed in this work. By considering parameter effects and intrinsic curvature it is shown that the new parametrization results in a linear effect of the shape parameter. Also it has advantages in quantile estimation because of its ability to provide estimates for a wider range of data sets.
A less frequently used distribution in the field of reliability is the Beta distribution. The lack of frequency of its use is partly due to the difficulty in estimating its parameters. A simple, applicable method is developed here of estimating these parameters. This 'group method' involves estimating the two ends of the distribution. It is shown that this procedure can be used, together with other methods of estimating the two- parameter Beta distribution successfully to estimate the four-parameter Beta distribution.Inference for plant-capture
https://hdl.handle.net/10023/13741
When investigating the dynamics of an animal population, a primary objective is to obtain reasonable estimates of abundance or population size. This thesis concentrates on the problem of obtaining point estimates of abundance from capture-recapture data and on how such estimation can be improved by using the method of plant-capture. Plant-capture constitutes a natural generalisation of capture-recapture. In a plant-capture study a pre-marked population of known size is added to the target population of unknown size. The capture-recapture experiment is then carried out on the augmented population. Chapter 1 considers the addition of planted individuals to target populations which behave according to the standard capture-recapture model M₀. Chapter 2 investigates an analogous model based on sampling in continuous time. In each of these chapters, distributional results are derived under the assumption that the behaviour of the plants is indistinguishable from that of members of the target population. Maximum likelihood estimators and other new estimators are proposed for each model. The results suggest that the use of plants is beneficial, and furthermore that the new estimators perform more satisfactorily than the maximum likelihood estimators. Chapter 3 introduces, initially in the absence of plants, a new class of estimators, described as coverage adjusted estimators, for the standard capture-recapture model M[sub]h. These new estimators are shown, through simulation and real life data, to compare favourably with estimators that have previously been proposed. Plant-capture versions of these new estimators are then derived and the usefulness of the plants is demonstrated through simulation. Chapter 4 describes how the approach taken in chapter 3 can be modified to produce a new estimator for the analogous continuous time model. This estimator is then shown through simulation to be preferable to estimators that have previously been proposed.
Thu, 01 Jan 1998 00:00:00 GMThttps://hdl.handle.net/10023/137411998-01-01T00:00:00ZAshbridge, JonathanWhen investigating the dynamics of an animal population, a primary objective is to obtain reasonable estimates of abundance or population size. This thesis concentrates on the problem of obtaining point estimates of abundance from capture-recapture data and on how such estimation can be improved by using the method of plant-capture. Plant-capture constitutes a natural generalisation of capture-recapture. In a plant-capture study a pre-marked population of known size is added to the target population of unknown size. The capture-recapture experiment is then carried out on the augmented population. Chapter 1 considers the addition of planted individuals to target populations which behave according to the standard capture-recapture model M₀. Chapter 2 investigates an analogous model based on sampling in continuous time. In each of these chapters, distributional results are derived under the assumption that the behaviour of the plants is indistinguishable from that of members of the target population. Maximum likelihood estimators and other new estimators are proposed for each model. The results suggest that the use of plants is beneficial, and furthermore that the new estimators perform more satisfactorily than the maximum likelihood estimators. Chapter 3 introduces, initially in the absence of plants, a new class of estimators, described as coverage adjusted estimators, for the standard capture-recapture model M[sub]h. These new estimators are shown, through simulation and real life data, to compare favourably with estimators that have previously been proposed. Plant-capture versions of these new estimators are then derived and the usefulness of the plants is demonstrated through simulation. Chapter 4 describes how the approach taken in chapter 3 can be modified to produce a new estimator for the analogous continuous time model. This estimator is then shown through simulation to be preferable to estimators that have previously been proposed.The asymptotic distribution and robustness of the likelihood ratio and score test statistics
https://hdl.handle.net/10023/13738
Cordeiro & Ferrari (1991) use the asymptotic expansion of Harris (1985) for the moment generating function of the score statistic to produce a generalization of Bartlett adjustment for application to the score statistic. It is shown here that Harris's expansion is not invariant under reparameterization and an invariant expansion is derived using a method based on the expected likelihood yoke. A necessary and sufficient condition for the existence of a generalized Bartlett adjustment for an arbitrary statistic is given in terms of its moment generating function. Generalized Bartlett adjustments to the likelihood ratio and score test statistics are derived in the case where the interest parameter is one-dimensional under the assumption of a mis-specified model, where the true distribution is not assumed to be that under the null hypothesis.
Sun, 01 Jan 1995 00:00:00 GMThttps://hdl.handle.net/10023/137381995-01-01T00:00:00ZEmberson, E. A.Cordeiro & Ferrari (1991) use the asymptotic expansion of Harris (1985) for the moment generating function of the score statistic to produce a generalization of Bartlett adjustment for application to the score statistic. It is shown here that Harris's expansion is not invariant under reparameterization and an invariant expansion is derived using a method based on the expected likelihood yoke. A necessary and sufficient condition for the existence of a generalized Bartlett adjustment for an arbitrary statistic is given in terms of its moment generating function. Generalized Bartlett adjustments to the likelihood ratio and score test statistics are derived in the case where the interest parameter is one-dimensional under the assumption of a mis-specified model, where the true distribution is not assumed to be that under the null hypothesis.On the equivalence of Markov Algorithms and Turing machines and some consequent results
https://hdl.handle.net/10023/13736
Turing Machines and Markov Algorithms are, and were designed to be, the most powerful devices possible in the field of abstract automata: by their means any and every computable function can be computed.
Because of their equal, indeed maximal, strength, it was naturally assumed that these devices should be equivalent. Nonetheless a formal, exact proof of this universally presumed equivalence was lacking.
The present dissertation rectifies that omission by developing the desired complete, rigorous proof of the equivalence between Turing Machines and Markov Algorithms. The demonstration is being conducted in a constructionist way: for any given Markov Algorithm it is shown that a Turing Machine can be constructed capable of performing exactly what the Algorithm can do and nothing more, and vice versa.
The proof consists in the theoretical construction, given an arbitrary Markov Algorithm, of a Turing Machine behaving in exactly the same way as the Algorithm for all possible inputs; and conversely. Furthermore, the proof is given concrete shape by designing a computer program which can actually carry out the said theoretical constructions.
The equivalence between TM and MA as proven in the first part of our thesis, is being used in the second part for establishing some important consequent results: Thus the equivalence of Deterministic and Nondeterministic MA, of TM and Type 0 Grammars, and of Labelled and Unlabelled MA is concisely shown, and the use of TM as recognizers for type 1 and 3 grammars exclusively is exhibited. It is interesting that, by utilizing the equivalence of TM and MA, it was made possible that the proofs of these latter results be based on primitive principles.
Mon, 01 Jan 1979 00:00:00 GMThttps://hdl.handle.net/10023/137361979-01-01T00:00:00ZPapathanassiou, EleftheriosTuring Machines and Markov Algorithms are, and were designed to be, the most powerful devices possible in the field of abstract automata: by their means any and every computable function can be computed.
Because of their equal, indeed maximal, strength, it was naturally assumed that these devices should be equivalent. Nonetheless a formal, exact proof of this universally presumed equivalence was lacking.
The present dissertation rectifies that omission by developing the desired complete, rigorous proof of the equivalence between Turing Machines and Markov Algorithms. The demonstration is being conducted in a constructionist way: for any given Markov Algorithm it is shown that a Turing Machine can be constructed capable of performing exactly what the Algorithm can do and nothing more, and vice versa.
The proof consists in the theoretical construction, given an arbitrary Markov Algorithm, of a Turing Machine behaving in exactly the same way as the Algorithm for all possible inputs; and conversely. Furthermore, the proof is given concrete shape by designing a computer program which can actually carry out the said theoretical constructions.
The equivalence between TM and MA as proven in the first part of our thesis, is being used in the second part for establishing some important consequent results: Thus the equivalence of Deterministic and Nondeterministic MA, of TM and Type 0 Grammars, and of Labelled and Unlabelled MA is concisely shown, and the use of TM as recognizers for type 1 and 3 grammars exclusively is exhibited. It is interesting that, by utilizing the equivalence of TM and MA, it was made possible that the proofs of these latter results be based on primitive principles.Some contributions to the theory of mathematical programming
https://hdl.handle.net/10023/13734
As stated earlier the Simplex Method (or its variations e.g. Dual Simplex Method) has thus far been the most effective and widely used general method for the solution of linear programming problems. The Simplex Method in its various forms starts initially with a basic feasible solution and continues its moves in different iterations within the feasible region till it finds the optimal solution. The only other notable variation of the Simplex Method, namely the Dual Simplex Method, on the other hand, by virtue of the special formulation of the linear programming problem, starts with an in-feasible solution and continues to move in the in-feasible region till it finds the optimal solution at which it enters the feasible region. In other respects both the Simplex and the Dual Simplex Methods follow essentially the same principle for obtaining the optimal solution. The rigorous mathematical features have been widely discussed in the literature [12, 16, 34, 35, 38, 68, 77] and only those formal aspects of this topic which are closely related to the subject of this thesis will be outlined.
The Multiplex Method, though reported in the literature [30, 15, 69, 71, 29, 32], is not so well known and has also not been widely coded on electronic computers. It had earlier been programmed for the English Electric’s Computer ‘DEUCE’ by the author [72] and Ferranti’s ‘MERCURY’ by Ole-John Dahl in 1960 [15]. Later both the above mentioned computers were obsolete and the efforts presently concentrate on coding it for UNIVAC 1100 and IBM 360. The Multiplex Method, as such, has been included in the present thesis and discussed in some detail in chapter 2. The flow diagram and the algorithm for the method is given in section 2.4, chapter 2.
The main body of the thesis consists of developing a new linear programming method which has been called the Bounding Hyperplane Method – Part I. This is explained in detail in chapter 3. The method could initially start with either a basic feasible or in-feasible point and in its subsequent moves it may either alternate between the feasible and the in-feasible regions or get restricted to either of them depending upon the problem. It is applicable as a new phase which we call phase 0 to the Simple Method, particularly in situations where an initial basic feasible point is not available. In such cases it either results in a feasible point at the end of phase 0 or else yields a ‘better’ in-feasible point for phase 1 operations of the Simplex Method. Moreover, it is found that the number of iterations required to reach either the former by the application of phase 0 or the latter by the application of first phase 0 and then phase 1 are, in general, less than those required by following phase 1 alone. This is explained with illustrations in Chapter 6. Even when applied alone the method, in general, yields the optimal solution in fewer iterations as compared with the Simplex Method. This is illustrated with examples in chapter 3.
We also develop and illustrate a powerful but straight-forward method whereby we first find the solution to the equality constraints and (if the former does not yield an inconsistent solution point) then the transformations to the latter are obtained from the equality solution tableau corresponding to the former. This results in reducing the iteration time appreciably for each iteration of the method. It has been called the B.H.P.M. – part II and is discussed in chapter 4.
To estimate the time taken by the B.H.P and the Simplex Method, the two codes (written in Fortran) have been run on a number of problems taken from the literature. The results have been summarised in chapter 7.
Finally, the suggestions for further research towards i. the extensions of the B.H.P.M. to the quadratic programming problem where the function in (1.1.1) is positive semi-definite, and (ii) the accuracy of computations in linear programming, in general, are discussed in sections 8.1 and 8.2 respectively of chapter 8.
Thu, 01 Jan 1970 00:00:00 GMThttps://hdl.handle.net/10023/137341970-01-01T00:00:00ZSaksena, Chandra P.As stated earlier the Simplex Method (or its variations e.g. Dual Simplex Method) has thus far been the most effective and widely used general method for the solution of linear programming problems. The Simplex Method in its various forms starts initially with a basic feasible solution and continues its moves in different iterations within the feasible region till it finds the optimal solution. The only other notable variation of the Simplex Method, namely the Dual Simplex Method, on the other hand, by virtue of the special formulation of the linear programming problem, starts with an in-feasible solution and continues to move in the in-feasible region till it finds the optimal solution at which it enters the feasible region. In other respects both the Simplex and the Dual Simplex Methods follow essentially the same principle for obtaining the optimal solution. The rigorous mathematical features have been widely discussed in the literature [12, 16, 34, 35, 38, 68, 77] and only those formal aspects of this topic which are closely related to the subject of this thesis will be outlined.
The Multiplex Method, though reported in the literature [30, 15, 69, 71, 29, 32], is not so well known and has also not been widely coded on electronic computers. It had earlier been programmed for the English Electric’s Computer ‘DEUCE’ by the author [72] and Ferranti’s ‘MERCURY’ by Ole-John Dahl in 1960 [15]. Later both the above mentioned computers were obsolete and the efforts presently concentrate on coding it for UNIVAC 1100 and IBM 360. The Multiplex Method, as such, has been included in the present thesis and discussed in some detail in chapter 2. The flow diagram and the algorithm for the method is given in section 2.4, chapter 2.
The main body of the thesis consists of developing a new linear programming method which has been called the Bounding Hyperplane Method – Part I. This is explained in detail in chapter 3. The method could initially start with either a basic feasible or in-feasible point and in its subsequent moves it may either alternate between the feasible and the in-feasible regions or get restricted to either of them depending upon the problem. It is applicable as a new phase which we call phase 0 to the Simple Method, particularly in situations where an initial basic feasible point is not available. In such cases it either results in a feasible point at the end of phase 0 or else yields a ‘better’ in-feasible point for phase 1 operations of the Simplex Method. Moreover, it is found that the number of iterations required to reach either the former by the application of phase 0 or the latter by the application of first phase 0 and then phase 1 are, in general, less than those required by following phase 1 alone. This is explained with illustrations in Chapter 6. Even when applied alone the method, in general, yields the optimal solution in fewer iterations as compared with the Simplex Method. This is illustrated with examples in chapter 3.
We also develop and illustrate a powerful but straight-forward method whereby we first find the solution to the equality constraints and (if the former does not yield an inconsistent solution point) then the transformations to the latter are obtained from the equality solution tableau corresponding to the former. This results in reducing the iteration time appreciably for each iteration of the method. It has been called the B.H.P.M. – part II and is discussed in chapter 4.
To estimate the time taken by the B.H.P and the Simplex Method, the two codes (written in Fortran) have been run on a number of problems taken from the literature. The results have been summarised in chapter 7.
Finally, the suggestions for further research towards i. the extensions of the B.H.P.M. to the quadratic programming problem where the function in (1.1.1) is positive semi-definite, and (ii) the accuracy of computations in linear programming, in general, are discussed in sections 8.1 and 8.2 respectively of chapter 8.Involutive automorphisms and real forms of Kac-Moody algebras
https://hdl.handle.net/10023/13731
Involutive automorphisms of complex affine Kac-Moody algebras (in particular, their conjugacy classes within the group of all automorphisms) and their compact real forms are studied, using the matrix formulation which was developed by Cornwell. The initial study of the a⁽¹⁾ series of affine untwisted Kac-Moody algebras is extended to include the complex affine untwisted Kac-Moody algebras B⁽¹⁾, C⁽¹⁾ and D⁽¹⁾. From the information obtained, explicit bases for real forms of these Kac-Moody algebras are then constructed. A scheme for naming some real forms is suggested. Further work is included which examines the involutive automorphisms and the real forms of A₂⁽²⁾and the algebra G⁽¹⁾₂ (which is based upon an exceptional simple Lie algebra). The work involving the algebra A₂⁽²⁾is part of work towards extending the matrix formulation to twisted Kac-Moody algebras. The analysis also acts as a practical test of this method, and from it we may infer different ways of using the formulation to eventually obtain a complete picture of the conjugacy classes of the involutive automorphisms of all the affine Kac-Moody algebras.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/137311996-01-01T00:00:00ZClarke, StefanInvolutive automorphisms of complex affine Kac-Moody algebras (in particular, their conjugacy classes within the group of all automorphisms) and their compact real forms are studied, using the matrix formulation which was developed by Cornwell. The initial study of the a⁽¹⁾ series of affine untwisted Kac-Moody algebras is extended to include the complex affine untwisted Kac-Moody algebras B⁽¹⁾, C⁽¹⁾ and D⁽¹⁾. From the information obtained, explicit bases for real forms of these Kac-Moody algebras are then constructed. A scheme for naming some real forms is suggested. Further work is included which examines the involutive automorphisms and the real forms of A₂⁽²⁾and the algebra G⁽¹⁾₂ (which is based upon an exceptional simple Lie algebra). The work involving the algebra A₂⁽²⁾is part of work towards extending the matrix formulation to twisted Kac-Moody algebras. The analysis also acts as a practical test of this method, and from it we may infer different ways of using the formulation to eventually obtain a complete picture of the conjugacy classes of the involutive automorphisms of all the affine Kac-Moody algebras.Subalgebras of free nilpotent and polynilpotent lie algebras
https://hdl.handle.net/10023/13729
In this thesis we study subalgebras in free nilpotent and polynilpotent Lie algebras. Chapter 1 sets up the notation and includes definitions and elementary properties of free and certain reduced free Lie algebras that we use throughout this thesis. We also describe a Hall basis of a free Lie algebra as in [4] and a basis for a free polynilpotent Lie algebra which was developed in [24].
In Chapter 2 we first consider the class of nilpotency of subalbebras of free nilpotent Lie algebras starting with two-generator subalgebras. Then we study those subalgebras in a free nilpotent Lie algebra which, are themselves free nilpotent. We give necessary and sufficient conditions in the case of two-generator subalgebras.
Chapter 3 extends the results obtained in Chapter 2 to the polynilpotent case. First we look at two-generator subalgebras of a free polynilpotent Lie algebra. Then we consider more general subalgebras. Finally we study those subalgebras which are themselves free polynilpotent and give necessary and sufficient conditions for two-generator subalgebras to be free polynilpotent.
In Chapter 4 we first study certain properties of ideals in free, free nilpotent and free polynilpotent Lie algebras and establish the fact that in a free polynilpotent Lie algebra a nonzero ideal which is finitely-generated as a subalgebra must be equal to the whole algebra. Then we consider the quotient Lie algebra of a lower central term of a free Lie algebra by a term of the lower central series of an ideal. We then generalize the results to cover the free nilpotent and free polynilpotent cases. In the last section of Chapter 4 we consider ideals of free nilpotent (and later polynilpotent) Lie algebras as free nilpotent (polynilpotent) subalgebras and establish the fact that in most non-trivial cases such an ideal cannot be free nilpotent (polynilpotent).
In the last chapter we consider the m+k-th term of the lower central series of a free Lie algebra as a subalgebra of the m-th term for m ⩽ k and generalize the results proved in [25]. We give reasons for the failure of these results in the case m > k.
Sat, 01 Jan 1977 00:00:00 GMThttps://hdl.handle.net/10023/137291977-01-01T00:00:00ZBoral, MelihIn this thesis we study subalgebras in free nilpotent and polynilpotent Lie algebras. Chapter 1 sets up the notation and includes definitions and elementary properties of free and certain reduced free Lie algebras that we use throughout this thesis. We also describe a Hall basis of a free Lie algebra as in [4] and a basis for a free polynilpotent Lie algebra which was developed in [24].
In Chapter 2 we first consider the class of nilpotency of subalbebras of free nilpotent Lie algebras starting with two-generator subalgebras. Then we study those subalgebras in a free nilpotent Lie algebra which, are themselves free nilpotent. We give necessary and sufficient conditions in the case of two-generator subalgebras.
Chapter 3 extends the results obtained in Chapter 2 to the polynilpotent case. First we look at two-generator subalgebras of a free polynilpotent Lie algebra. Then we consider more general subalgebras. Finally we study those subalgebras which are themselves free polynilpotent and give necessary and sufficient conditions for two-generator subalgebras to be free polynilpotent.
In Chapter 4 we first study certain properties of ideals in free, free nilpotent and free polynilpotent Lie algebras and establish the fact that in a free polynilpotent Lie algebra a nonzero ideal which is finitely-generated as a subalgebra must be equal to the whole algebra. Then we consider the quotient Lie algebra of a lower central term of a free Lie algebra by a term of the lower central series of an ideal. We then generalize the results to cover the free nilpotent and free polynilpotent cases. In the last section of Chapter 4 we consider ideals of free nilpotent (and later polynilpotent) Lie algebras as free nilpotent (polynilpotent) subalgebras and establish the fact that in most non-trivial cases such an ideal cannot be free nilpotent (polynilpotent).
In the last chapter we consider the m+k-th term of the lower central series of a free Lie algebra as a subalgebra of the m-th term for m ⩽ k and generalize the results proved in [25]. We give reasons for the failure of these results in the case m > k.Finite difference solutions of the Von Mises equation
https://hdl.handle.net/10023/13727
Prandtl in 1904 discovered that the flow of a fluid over a thin obstacle can be adequately represented by an approximate set of equations, much simpler than the complex Navier-Stokes equations which govern the motion of fluid.
A particularly simple for of these equations, for the two-dimensional steady flow of a fluid past a flat plate, are the Von Mises Boundary layer equations. Unfortunately the Von Mises transformation introduces a singularity at the plate and this discouraged the use of the equations as a means for obtaining numerical solutions of boundary layer problems in incompressible and compressible flow.
In this thesis, we show that this difficulty can be overcome and the Von Mises equations are used as a basis for a finite difference evaluation of the velocity and temperature in the boundary layer adjacent to a flat plate, particular attention being given to conditions near the plate and more especially to the separation point.
In the section on compressible flow, the calculations also yield a check on certain common simplifying assumptions.
Wed, 01 Jan 1958 00:00:00 GMThttps://hdl.handle.net/10023/137271958-01-01T00:00:00ZThomson, John YoungPrandtl in 1904 discovered that the flow of a fluid over a thin obstacle can be adequately represented by an approximate set of equations, much simpler than the complex Navier-Stokes equations which govern the motion of fluid.
A particularly simple for of these equations, for the two-dimensional steady flow of a fluid past a flat plate, are the Von Mises Boundary layer equations. Unfortunately the Von Mises transformation introduces a singularity at the plate and this discouraged the use of the equations as a means for obtaining numerical solutions of boundary layer problems in incompressible and compressible flow.
In this thesis, we show that this difficulty can be overcome and the Von Mises equations are used as a basis for a finite difference evaluation of the velocity and temperature in the boundary layer adjacent to a flat plate, particular attention being given to conditions near the plate and more especially to the separation point.
In the section on compressible flow, the calculations also yield a check on certain common simplifying assumptions.Semigroups of singular endomorphisms of vector space
https://hdl.handle.net/10023/13725
In 1967, J. A. Erdős showed, using a matrix theory approach that the semigroup Sing[sub]n of singular endomorphisms of an n-dimensional vector space is generated by the set E of idempotent endomorphisms of rank n - 1. This thesis gives an alternative proof using a linear algebra and semigroup theory approach. It is also shown that not all the elements of E are needed to generate Sing[sub]n. Necessary conditions for a subset of E to generate found; these conditions are shown to be sufficient if the vector space is defined over a finite field. In this case, the minimum order of all subsets of E that generate Sing[sub]n is found. The problem of determining the number of subsets of E that generate Sing[sub]n and have this minimum order is considered; it is completely solved when the vector space is two-dimensional. From the proof given by Erdős, it could be deduced that any element of Sing[sub]n could be expressed as the product of, at most, 2n elements of E. It is shown here that this bound may be reduced to n, and that this is best possible. It is also shown that, if E⁺ is the set of all idempotent of Singn, then (E⁺)ⁿ⁻¹ is strictly contained in Sing[sub]n. Finally, it is shown that Erdős's result cannot be extended to the semigroup Sing of continuous singular endomorphisms of a separable Hilbert space. The sub semigroup of Sing generated by the idempotent of Sing is determined and is, clearly, strictly contained in Sing.
Tue, 01 Jan 1980 00:00:00 GMThttps://hdl.handle.net/10023/137251980-01-01T00:00:00ZDawlings, Robert J. H.In 1967, J. A. Erdős showed, using a matrix theory approach that the semigroup Sing[sub]n of singular endomorphisms of an n-dimensional vector space is generated by the set E of idempotent endomorphisms of rank n - 1. This thesis gives an alternative proof using a linear algebra and semigroup theory approach. It is also shown that not all the elements of E are needed to generate Sing[sub]n. Necessary conditions for a subset of E to generate found; these conditions are shown to be sufficient if the vector space is defined over a finite field. In this case, the minimum order of all subsets of E that generate Sing[sub]n is found. The problem of determining the number of subsets of E that generate Sing[sub]n and have this minimum order is considered; it is completely solved when the vector space is two-dimensional. From the proof given by Erdős, it could be deduced that any element of Sing[sub]n could be expressed as the product of, at most, 2n elements of E. It is shown here that this bound may be reduced to n, and that this is best possible. It is also shown that, if E⁺ is the set of all idempotent of Singn, then (E⁺)ⁿ⁻¹ is strictly contained in Sing[sub]n. Finally, it is shown that Erdős's result cannot be extended to the semigroup Sing of continuous singular endomorphisms of a separable Hilbert space. The sub semigroup of Sing generated by the idempotent of Sing is determined and is, clearly, strictly contained in Sing.Formal languages and idempotent semigroups
https://hdl.handle.net/10023/13724
The structure of the lattice 𝗟𝗕 of varieties of idempotent semigroups or bands (as universal algebras) was determined by Birjukov, Fennemore and Gerhard. Wis- math determined the structure of a related lattice: the lattice LBM of varieties of band monoids. In the first two parts we study several questions about these varieties.
In Part I we compute the cardinalities of the Green classes of the free objects in each variety of 𝗟𝗕 [𝗟𝗕𝗠]. These cardinalities constitute a useful piece of information in the study of several questions about these varieties and some of the conclusions obtained here are used in parts II and III.
Part II concerns expansions of bands [band monoids]. More precisely, we compute here the cut-down to generators of the Rhodes expansions of the free objects in the varieties of 𝗟𝗕. We define Rhodes expansion of a monoid, its cut-down to generators and we compute the cut-down to generators of the Rhodes expansions of the free objects in the varieties of 𝗟𝗕𝗠.
In Part III we deal with Eilenberg varieties of band monoids. The last chapter is particularly concerned with the description of the varieties of languages corresponding to these varieties.
Tue, 01 Jan 1991 00:00:00 GMThttps://hdl.handle.net/10023/137241991-01-01T00:00:00ZSezinando, Helena Maria da EncarnaçãoThe structure of the lattice 𝗟𝗕 of varieties of idempotent semigroups or bands (as universal algebras) was determined by Birjukov, Fennemore and Gerhard. Wis- math determined the structure of a related lattice: the lattice LBM of varieties of band monoids. In the first two parts we study several questions about these varieties.
In Part I we compute the cardinalities of the Green classes of the free objects in each variety of 𝗟𝗕 [𝗟𝗕𝗠]. These cardinalities constitute a useful piece of information in the study of several questions about these varieties and some of the conclusions obtained here are used in parts II and III.
Part II concerns expansions of bands [band monoids]. More precisely, we compute here the cut-down to generators of the Rhodes expansions of the free objects in the varieties of 𝗟𝗕. We define Rhodes expansion of a monoid, its cut-down to generators and we compute the cut-down to generators of the Rhodes expansions of the free objects in the varieties of 𝗟𝗕𝗠.
In Part III we deal with Eilenberg varieties of band monoids. The last chapter is particularly concerned with the description of the varieties of languages corresponding to these varieties.Random-walk theory and statistical mechanics of lattice systems
https://hdl.handle.net/10023/13722
It has been found elsewhere that when approximate relations for the two-particle correlation functions of classical statistical mechanics, such as the Percus-Yevick and the mean-spherical approximations, are applied to the lattice gas models with nearest-neighbour interactions simple expressions are obtained for the total correlation function in terms of the lattice Green's function. Since many of the properties of random walks on a lattice can be described by the lattice Green's function, it follows that these systems, at least when treated under these approximations, may be analysed in terms of the language of random walks.
Here the theory of random walks on lattices is appropriately extended to show that the relationship between the correlation functions and the lattice Green's function is not dependent upon the employment of these approximations, nor to a particular range or form of the potential function. Instead, this relationship is shown to be an alternative form of the Ornstein-Zernike relation between the direct and total correlation functions. The direct correlation function is directly related to the probability of a single step, whereas the total correlation function is given by the first-passage- time probabilities of the random walks. Thermodynamic properties, such as the isothermal compressibility, are also interpreted in terms of random-walk concepts.
The random-walk formulation is then extended to include the study of ordered phases in lattice-gas models and hence order-disorder transitions in these systems. Also, an asymptotic form for the lattice Green's function is derived to illustrate how the form of decay of the total correlation function at large distances depends on the properties of the potential function.
To show that the random-walk interpretation of the Ornstein-Zernike relation is not restricted to lattice systems, we define analogous random-walk functions for continuous space. These lead to a straight-forward generalization of most expressions for discrete space-; the relationship between the continuous-space total correlation and Green's functions has the same form as that for the lattice systems. We also explore the possibility of obtaining random-walk properties of a (lattice or continuous-space) system, not from the existing approximate expressions for the direct correlation function, but instead from a generalised Ornstein-Zernike relation based on a maximum principle of classical statistical mechanics.
Finally, we choose a few specific lattice-gas models to show how the method describes their different properties, such as the behaviour of the total correlation function or that of an order- disorder phase transition.
Tue, 01 Jan 1974 00:00:00 GMThttps://hdl.handle.net/10023/137221974-01-01T00:00:00ZNieto, Alberto RobledoIt has been found elsewhere that when approximate relations for the two-particle correlation functions of classical statistical mechanics, such as the Percus-Yevick and the mean-spherical approximations, are applied to the lattice gas models with nearest-neighbour interactions simple expressions are obtained for the total correlation function in terms of the lattice Green's function. Since many of the properties of random walks on a lattice can be described by the lattice Green's function, it follows that these systems, at least when treated under these approximations, may be analysed in terms of the language of random walks.
Here the theory of random walks on lattices is appropriately extended to show that the relationship between the correlation functions and the lattice Green's function is not dependent upon the employment of these approximations, nor to a particular range or form of the potential function. Instead, this relationship is shown to be an alternative form of the Ornstein-Zernike relation between the direct and total correlation functions. The direct correlation function is directly related to the probability of a single step, whereas the total correlation function is given by the first-passage- time probabilities of the random walks. Thermodynamic properties, such as the isothermal compressibility, are also interpreted in terms of random-walk concepts.
The random-walk formulation is then extended to include the study of ordered phases in lattice-gas models and hence order-disorder transitions in these systems. Also, an asymptotic form for the lattice Green's function is derived to illustrate how the form of decay of the total correlation function at large distances depends on the properties of the potential function.
To show that the random-walk interpretation of the Ornstein-Zernike relation is not restricted to lattice systems, we define analogous random-walk functions for continuous space. These lead to a straight-forward generalization of most expressions for discrete space-; the relationship between the continuous-space total correlation and Green's functions has the same form as that for the lattice systems. We also explore the possibility of obtaining random-walk properties of a (lattice or continuous-space) system, not from the existing approximate expressions for the direct correlation function, but instead from a generalised Ornstein-Zernike relation based on a maximum principle of classical statistical mechanics.
Finally, we choose a few specific lattice-gas models to show how the method describes their different properties, such as the behaviour of the total correlation function or that of an order- disorder phase transition.Contributions to the theory of Ockham algebras
https://hdl.handle.net/10023/13720
In the first part of this thesis we consider particular ordered sets (connected and of small height) and determine the cardinality of the corresponding dual MS - algebra and of its set of fixed points.
The remainder of the thesis is devoted to a study of congruences of Ockham algebras and a generalised variety K𝜔 of Ockham algebras that contains all of the Berman varieties K[sub]p,[sub]q. In particular we consider the congruences [sub]i(i = 1, 2,...) defined on an Ockham algebra (L; f) by
(x, y) ∊ [sub]i ⇔ fⁱ(x)= fⁱ(y)
and show that (L; f) ∊ K𝜔 is subdirectly irreducible if and only if the lattice of congruences of L reduces to the chain
𝜔 = 𝝫₀ ≤ 𝝫₁≤ 𝝫₂≤ … ≤𝝫𝜔<𝞲
Where 𝝫𝜔 = ⌵ [sub]i≥0𝝫i. Finally we obtain a characterisation of the finite simple Ockham algebras.
Wed, 01 Jan 1997 00:00:00 GMThttps://hdl.handle.net/10023/137201997-01-01T00:00:00ZFang, JieIn the first part of this thesis we consider particular ordered sets (connected and of small height) and determine the cardinality of the corresponding dual MS - algebra and of its set of fixed points.
The remainder of the thesis is devoted to a study of congruences of Ockham algebras and a generalised variety K𝜔 of Ockham algebras that contains all of the Berman varieties K[sub]p,[sub]q. In particular we consider the congruences [sub]i(i = 1, 2,...) defined on an Ockham algebra (L; f) by
(x, y) ∊ [sub]i ⇔ fⁱ(x)= fⁱ(y)
and show that (L; f) ∊ K𝜔 is subdirectly irreducible if and only if the lattice of congruences of L reduces to the chain
𝜔 = 𝝫₀ ≤ 𝝫₁≤ 𝝫₂≤ … ≤𝝫𝜔<𝞲
Where 𝝫𝜔 = ⌵ [sub]i≥0𝝫i. Finally we obtain a characterisation of the finite simple Ockham algebras.The descent algebras of Coxeter groups
https://hdl.handle.net/10023/13713
A descent algebra is a subalgebra of the group algebra of a Coxeter group. They were first defined over a field of characteristic zero. In this thesis, the main areas of research to be addressed are;
1. The formulation of a rule for multiplying two elements of descent algebra of the Coxeter groups of type D.
2. The identification of properties exhibited by descent algebras over a field of prime characteristic.
In addressing the first, a framework which exploits the specific properties of Coxeter groups is set up. With this framework, a new justification is given for existing rules for multiplying together two elements in the descent algebras of the Coxeter groups of type A and B. This framework is then used to derive a new multiplication rule for the descent algebra of the Coxeter groups of type D.
To address the second, a descent algebra over a field of prime characteristic, p, is defined. A homomorphism into the algebra of generalised p-modular characters is then described. This homomorphism is then used to obtain the radical, and allows the irreducible modules of the descent algebra to be determined.
Results from the two areas addressed are then exploited to give an explicit description of the radical of the descent algebra of the symmetric groups, over a finite field. In this instance, the nilpotency index of the radical and the irreducible representations are also described. Similarly, the descent algebra of the hyper-octahedral groups, over a finite field, has its radical, nilpotency index, and irreducible representations explicitly determined.
Wed, 01 Jan 1997 00:00:00 GMThttps://hdl.handle.net/10023/137131997-01-01T00:00:00ZVan WIlligenburg, StephanieA descent algebra is a subalgebra of the group algebra of a Coxeter group. They were first defined over a field of characteristic zero. In this thesis, the main areas of research to be addressed are;
1. The formulation of a rule for multiplying two elements of descent algebra of the Coxeter groups of type D.
2. The identification of properties exhibited by descent algebras over a field of prime characteristic.
In addressing the first, a framework which exploits the specific properties of Coxeter groups is set up. With this framework, a new justification is given for existing rules for multiplying together two elements in the descent algebras of the Coxeter groups of type A and B. This framework is then used to derive a new multiplication rule for the descent algebra of the Coxeter groups of type D.
To address the second, a descent algebra over a field of prime characteristic, p, is defined. A homomorphism into the algebra of generalised p-modular characters is then described. This homomorphism is then used to obtain the radical, and allows the irreducible modules of the descent algebra to be determined.
Results from the two areas addressed are then exploited to give an explicit description of the radical of the descent algebra of the symmetric groups, over a finite field. In this instance, the nilpotency index of the radical and the irreducible representations are also described. Similarly, the descent algebra of the hyper-octahedral groups, over a finite field, has its radical, nilpotency index, and irreducible representations explicitly determined.Certain classes of group presentations
https://hdl.handle.net/10023/13709
In Chapter two we look at the class
F(n) = <R, S | Rⁿ = Sⁿ = (Rᵃ¹Sᵇ¹)ˣ¹(Rᶜ¹Sᵈ¹)ʸ¹(Rᵃ²Sᵇ²)ˣ² (Rᶜ²Sᵈ²)ʸ² …(RᵃᵐSᵇᵐ)ˣᵐ (RᶜᵐSᵈᵐ)ʸᵐ = 1 >.
For some values of n, a[sub]i , b[sub]i, d[sub]i, x[sub]i, y[sub]i we give results on these groups where we have been able to determine their order, either finite or infinite. In the last section in Chapter two we study two classes of groups generated by A and B and subject to the following relations:
Relations for class 1:
A⁴ = 1, B⁴ = 1, (B(AB)²)⁴ = 1, (B(BA)⁶)⁴ = 1, (B(BA)¹⁴)⁴ = 1, …,
B(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-2)⁴ = 1
A⁻¹B⁻¹)²⁽ⁿ⁻³⁾ᐟ²B(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-2)B(BA)⁽²⁽ⁿ⁻³⁾ᐟ²B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁻¹⁾ᐟ²-2) B⁻¹
A⁻¹B⁻¹)²⁽ⁿ⁺¹⁾ᐟ²-3) A(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-1)B⁻¹= 1
(BA)²⁽ⁿ⁻¹⁾ᐟ² B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁻¹⁾ᐟ²-2) B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁺¹⁾ᐟ²-3) A² =1
Relations for class 2:
A⁴ = 1, B⁴ = 1, (B(AB)²)⁴ = 1, (B(BA)⁶)⁴ = 1, (B(BA)¹⁴)⁴ = 1, …, B(BA)⁽²⁽ⁿᐟ²⁻²⁾)⁴ = 1 , B⁻¹(BA)² ⁽ⁿ⁻²⁾ᐟ²B(BA) ⁽²ⁿᐟ²⁻²⁾ B(A⁻¹B⁻¹)²⁽ⁿ⁻²⁾ᐟ²-1) = 1, (BA) ⁽²ⁿᐟ²+2⁽ⁿ⁻²⁾ᐟ²+2)B(BA) ⁽²ⁿᐟ²-2)B(A⁻¹B⁻¹)²⁽ⁿ⁻²⁾ᐟ²-1)A² =1.
The groups in the first class turn out to be the cyclic group of order 2 and the groups in the second class turn out to be metabelian groups of order 4. (2ⁿᐟ²-1)² . Moreover the derived group of the groups in the second class is the direct product of two copies of a cyclic group of order (2ⁿᐟ²-1)². In Chapter three we study the groups with a presentation of the form:
<A,B|A⁴ = 1, Bⁿ = 1, AⁱBʲAᵏBᵗ =1
and determine all possibilities with conditions: j+t = 0 and i,k ∊ { + 1, 2 }.
Also in the second section of Chapter three we study the groups with a presentation of the form:
<A,B | A⁴ = 1, Bⁿ =1, AⁱBʲAᵏBᵗA ᵐBᵖ =1>
and determine some of the possibilities with conditions: j = l,t = l,p = -2 and i,k,m ∊ ℤ. In Chapter four we give new efficient presentations for the groups PSL(2,p), where p is an odd prime, p ∊ { 5,7,11,13,17,19,23,29,31,37, 41,43,53,59,79,83,89,109,139,229 }. We give permutation generators for these groups which satisfy our efficient presentation. Also we give new efficient presentations for PSL(2,p), where p is a prime power and p ∊ { 9,25,27,49,169}. Also in Chapter four, permutation generators are given for these groups which satisfy our presentations. In Chapter five we give new efficient presentations for the groups SL(2,p), where p is an odd prime and p ∊ { 5,7,11,13,17,19,23,29,31,41, 43,53,79,89,109,139,229 }. Also we give new efficient presentations for the groups SL(2,p), where p is an prime power and p ∊ { 8,16,25,27,49,169 }. In Chapter six we study the class of groups with the presentation
<a,b |aᵖ =1, bᵐ⁺ᵖa⁻ᵐbᵐa⁻ᵐ =1, (ab)² = 1>
,p an odd number and m ∊ ℤ. For some values of p and m these groups have connections with the groups PSL(2,p). In Chapter 7 we attempt to show the efficiency of PSL(2, ℤ[sub]n ) x PSL(2, ℤ[sub]m). For some values of n and m we give efficient presentation for these groups. In the same chapter we also attempt to show the efficiency of PSL(2, ℤ [sub]p) x PSL(2,32). For some values of p we give an efficient presentation for these groups. In the last section of the thesis we give efficient presentations for the following direct products
(i) PSL(2,5) X PSL(2,3²)
(ii) PSL(2,7) X PSL(2,3²)
(iii) PSL(2,5) X PSL(2,3³)
Also in the last section of the thesis the structure of a perfect group of order 161280 is investigated.
Fri, 01 Jan 1993 00:00:00 GMThttps://hdl.handle.net/10023/137091993-01-01T00:00:00ZVatansever, BilalIn Chapter two we look at the class
F(n) = <R, S | Rⁿ = Sⁿ = (Rᵃ¹Sᵇ¹)ˣ¹(Rᶜ¹Sᵈ¹)ʸ¹(Rᵃ²Sᵇ²)ˣ² (Rᶜ²Sᵈ²)ʸ² …(RᵃᵐSᵇᵐ)ˣᵐ (RᶜᵐSᵈᵐ)ʸᵐ = 1 >.
For some values of n, a[sub]i , b[sub]i, d[sub]i, x[sub]i, y[sub]i we give results on these groups where we have been able to determine their order, either finite or infinite. In the last section in Chapter two we study two classes of groups generated by A and B and subject to the following relations:
Relations for class 1:
A⁴ = 1, B⁴ = 1, (B(AB)²)⁴ = 1, (B(BA)⁶)⁴ = 1, (B(BA)¹⁴)⁴ = 1, …,
B(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-2)⁴ = 1
A⁻¹B⁻¹)²⁽ⁿ⁻³⁾ᐟ²B(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-2)B(BA)⁽²⁽ⁿ⁻³⁾ᐟ²B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁻¹⁾ᐟ²-2) B⁻¹
A⁻¹B⁻¹)²⁽ⁿ⁺¹⁾ᐟ²-3) A(BA)⁽²⁽ⁿ⁻¹⁾ᐟ²-1)B⁻¹= 1
(BA)²⁽ⁿ⁻¹⁾ᐟ² B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁻¹⁾ᐟ²-2) B⁻¹(A⁻¹B⁻¹)²⁽ⁿ⁺¹⁾ᐟ²-3) A² =1
Relations for class 2:
A⁴ = 1, B⁴ = 1, (B(AB)²)⁴ = 1, (B(BA)⁶)⁴ = 1, (B(BA)¹⁴)⁴ = 1, …, B(BA)⁽²⁽ⁿᐟ²⁻²⁾)⁴ = 1 , B⁻¹(BA)² ⁽ⁿ⁻²⁾ᐟ²B(BA) ⁽²ⁿᐟ²⁻²⁾ B(A⁻¹B⁻¹)²⁽ⁿ⁻²⁾ᐟ²-1) = 1, (BA) ⁽²ⁿᐟ²+2⁽ⁿ⁻²⁾ᐟ²+2)B(BA) ⁽²ⁿᐟ²-2)B(A⁻¹B⁻¹)²⁽ⁿ⁻²⁾ᐟ²-1)A² =1.
The groups in the first class turn out to be the cyclic group of order 2 and the groups in the second class turn out to be metabelian groups of order 4. (2ⁿᐟ²-1)² . Moreover the derived group of the groups in the second class is the direct product of two copies of a cyclic group of order (2ⁿᐟ²-1)². In Chapter three we study the groups with a presentation of the form:
<A,B|A⁴ = 1, Bⁿ = 1, AⁱBʲAᵏBᵗ =1
and determine all possibilities with conditions: j+t = 0 and i,k ∊ { + 1, 2 }.
Also in the second section of Chapter three we study the groups with a presentation of the form:
<A,B | A⁴ = 1, Bⁿ =1, AⁱBʲAᵏBᵗA ᵐBᵖ =1>
and determine some of the possibilities with conditions: j = l,t = l,p = -2 and i,k,m ∊ ℤ. In Chapter four we give new efficient presentations for the groups PSL(2,p), where p is an odd prime, p ∊ { 5,7,11,13,17,19,23,29,31,37, 41,43,53,59,79,83,89,109,139,229 }. We give permutation generators for these groups which satisfy our efficient presentation. Also we give new efficient presentations for PSL(2,p), where p is a prime power and p ∊ { 9,25,27,49,169}. Also in Chapter four, permutation generators are given for these groups which satisfy our presentations. In Chapter five we give new efficient presentations for the groups SL(2,p), where p is an odd prime and p ∊ { 5,7,11,13,17,19,23,29,31,41, 43,53,79,89,109,139,229 }. Also we give new efficient presentations for the groups SL(2,p), where p is an prime power and p ∊ { 8,16,25,27,49,169 }. In Chapter six we study the class of groups with the presentation
<a,b |aᵖ =1, bᵐ⁺ᵖa⁻ᵐbᵐa⁻ᵐ =1, (ab)² = 1>
,p an odd number and m ∊ ℤ. For some values of p and m these groups have connections with the groups PSL(2,p). In Chapter 7 we attempt to show the efficiency of PSL(2, ℤ[sub]n ) x PSL(2, ℤ[sub]m). For some values of n and m we give efficient presentation for these groups. In the same chapter we also attempt to show the efficiency of PSL(2, ℤ [sub]p) x PSL(2,32). For some values of p we give an efficient presentation for these groups. In the last section of the thesis we give efficient presentations for the following direct products
(i) PSL(2,5) X PSL(2,3²)
(ii) PSL(2,7) X PSL(2,3²)
(iii) PSL(2,5) X PSL(2,3³)
Also in the last section of the thesis the structure of a perfect group of order 161280 is investigated.Semigroups with length morphisms
https://hdl.handle.net/10023/13706
The class of metrical semigroups is defined as the set consisting of those semigroups which can be homomorphically mapped into the semigroup of natural numbers (without zero) under addition.
The finitely generated members of this class are characterised and the infinitely generated case is discussed. A semigroup is called locally metrical if every finitely generated subsemigroup is metrical.
The classical Green's relations are trivial on any metrical semigroup. Generalisations 𝓗+, 𝓛+ and 𝓡+ of the Green's relations are defined and it is shown that for any cancellative metrical semigroup, S, 𝓗 + is " as big as possible " if and only if S is isomorphic to a special type of semidirect product of 𝗡 and a group. Lyndon's characterisation of free groups by length functions is discussed andalink between length functions, metrical semigroups and semigroups embeddable into free semigroups is investigated. Next the maximal locally metrical ideal of a semigroup is discussed, and the class of t-compressible semigroups is defined as the set consisting of those semigroups that have no locally metrical ideal. The class of t-compressible semigroups is seen to contain the classes of regular and simple semigroups. Finally it is shown that a large class of semigroups can be decomposed into a chain of locally metrical ideals together with a t-compressible semigroup.
Thu, 01 Jan 1998 00:00:00 GMThttps://hdl.handle.net/10023/137061998-01-01T00:00:00ZSaunders, Bryan JamesThe class of metrical semigroups is defined as the set consisting of those semigroups which can be homomorphically mapped into the semigroup of natural numbers (without zero) under addition.
The finitely generated members of this class are characterised and the infinitely generated case is discussed. A semigroup is called locally metrical if every finitely generated subsemigroup is metrical.
The classical Green's relations are trivial on any metrical semigroup. Generalisations 𝓗+, 𝓛+ and 𝓡+ of the Green's relations are defined and it is shown that for any cancellative metrical semigroup, S, 𝓗 + is " as big as possible " if and only if S is isomorphic to a special type of semidirect product of 𝗡 and a group. Lyndon's characterisation of free groups by length functions is discussed andalink between length functions, metrical semigroups and semigroups embeddable into free semigroups is investigated. Next the maximal locally metrical ideal of a semigroup is discussed, and the class of t-compressible semigroups is defined as the set consisting of those semigroups that have no locally metrical ideal. The class of t-compressible semigroups is seen to contain the classes of regular and simple semigroups. Finally it is shown that a large class of semigroups can be decomposed into a chain of locally metrical ideals together with a t-compressible semigroup.Infinite transformation semigroups
https://hdl.handle.net/10023/13705
In this thesis some topics in the field of Infinite Transformation Semigroups are investigated.
In 1966 Howie considered the full transformation semigroup 𝓣 (x) on an infinite set x of cardinality m. For each 𝝰 in 𝓣 (x) he defined defect of 𝝰 = def 𝝰 and collapse of 𝝰= C(a) to be the sets X \ X 𝝰 and { 𝓍 ∊ x : (∃∊ x, y ≠ 𝓍) X𝝰 = Y𝝰 }, respectively. Later, in 1981 he introduced the set
S[sub]m̱ = {𝝰 ∊ 𝓣(x): |def 𝝰 | = | c(𝝰) | = | ran 𝝰 | = m, |y 𝝰 [super]-1 | <m,
(∀ y ∊ ran 𝝰) }
which is a subsemigroup of 𝓣 (x) provided the cardinal m is regular. Taking m to be a regular cardinal number, Howie proved that S[sub]m̱ is then a bisimple, idempotent-generated semigroup of depth 4. Next he considered the congruence defined in S[sub]m̱ by
△[sub]m̱ = {(𝝰, β) ∊ S[sub]m̱ x S[sub]m̱ : max (|D(𝝰, β) 𝝰| , | D((𝝰, β) β | ) < m̱ }
where D(𝝰, β) = { 𝓍 ∊ X : 𝓍 𝝰 ≠ 𝓍β } and showed that S[sub]m̱* = S[sub]m̱/ △[sub]m̱ is a bisimple, congruence-free and idempotent-generated semigroup of depth 4.
In this thesis comparable results are obtained for the semigroup P[sub]m̱ which is the top principal factor of the semigroup
𝓠[sub]m̱ = {𝝰 ∊ 𝓣(x): |def 𝝰 | = | c(𝝰) | = m̱}
Here it is no longer necessary to restrict to a regular cardinal m̱. The set S[sub]m̱ considered by Howie fails to be a subsemigroup of 𝓣 (𝓍) if m̱ is not regular. It is shown that in this case <S[sub]m̱ > = O[sub]m̱ . In the case where m̱ = 𝓍₀ (a regular cardinal) it is shown that △[sub]𝓍₀ is the only proper congruence on S[sub]m̱.
Within the symmetric inverse semigroup 𝓣(𝓍), the Baer-Levi semigroup B of type (m̱, m̱) on X is considered and a dual B* found. The products BB* and B*B are investigated and the semigroup Km̱ = <B*B> is described. The top principal factor of Km̱ is denoted by Lm̱ and it is shown that Lm̱ = B*B ⋃ {O}. On the set Lm̱ a congruence δ[sub]m̱, closely analogous to the congruence △[sub]m̱ defined above, is considered, and it is shown that Lm̱ / δ[sub]m̱ is a o-bisimple, inverse and nilpotent-generated semigroup.
Finally, two embedding theorems for inverse semigroups and semigroups in general are presented. The cardinalities of some of the semigroups introduced in this thesis are studied.
Sat, 01 Jan 1983 00:00:00 GMThttps://hdl.handle.net/10023/137051983-01-01T00:00:00ZMarques, Maria PaulaIn this thesis some topics in the field of Infinite Transformation Semigroups are investigated.
In 1966 Howie considered the full transformation semigroup 𝓣 (x) on an infinite set x of cardinality m. For each 𝝰 in 𝓣 (x) he defined defect of 𝝰 = def 𝝰 and collapse of 𝝰= C(a) to be the sets X \ X 𝝰 and { 𝓍 ∊ x : (∃∊ x, y ≠ 𝓍) X𝝰 = Y𝝰 }, respectively. Later, in 1981 he introduced the set
S[sub]m̱ = {𝝰 ∊ 𝓣(x): |def 𝝰 | = | c(𝝰) | = | ran 𝝰 | = m, |y 𝝰 [super]-1 | <m,
(∀ y ∊ ran 𝝰) }
which is a subsemigroup of 𝓣 (x) provided the cardinal m is regular. Taking m to be a regular cardinal number, Howie proved that S[sub]m̱ is then a bisimple, idempotent-generated semigroup of depth 4. Next he considered the congruence defined in S[sub]m̱ by
△[sub]m̱ = {(𝝰, β) ∊ S[sub]m̱ x S[sub]m̱ : max (|D(𝝰, β) 𝝰| , | D((𝝰, β) β | ) < m̱ }
where D(𝝰, β) = { 𝓍 ∊ X : 𝓍 𝝰 ≠ 𝓍β } and showed that S[sub]m̱* = S[sub]m̱/ △[sub]m̱ is a bisimple, congruence-free and idempotent-generated semigroup of depth 4.
In this thesis comparable results are obtained for the semigroup P[sub]m̱ which is the top principal factor of the semigroup
𝓠[sub]m̱ = {𝝰 ∊ 𝓣(x): |def 𝝰 | = | c(𝝰) | = m̱}
Here it is no longer necessary to restrict to a regular cardinal m̱. The set S[sub]m̱ considered by Howie fails to be a subsemigroup of 𝓣 (𝓍) if m̱ is not regular. It is shown that in this case <S[sub]m̱ > = O[sub]m̱ . In the case where m̱ = 𝓍₀ (a regular cardinal) it is shown that △[sub]𝓍₀ is the only proper congruence on S[sub]m̱.
Within the symmetric inverse semigroup 𝓣(𝓍), the Baer-Levi semigroup B of type (m̱, m̱) on X is considered and a dual B* found. The products BB* and B*B are investigated and the semigroup Km̱ = <B*B> is described. The top principal factor of Km̱ is denoted by Lm̱ and it is shown that Lm̱ = B*B ⋃ {O}. On the set Lm̱ a congruence δ[sub]m̱, closely analogous to the congruence △[sub]m̱ defined above, is considered, and it is shown that Lm̱ / δ[sub]m̱ is a o-bisimple, inverse and nilpotent-generated semigroup.
Finally, two embedding theorems for inverse semigroups and semigroups in general are presented. The cardinalities of some of the semigroups introduced in this thesis are studied.Idempotents, nilpotents, rank and order in finite transformation semigroups
https://hdl.handle.net/10023/13703
Let E, E₁ denote, respectively, the set of singular idempotents in T[sub]n (the semigroup of all full transformations on a finite set X[sub]n = {1,..., n}) and the set of idempotents of defect 1. For a singular element 𝑎 in Tn, let k(𝑎),k₁ (𝑎) be defined by the properties
𝑎 ∈ Eᵏ⁽ᵃ⁾, 𝑎 ∉ Eᵏ⁽ᵃ⁾⁻¹,
𝑎 ∈ E₁ᵏ¹⁽ᵃ⁾, 𝑎 ∉ E₁ᵏ¹⁽ᵃ⁾⁻¹.
In this Thesis, we obtain results analogous to those of Iwahori (1977), Howie (1980), Saito (1989) and Howie, Lusk and McFadden (1990) concerning the values of k(𝑎) and k₁(𝑎) for the partial transformation semigroup P[sub]n. The analogue of Howie and McFadden's (1990) result on the rank of the semigroup K(n,r) = { 𝑎 ∈ T [sub]n: |im 𝑎 | ≤ r,2 ≤ r ≤ n-1} is also obtained.
The nilpotent-generated subsemigroup of P[sub]n was characterised by Sullivan in 1987. In this work, we have obtained its depth and rank.
Nilpotents in IO[sub]n and PO[sub]n (the semigroup of all partial one-one order-preserving maps, and all partial order-preserving maps) are studied. A characterisation of their nilpotent-generated subsemigroups is obtained. So also are their depth and rank. We have also characterised their nilpotent-generated subsemigroup for the infinite set X = {1,2,...}. The rank of the semigroup L(n,r) = {a ∈ S : |im 𝑎 | ≤r, 1 ≤ r ≤ n - 2} is investigated for S = O[sub]n,PO[sub]n,SPO[sub]n and I[sub]n (where O[sub]n is the semigroup of all order-preserving full transformations, SPO[sub]n the semigroup of all strictly partial order- preserving maps, and In the semigroup of one-one partial transformation).
Wed, 01 Jan 1992 00:00:00 GMThttps://hdl.handle.net/10023/137031992-01-01T00:00:00ZGarba, Goje UbaLet E, E₁ denote, respectively, the set of singular idempotents in T[sub]n (the semigroup of all full transformations on a finite set X[sub]n = {1,..., n}) and the set of idempotents of defect 1. For a singular element 𝑎 in Tn, let k(𝑎),k₁ (𝑎) be defined by the properties
𝑎 ∈ Eᵏ⁽ᵃ⁾, 𝑎 ∉ Eᵏ⁽ᵃ⁾⁻¹,
𝑎 ∈ E₁ᵏ¹⁽ᵃ⁾, 𝑎 ∉ E₁ᵏ¹⁽ᵃ⁾⁻¹.
In this Thesis, we obtain results analogous to those of Iwahori (1977), Howie (1980), Saito (1989) and Howie, Lusk and McFadden (1990) concerning the values of k(𝑎) and k₁(𝑎) for the partial transformation semigroup P[sub]n. The analogue of Howie and McFadden's (1990) result on the rank of the semigroup K(n,r) = { 𝑎 ∈ T [sub]n: |im 𝑎 | ≤ r,2 ≤ r ≤ n-1} is also obtained.
The nilpotent-generated subsemigroup of P[sub]n was characterised by Sullivan in 1987. In this work, we have obtained its depth and rank.
Nilpotents in IO[sub]n and PO[sub]n (the semigroup of all partial one-one order-preserving maps, and all partial order-preserving maps) are studied. A characterisation of their nilpotent-generated subsemigroups is obtained. So also are their depth and rank. We have also characterised their nilpotent-generated subsemigroup for the infinite set X = {1,2,...}. The rank of the semigroup L(n,r) = {a ∈ S : |im 𝑎 | ≤r, 1 ≤ r ≤ n - 2} is investigated for S = O[sub]n,PO[sub]n,SPO[sub]n and I[sub]n (where O[sub]n is the semigroup of all order-preserving full transformations, SPO[sub]n the semigroup of all strictly partial order- preserving maps, and In the semigroup of one-one partial transformation).On a family of semigroup congruences
https://hdl.handle.net/10023/13699
We introduce in this thesis a new family of semigroup congruences, and we set out to prove that it is worth studying them for the following very important reasons:
(a) that it provides an alternative way of studying algebraic structures of semigroups, thus shedding new light over semigroup structures already known, and it also provides new information about other structures not formerly understood;
(b) that it is useful for constructing new semigroups, hence producing new and interesting classes of semigroups from known classes; and
(c) that it is useful for classifying semigroups, particularly in describing lattices formed by semigroup species such as varieties, pseudovarieties, existence varieties etc.
This interesting family of congruences is described as follows: for any semigroup S, and any ordered pair (n,m) of non-negative integers, define ⦵(n,m) = {(a,b): uav = ubv, for all ⋿Sn and v ⋿Sm}, and we make the convention that S¹ = S and that S0 denotes the set containing only the empty word. The particular cases ⦵(0,1), ⦵(1,0) and ⦵(0,0) were considered by the author in his M.Sc. thesis (1991). In fact, one can recognise ⦵(1,0) to be the well known kernel of the right regular representation of S. It turns out that if S is reductive (for example, if S is a monoid), then ⦵(i,j) is equal to ⦵(0,0) - the identity relation on S, for every (i,j).
After developing the tools required for the latter part of the thesis in Chapters 0-2, in Chapter 3 we introduce a new class of semigroups - the class of all structurally regular semigroups. Making use of a new Mal'tsev-type product, in Chapters 4,5,6 and 7, we describe the lattices formed by certain varieties of structurally regular semigroups.
Many interesting open problems are posed throughout the thesis, and brief literature reviews are inserted in the text where appropriate.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/136991996-01-01T00:00:00ZKopamu, Samuel Joseph LyambianWe introduce in this thesis a new family of semigroup congruences, and we set out to prove that it is worth studying them for the following very important reasons:
(a) that it provides an alternative way of studying algebraic structures of semigroups, thus shedding new light over semigroup structures already known, and it also provides new information about other structures not formerly understood;
(b) that it is useful for constructing new semigroups, hence producing new and interesting classes of semigroups from known classes; and
(c) that it is useful for classifying semigroups, particularly in describing lattices formed by semigroup species such as varieties, pseudovarieties, existence varieties etc.
This interesting family of congruences is described as follows: for any semigroup S, and any ordered pair (n,m) of non-negative integers, define ⦵(n,m) = {(a,b): uav = ubv, for all ⋿Sn and v ⋿Sm}, and we make the convention that S¹ = S and that S0 denotes the set containing only the empty word. The particular cases ⦵(0,1), ⦵(1,0) and ⦵(0,0) were considered by the author in his M.Sc. thesis (1991). In fact, one can recognise ⦵(1,0) to be the well known kernel of the right regular representation of S. It turns out that if S is reductive (for example, if S is a monoid), then ⦵(i,j) is equal to ⦵(0,0) - the identity relation on S, for every (i,j).
After developing the tools required for the latter part of the thesis in Chapters 0-2, in Chapter 3 we introduce a new class of semigroups - the class of all structurally regular semigroups. Making use of a new Mal'tsev-type product, in Chapters 4,5,6 and 7, we describe the lattices formed by certain varieties of structurally regular semigroups.
Many interesting open problems are posed throughout the thesis, and brief literature reviews are inserted in the text where appropriate.Nonstandard quantum groups: twisting constructions and noncommutative differential geometry
https://hdl.handle.net/10023/13693
The general subject of this thesis is quantum groups. The major original results are obtained in the particular areas of twisting constructions and noncommutative differential geometry.
Chapters 1 and 2 are intended to explain to the reader what are quantum groups. They are written in the form of a series of linked results and definitions. Chapter 1 reviews the theory of Lie algebras and Lie groups, focusing attention in particular on the classical Lie algebras and groups. Though none of the quoted results are due to the author, such a review, aimed specifically at setting up the paradigm which provides essential guidance in the theory of quantum groups, does not seem to have appeared already. In Chapter 2 the elements of the quantum group theory are recalled. Once again, almost none of the results are due to the author, though in Section 2.10, some results concerning the nonstandard Jordanian group are presented, by way of a worked example, which have not been published.
Chapter 3 concerns twisting constructions. We introduce a new class of 2-cocycles defined explicitly on the generators of certain multiparameter standard quantum groups. These allow us, through the process of twisting the familiar standard quantum groups, to generate new as well as previously known examples of non-standard quantum groups. In particular we are able to construct generalisations of both the Cremmer-Gervais deformation of SL(3) and the so called esoteric quantum groups of Fronsdal and Galindo in an explicit and straightforward manner.
In Chapter 4 we consider the differential calculus on Hopf algebras as introduced by Woronowicz. We classify all 4-dimensional first order bicovariant calculi on the Jordanian quantum group GL[sub]h,[sub]g(2) and all 3-dimensional first order bicovariant calculi on the Jordanian quantum group SL[sub]h(2). In both cases we assume that the bicovariant bimodules are generated as left modules by the differentials of the quantum group generators. It is found that there are 3 1-parameter families of 4-dimensional bicovariant first order calculi on GL[sub]h,[sub]g(2) and that there is a single, unique, 3-dimensional bicovariant calculus on SL[sub]h(2). This 3-dimensional calculus may be obtained through a classical-like reduction from any one of the three families of 4-dimensional calculi on GL[sub]h,[sub]g(2). Details of the higher order calculi and also the quantum Lie algebras are presented for all calculi. The quantum Lie algebra obtained from the bicovariant calculus on SL[sub]h(2) is shown to be isomorphic to the quantum Lie algebra we obtain as an ad-submodule within the Jordanian universal enveloping algebra U[sub]h(sl[sub]2(C)) and also through a consideration of the decomposition of the tensor product of two copies of the deformed adjoint module. We also obtain the quantum Killing form for this quantum Lie algebra.
Thu, 01 Jan 1998 00:00:00 GMThttps://hdl.handle.net/10023/136931998-01-01T00:00:00ZJacobs, Andrew D.The general subject of this thesis is quantum groups. The major original results are obtained in the particular areas of twisting constructions and noncommutative differential geometry.
Chapters 1 and 2 are intended to explain to the reader what are quantum groups. They are written in the form of a series of linked results and definitions. Chapter 1 reviews the theory of Lie algebras and Lie groups, focusing attention in particular on the classical Lie algebras and groups. Though none of the quoted results are due to the author, such a review, aimed specifically at setting up the paradigm which provides essential guidance in the theory of quantum groups, does not seem to have appeared already. In Chapter 2 the elements of the quantum group theory are recalled. Once again, almost none of the results are due to the author, though in Section 2.10, some results concerning the nonstandard Jordanian group are presented, by way of a worked example, which have not been published.
Chapter 3 concerns twisting constructions. We introduce a new class of 2-cocycles defined explicitly on the generators of certain multiparameter standard quantum groups. These allow us, through the process of twisting the familiar standard quantum groups, to generate new as well as previously known examples of non-standard quantum groups. In particular we are able to construct generalisations of both the Cremmer-Gervais deformation of SL(3) and the so called esoteric quantum groups of Fronsdal and Galindo in an explicit and straightforward manner.
In Chapter 4 we consider the differential calculus on Hopf algebras as introduced by Woronowicz. We classify all 4-dimensional first order bicovariant calculi on the Jordanian quantum group GL[sub]h,[sub]g(2) and all 3-dimensional first order bicovariant calculi on the Jordanian quantum group SL[sub]h(2). In both cases we assume that the bicovariant bimodules are generated as left modules by the differentials of the quantum group generators. It is found that there are 3 1-parameter families of 4-dimensional bicovariant first order calculi on GL[sub]h,[sub]g(2) and that there is a single, unique, 3-dimensional bicovariant calculus on SL[sub]h(2). This 3-dimensional calculus may be obtained through a classical-like reduction from any one of the three families of 4-dimensional calculi on GL[sub]h,[sub]g(2). Details of the higher order calculi and also the quantum Lie algebras are presented for all calculi. The quantum Lie algebra obtained from the bicovariant calculus on SL[sub]h(2) is shown to be isomorphic to the quantum Lie algebra we obtain as an ad-submodule within the Jordanian universal enveloping algebra U[sub]h(sl[sub]2(C)) and also through a consideration of the decomposition of the tensor product of two copies of the deformed adjoint module. We also obtain the quantum Killing form for this quantum Lie algebra.Computing with simple groups: maximal subgroups and presentations
https://hdl.handle.net/10023/13692
For the non-abelian simple groups G of order up to 10⁶ , excluding the groups PSL(2,q), q > 9, the presentations in terms of an involution a and an element b of minimal order (with respect to a) such that G=<a,b> are well known. The presentations are complete in the sense that any pair (x,y) of generators of G satisfying x²=yᵐ=1, with m minimal, will satisfy the defining relations of just one presentation in the list. There are 106 such presentations.
Using a computer, we give generators for each maximal subgroup of the groups G. For each presentation of G, the generators of maximal subgroups are given as words in the group generators. Similarly generators for a Sylow p-subgroup of G, for each p, are given. For each group G, we give a representative for each conjugacy class of the group as a word in the group generators.
Minimal presentations for each Sylow p-subgroup of the groups G, and for most of the maximal subgroups of G are constructed. To obtain such presentations, the Schur multipliers of the underlying groups are calculated.
The same tasks are carried out for those groups PSL(2,q) of order less than 10⁶ which are included in the "ATLAS of finite groups". For these groups we consider a presentation on two generators x, y with x²=y³=1.
A finite group G is said to be efficient if it has a presentation on d generators and d+rank(M(G)) relations (for some d) where M(G) is the Schur multiplier of G. We show that the simple groups J₁, PSU(3,5) and M₂₂ are efficient. We also give efficient presentations for the direct products A₅xA₆, A₅xA₆,A₆xA₇ where Ĥ denotes the covering group of H.
Sun, 01 Jan 1989 00:00:00 GMThttps://hdl.handle.net/10023/136921989-01-01T00:00:00ZJamali, Ali-RezaFor the non-abelian simple groups G of order up to 10⁶ , excluding the groups PSL(2,q), q > 9, the presentations in terms of an involution a and an element b of minimal order (with respect to a) such that G=<a,b> are well known. The presentations are complete in the sense that any pair (x,y) of generators of G satisfying x²=yᵐ=1, with m minimal, will satisfy the defining relations of just one presentation in the list. There are 106 such presentations.
Using a computer, we give generators for each maximal subgroup of the groups G. For each presentation of G, the generators of maximal subgroups are given as words in the group generators. Similarly generators for a Sylow p-subgroup of G, for each p, are given. For each group G, we give a representative for each conjugacy class of the group as a word in the group generators.
Minimal presentations for each Sylow p-subgroup of the groups G, and for most of the maximal subgroups of G are constructed. To obtain such presentations, the Schur multipliers of the underlying groups are calculated.
The same tasks are carried out for those groups PSL(2,q) of order less than 10⁶ which are included in the "ATLAS of finite groups". For these groups we consider a presentation on two generators x, y with x²=y³=1.
A finite group G is said to be efficient if it has a presentation on d generators and d+rank(M(G)) relations (for some d) where M(G) is the Schur multiplier of G. We show that the simple groups J₁, PSU(3,5) and M₂₂ are efficient. We also give efficient presentations for the direct products A₅xA₆, A₅xA₆,A₆xA₇ where Ĥ denotes the covering group of H.Semigroup presentations
https://hdl.handle.net/10023/13689
In this thesis we consider the following two fundamental problems for semigroup presentations:
1. Given a semigroup find a presentation defining it.
2. Given a presentation describe the semigroup defined by it.
We also establish other related results.
After an introduction in Chapter 1, we consider the first problem in Chapter 2, and establish a presentation for the commutative semigroup of integers Zpt. Dually, in Chapter 3 we consider the second problem and study presentations of semigroups related to the direct product of cyclic groups. In Chapter 4 we study presentations of semigroups related to dihedral groups and establish their V-classes structure in Chapter 5. In Chapter 6 we establish some results related to the Schutzenberger group which were suggested by our studies of the semigroup presentations in Chapters 3 and 4. Finally, in Chapter 7 we define and study new classes of semigroups which we call R, L-semi-commutative and semi-commutative semigroups and they were also suggested by our studies of the semigroup presentations in Chapters 3 and 4.
Wed, 01 Jan 1997 00:00:00 GMThttps://hdl.handle.net/10023/136891997-01-01T00:00:00ZIbrahim, Mohammed Ali FayaIn this thesis we consider the following two fundamental problems for semigroup presentations:
1. Given a semigroup find a presentation defining it.
2. Given a presentation describe the semigroup defined by it.
We also establish other related results.
After an introduction in Chapter 1, we consider the first problem in Chapter 2, and establish a presentation for the commutative semigroup of integers Zpt. Dually, in Chapter 3 we consider the second problem and study presentations of semigroups related to the direct product of cyclic groups. In Chapter 4 we study presentations of semigroups related to dihedral groups and establish their V-classes structure in Chapter 5. In Chapter 6 we establish some results related to the Schutzenberger group which were suggested by our studies of the semigroup presentations in Chapters 3 and 4. Finally, in Chapter 7 we define and study new classes of semigroups which we call R, L-semi-commutative and semi-commutative semigroups and they were also suggested by our studies of the semigroup presentations in Chapters 3 and 4.Algorithms for subgroup presentations: computer implementation and applications
https://hdl.handle.net/10023/13684
One of the main algorithms of computational group theory is the Todd-Coxeter coset enumeration algorithm, which provides a systematic method for finding the index of a subgroup of a finitely presented group. This has been extended in various ways to provide not only the index of a subgroup, but also a presentation for the subgroup. These methods tie in with a technique introduced by Reidemeister in the 1920's and later improved by Schreier, now known as the Reidemeister-Schreier algorithm.
In this thesis we discuss some of these variants of the Todd-Coxeter algorithm and their inter-relation, and also look at existing computer implementations of these different techniques. We then go on to describe a new package for coset methods which incorporates various types of coset enumeration, including modified Todd- Coxeter methods and the Reidemeister-Schreier process. This also has the capability of carrying out Tietze transformation simplification. Statistics obtained from running the new package on a collection of test examples are given, and the various techniques compared.
Finally, we use these algorithms, both theoretically and as computer implementations, to investigate a particular class of finitely presented groups defined by the presentation: < a, b | aⁿ = b² = (ab-1) ß =1, ab² = ba²>. Some interesting results have been discovered about these groups for various values of β and n. For example, if n is odd, the groups turn out to be finite and metabelian, and if β= 3 or β= 4 the derived group has an order which is dependent on the values of n (mod 8) and n (mod 12) respectively.
Tue, 01 Jan 1991 00:00:00 GMThttps://hdl.handle.net/10023/136841991-01-01T00:00:00ZHeggie, Patricia, M.One of the main algorithms of computational group theory is the Todd-Coxeter coset enumeration algorithm, which provides a systematic method for finding the index of a subgroup of a finitely presented group. This has been extended in various ways to provide not only the index of a subgroup, but also a presentation for the subgroup. These methods tie in with a technique introduced by Reidemeister in the 1920's and later improved by Schreier, now known as the Reidemeister-Schreier algorithm.
In this thesis we discuss some of these variants of the Todd-Coxeter algorithm and their inter-relation, and also look at existing computer implementations of these different techniques. We then go on to describe a new package for coset methods which incorporates various types of coset enumeration, including modified Todd- Coxeter methods and the Reidemeister-Schreier process. This also has the capability of carrying out Tietze transformation simplification. Statistics obtained from running the new package on a collection of test examples are given, and the various techniques compared.
Finally, we use these algorithms, both theoretically and as computer implementations, to investigate a particular class of finitely presented groups defined by the presentation: < a, b | aⁿ = b² = (ab-1) ß =1, ab² = ba²>. Some interesting results have been discovered about these groups for various values of β and n. For example, if n is odd, the groups turn out to be finite and metabelian, and if β= 3 or β= 4 the derived group has an order which is dependent on the values of n (mod 8) and n (mod 12) respectively.On the efficiency of finite groups
https://hdl.handle.net/10023/13682
In Chapter 2 of this thesis we look at methods for finding efficient presentations of the transitive permutation groups of degree ≤ 12. Chapter 3 gives efficient presentations for certain direct products of groups including PSL(2, P)2 SL(2, p) X SL(2, 8), PSL(2, p) x C2, for prime p ≥ 5 and PSL(2, 25)3. Chapter 4 introduces a new class of inefficient groups and Chapter 5 gives a brief survey of some of the open problems relating to the efficiency of finite groups.
Mon, 01 Jan 1996 00:00:00 GMThttps://hdl.handle.net/10023/136821996-01-01T00:00:00ZBrookes, MelanieIn Chapter 2 of this thesis we look at methods for finding efficient presentations of the transitive permutation groups of degree ≤ 12. Chapter 3 gives efficient presentations for certain direct products of groups including PSL(2, P)2 SL(2, p) X SL(2, 8), PSL(2, p) x C2, for prime p ≥ 5 and PSL(2, 25)3. Chapter 4 introduces a new class of inefficient groups and Chapter 5 gives a brief survey of some of the open problems relating to the efficiency of finite groups.Application of the Todd-Coxeter coset enumeration algorithm
https://hdl.handle.net/10023/13508
This thesis is concerned with a topic in combinatorial group theory and, in particular, with a study of some groups with finite presentations. After preliminary definitions and theorems we describe the Todd-Coxeter coset enumeration algorithm and the modified Todd-Coxeter algorithm which shows that, given a finitely generated subgroup H of finite index in a finitely presented group G, we can find a presentation for H. We then give elementary examples illustrating the algorithms and include a discussion on the computer programmes that are to be used. In the main part of the thesis we investigate two classes of cyclically presented groups. Supposewhere w1 = w is a word in a1,a2,...,an, and wi+1 is obtained from wi by applying the permutation (1 2 ... n) to the suffices of the a's. The first class we investigate are the groups that is the groups G(l,m,n) are groups of type G2 (w). Secondly we investigate the Fibonacci-type groups H(r,n,k,s,h) obtained when, for some integers r,s,h > 1, k > O, the word w is given by Fibonacci groups being the special case given by k = s = h = 1. For both classes we begin by giving some homomorphisms and isomorphisms that may be obtained. We show, using the Todd-Coxeter algorithm when appropriate, that the six groups G(2,2,3), G(2,2,-3), G(-l,-l,4), G(2,3,-2), G(-2,2,-1) and G(-2,3,l) are finite non-metacyclic groups of deficiency zero, having orders 215.33, 28.33, 29.3.5, 23.33.7, 23.3.5.11 amd 23.36 respectively. We also show that the groups G(1-n, 6, n) where n = 1 mod 5 give an infinite series of non-metacyclic groups. We consider the structure of the non-metacyclic groups H(3,6,1,1,1) and H(3,6,5,l,2) both of order 1512, showing that neither is isomorphic to G(2, 3, -2) another non-metacyclic group of order 1512. In a paper on the Fibonacci groups D.L. Johnson, J.W. Wamsley and D. Wright pose two questions relating to the Fibonacci groups for the case r = 1 mod n, namely to find 2-generator 2-relation presentations for them and also their orders. We answer these questions and generalise the results to the class H(r,n,k,s,1) where it is shown that H(r,n,k,s,1) is metacyclic if (i) r = s mod n, (ii) (r,n) = 1, (iii) (r + k - 1, n) - 1, and a 2-generator 2-relation presentation is found for these groups. Further if (iv) (r,s) = 1, then we show that H(r,n,k,s,1) is a finite metacyclic group of order rn - sn. A possible generalisation to the groups H(r,n,k,s,h) is considered. Finally the metacyclic groups H(r,4,1,2,1), r odd are discussed.
Wed, 01 Jan 1975 00:00:00 GMThttps://hdl.handle.net/10023/135081975-01-01T00:00:00ZCampbell, C. M. (Colin Matthew)This thesis is concerned with a topic in combinatorial group theory and, in particular, with a study of some groups with finite presentations. After preliminary definitions and theorems we describe the Todd-Coxeter coset enumeration algorithm and the modified Todd-Coxeter algorithm which shows that, given a finitely generated subgroup H of finite index in a finitely presented group G, we can find a presentation for H. We then give elementary examples illustrating the algorithms and include a discussion on the computer programmes that are to be used. In the main part of the thesis we investigate two classes of cyclically presented groups. Supposewhere w1 = w is a word in a1,a2,...,an, and wi+1 is obtained from wi by applying the permutation (1 2 ... n) to the suffices of the a's. The first class we investigate are the groups that is the groups G(l,m,n) are groups of type G2 (w). Secondly we investigate the Fibonacci-type groups H(r,n,k,s,h) obtained when, for some integers r,s,h > 1, k > O, the word w is given by Fibonacci groups being the special case given by k = s = h = 1. For both classes we begin by giving some homomorphisms and isomorphisms that may be obtained. We show, using the Todd-Coxeter algorithm when appropriate, that the six groups G(2,2,3), G(2,2,-3), G(-l,-l,4), G(2,3,-2), G(-2,2,-1) and G(-2,3,l) are finite non-metacyclic groups of deficiency zero, having orders 215.33, 28.33, 29.3.5, 23.33.7, 23.3.5.11 amd 23.36 respectively. We also show that the groups G(1-n, 6, n) where n = 1 mod 5 give an infinite series of non-metacyclic groups. We consider the structure of the non-metacyclic groups H(3,6,1,1,1) and H(3,6,5,l,2) both of order 1512, showing that neither is isomorphic to G(2, 3, -2) another non-metacyclic group of order 1512. In a paper on the Fibonacci groups D.L. Johnson, J.W. Wamsley and D. Wright pose two questions relating to the Fibonacci groups for the case r = 1 mod n, namely to find 2-generator 2-relation presentations for them and also their orders. We answer these questions and generalise the results to the class H(r,n,k,s,1) where it is shown that H(r,n,k,s,1) is metacyclic if (i) r = s mod n, (ii) (r,n) = 1, (iii) (r + k - 1, n) - 1, and a 2-generator 2-relation presentation is found for these groups. Further if (iv) (r,s) = 1, then we show that H(r,n,k,s,1) is a finite metacyclic group of order rn - sn. A possible generalisation to the groups H(r,n,k,s,h) is considered. Finally the metacyclic groups H(r,4,1,2,1), r odd are discussed.Centralisers and normalisers in symmetric and alternating groups
https://hdl.handle.net/10023/13507
In this thesis, we analyse the structure of the centraliser of an element and of the normaliser of a cyclic subgroup in both Sn and An. We show that the centraliser in Sn of a permutation can be written as a direct product of centralisers of regular permutations and that the centraliser of a regular permutation is a wreath product. In certain cases we prove that this wreath product splits as a direct product and we analyse the centre of the subgroup. We calculate the centraliser of a general permutation in An and show how this is related to the centralisers of regular permutations. We investigate the normaliser of the cyclic subgroup generated by an element of Sn and show how this is related to the centraliser of the permutation. We calculate the centre of the normaliser and investigate when the normaliser splits as a direct product. We carry out a similar investigation for normalisers of cyclic subgroups of An and investigate the relationship between normalisers in An and Sn. We give presentations for both centralisers and normalisers.
Thu, 01 Jan 1998 00:00:00 GMThttps://hdl.handle.net/10023/135071998-01-01T00:00:00ZBilgiç, HuseyinIn this thesis, we analyse the structure of the centraliser of an element and of the normaliser of a cyclic subgroup in both Sn and An. We show that the centraliser in Sn of a permutation can be written as a direct product of centralisers of regular permutations and that the centraliser of a regular permutation is a wreath product. In certain cases we prove that this wreath product splits as a direct product and we analyse the centre of the subgroup. We calculate the centraliser of a general permutation in An and show how this is related to the centralisers of regular permutations. We investigate the normaliser of the cyclic subgroup generated by an element of Sn and show how this is related to the centraliser of the permutation. We calculate the centre of the normaliser and investigate when the normaliser splits as a direct product. We carry out a similar investigation for normalisers of cyclic subgroups of An and investigate the relationship between normalisers in An and Sn. We give presentations for both centralisers and normalisers.Some applications of computer algebra and interval mathematics
https://hdl.handle.net/10023/13502
This thesis contains some applications of Computer Algebra to unconstrained optimization and some applications of Interval Mathematics to the problem of simultaneously bounding the simple zeros of polynomials. Chapter 1 contains a brief introduction to Computer Algebra and Interval Mathematics, and several of the fundamental results from Interval Mathematics which are used in Chapters 4 and 5. Chapter 2 contains a survey of those features of the symbol manipulation package ALgLIB[Shew-85] which it is necessary to understand in order to use ALgLIB as explained in Chapter 3. Chapter 3 contains a description of Sisser's method [Sis-82a] for unconstrained minimization and several modifications thereof which are implemented using the pseudo-code of Dennis and Schnabel [DenS-83], and ALgLIB, Chapter 3 also contains numerical results corresponding to Sisser's method and its modifications for 7 examples. Chapter 4 contains a new algorithm PRSS for the simultaneous estimation of polynomial zeros and the corresponding interval form IRSS for simultaneously bounding real polynomial zeros. Comparisons are made with some related existing algorithms. Numerical results of the comparisons are also given in this chapter. Chapter 5 contains an application of an idea due to Neumaier [Neu-85] to the problem of constructing interval versions of point iterative procedures for the estimation of simple zeros of analytic functions. In particular, interval versions of some point iterative procedures for the simultaneous estimation of simple (complex) polynomial zeros are described. Finally, numerical results are given to show the efficiency of the new algorithm.
Fri, 01 Jan 1988 00:00:00 GMThttps://hdl.handle.net/10023/135021988-01-01T00:00:00ZMonsi, Mansor BinThis thesis contains some applications of Computer Algebra to unconstrained optimization and some applications of Interval Mathematics to the problem of simultaneously bounding the simple zeros of polynomials. Chapter 1 contains a brief introduction to Computer Algebra and Interval Mathematics, and several of the fundamental results from Interval Mathematics which are used in Chapters 4 and 5. Chapter 2 contains a survey of those features of the symbol manipulation package ALgLIB[Shew-85] which it is necessary to understand in order to use ALgLIB as explained in Chapter 3. Chapter 3 contains a description of Sisser's method [Sis-82a] for unconstrained minimization and several modifications thereof which are implemented using the pseudo-code of Dennis and Schnabel [DenS-83], and ALgLIB, Chapter 3 also contains numerical results corresponding to Sisser's method and its modifications for 7 examples. Chapter 4 contains a new algorithm PRSS for the simultaneous estimation of polynomial zeros and the corresponding interval form IRSS for simultaneously bounding real polynomial zeros. Comparisons are made with some related existing algorithms. Numerical results of the comparisons are also given in this chapter. Chapter 5 contains an application of an idea due to Neumaier [Neu-85] to the problem of constructing interval versions of point iterative procedures for the estimation of simple zeros of analytic functions. In particular, interval versions of some point iterative procedures for the simultaneous estimation of simple (complex) polynomial zeros are described. Finally, numerical results are given to show the efficiency of the new algorithm.Proof diagrams and term rewriting with applications to computational algebra
https://hdl.handle.net/10023/13498
In this thesis lessons learned from the use of computer algebra systems and machine assisted theorem provers are developed in order to give an insight into both the problems and their solutions. Many algorithms in computational algebra and automated deduction (for example Grobner basis computations and Knuth-Bendix completion) tend to produce redundant facts and can contain more than one proof of any particular fact. This thesis introduces proof diagrams in order to compare and contrast the proofs of facts which such procedures generate. Proof diagrams make it possible to analyse the effect of heuristics which can be used to guide implementations of such algorithms. An extended version of an inference system for Knuth-Bendix completion is introduced. It is possible to see that this extension characterises the applicability of critical pair criteria, which are heuristics used in completion. We investigate a number of executions of a completion procedure by analysing the associated proof diagrams. This leads to a better understanding of the heuristics used to control these examples. Derived rales of inference are also investigated in this thesis. This is done in the formalism of proof diagrams. Rewrite rules for proof diagrams are defined: this is motivated by the notion of a transformation tactic in the Nuprl proof development system. A method to automatically extract 'useful' derived inference rales is also discussed. 'Off the shelf' theorem provers, such as the Larch Prover and Otter, are compared to specialised programs from computational group theory. This analysis makes it possible to see where methods from automated deduction can improve on the tools which group theorists currently use. Problems which can be attacked with theorem provers but not with currently used specialised programs are also indicated. Tietze transformations, from group theory, are discussed. This makes it possible to link ideas used in Knuth-Bendix completion programs and group presentation simplification programs. Tietze transformations provide heuristics for more efficient and effective implementations of these programs.
Wed, 01 Jan 1997 00:00:00 GMThttps://hdl.handle.net/10023/134981997-01-01T00:00:00ZShand, DuncanIn this thesis lessons learned from the use of computer algebra systems and machine assisted theorem provers are developed in order to give an insight into both the problems and their solutions. Many algorithms in computational algebra and automated deduction (for example Grobner basis computations and Knuth-Bendix completion) tend to produce redundant facts and can contain more than one proof of any particular fact. This thesis introduces proof diagrams in order to compare and contrast the proofs of facts which such procedures generate. Proof diagrams make it possible to analyse the effect of heuristics which can be used to guide implementations of such algorithms. An extended version of an inference system for Knuth-Bendix completion is introduced. It is possible to see that this extension characterises the applicability of critical pair criteria, which are heuristics used in completion. We investigate a number of executions of a completion procedure by analysing the associated proof diagrams. This leads to a better understanding of the heuristics used to control these examples. Derived rales of inference are also investigated in this thesis. This is done in the formalism of proof diagrams. Rewrite rules for proof diagrams are defined: this is motivated by the notion of a transformation tactic in the Nuprl proof development system. A method to automatically extract 'useful' derived inference rales is also discussed. 'Off the shelf' theorem provers, such as the Larch Prover and Otter, are compared to specialised programs from computational group theory. This analysis makes it possible to see where methods from automated deduction can improve on the tools which group theorists currently use. Problems which can be attacked with theorem provers but not with currently used specialised programs are also indicated. Tietze transformations, from group theory, are discussed. This makes it possible to link ideas used in Knuth-Bendix completion programs and group presentation simplification programs. Tietze transformations provide heuristics for more efficient and effective implementations of these programs.Tools and techniques for machine-assisted meta-theory
https://hdl.handle.net/10023/13382
Machine-assisted formal proofs are becoming commonplace in certain fields of mathematics and theoretical computer science. New formal systems and variations on old ones are constantly invented. The meta-theory of such systems, i.e. proofs about the system as opposed to proofs within the system, are mostly done informally with a pen and paper. Yet the meta-theory of deductive systems is an area which would obviously benefit from machine support for formal proof. Is the software currently available sufficiently powerful yet easy enough to use to make machine assistance for formal meta-theory a viable proposition? This thesis presents work done by the author on formalizing proof theory from [DP97a] in various formal systems: SEQUEL [Tar93, Tar97], Isabelle [Pau94] and Coq [BB+96]. SEQUEL and Isabelle were found to be difficult to use for this type of work. In particular, the lack of automated production of induction principles in SEQUEL and Isabelle undermined confidence in the resulting formal proofs. Coq was found to be suitable for the formalisation methodology first chosen: the use of nameless dummy variables (de Bruijn indices) as pioneered in [dB72]. A second approach (inspired by the work of McKinna and Pollack [vBJMR94, MP97]) formalising named variables was also the subject of some initial work, and a comparison of these two approaches is presented. The formalisation was restricted to the implicational fragment of propositional logic. The informal theory has been extended to cover full propositional logic by Dyckhoff and Pinto, and extension of the formalisation using de Bruijn indices would appear to present few difficulties. An overview of other work in this area, in terms of both the tools and formalisation methods, is also presented. The theory formalised differs from other such work in that other formalisations have involved only one calculus. [DP97a] involves the relationships between three different calculi. There is consequently a much greater requirement for equality reasoning in the formalisation. It is concluded that a formalisation of any significance is still difficult, particularly one involving multiple calculi. No tools currently exist that allow for the easy representation of even quite simple systems in a way that fits human intuitions while still allowing for automatic derivation of induction principles. New work on integrating higher order abstract syntax and induction may be the way forward, although such work is still in the early stages.
Wed, 01 Jan 1997 00:00:00 GMThttps://hdl.handle.net/10023/133821997-01-01T00:00:00ZAdams, Andrew, 1969-Machine-assisted formal proofs are becoming commonplace in certain fields of mathematics and theoretical computer science. New formal systems and variations on old ones are constantly invented. The meta-theory of such systems, i.e. proofs about the system as opposed to proofs within the system, are mostly done informally with a pen and paper. Yet the meta-theory of deductive systems is an area which would obviously benefit from machine support for formal proof. Is the software currently available sufficiently powerful yet easy enough to use to make machine assistance for formal meta-theory a viable proposition? This thesis presents work done by the author on formalizing proof theory from [DP97a] in various formal systems: SEQUEL [Tar93, Tar97], Isabelle [Pau94] and Coq [BB+96]. SEQUEL and Isabelle were found to be difficult to use for this type of work. In particular, the lack of automated production of induction principles in SEQUEL and Isabelle undermined confidence in the resulting formal proofs. Coq was found to be suitable for the formalisation methodology first chosen: the use of nameless dummy variables (de Bruijn indices) as pioneered in [dB72]. A second approach (inspired by the work of McKinna and Pollack [vBJMR94, MP97]) formalising named variables was also the subject of some initial work, and a comparison of these two approaches is presented. The formalisation was restricted to the implicational fragment of propositional logic. The informal theory has been extended to cover full propositional logic by Dyckhoff and Pinto, and extension of the formalisation using de Bruijn indices would appear to present few difficulties. An overview of other work in this area, in terms of both the tools and formalisation methods, is also presented. The theory formalised differs from other such work in that other formalisations have involved only one calculus. [DP97a] involves the relationships between three different calculi. There is consequently a much greater requirement for equality reasoning in the formalisation. It is concluded that a formalisation of any significance is still difficult, particularly one involving multiple calculi. No tools currently exist that allow for the easy representation of even quite simple systems in a way that fits human intuitions while still allowing for automatic derivation of induction principles. New work on integrating higher order abstract syntax and induction may be the way forward, although such work is still in the early stages.The Arabic translation of Theodosius's Sphaerica
https://hdl.handle.net/10023/13380
The thesis "The Arabic Translation of Theodosius's Sphaerica" is an edition of the Istanbul manuscript Topkapi Seray Ahmet III 3464.2. Included is a comparative apparatus of the Greek and Arabic texts showing possible correspondence between the posited Greek exemplar of the translator and the various Greek manuscript traditions reported by J.L. Heiberg in his critical edition of the text. Further differences are pointed out in the English Trajislation. There is also a glossary of terminology- giving listings from Greek to Arabic and Arabic to Greek. An appendix discussing the execution of the drawings in the Arabic manuscript and their relation to the Greek drawings as reported by Heiberg is also given. Other appendices include a chart representing the convention seemingly adopted by the translator for lettering the drawings, a listing of inconsistent grammatical usage found in the manuscript, parallel passages from the Greek text, the text of the present edition, the versions of al-Maghribi and al-Tusi, and a privately owned manuscript, and finally a list of interlinear sigla found on the first few folios of the manuscript the purpose of which is unclear.
Wed, 01 Jan 1975 00:00:00 GMThttps://hdl.handle.net/10023/133801975-01-01T00:00:00ZMartin, Thomas J.The thesis "The Arabic Translation of Theodosius's Sphaerica" is an edition of the Istanbul manuscript Topkapi Seray Ahmet III 3464.2. Included is a comparative apparatus of the Greek and Arabic texts showing possible correspondence between the posited Greek exemplar of the translator and the various Greek manuscript traditions reported by J.L. Heiberg in his critical edition of the text. Further differences are pointed out in the English Trajislation. There is also a glossary of terminology- giving listings from Greek to Arabic and Arabic to Greek. An appendix discussing the execution of the drawings in the Arabic manuscript and their relation to the Greek drawings as reported by Heiberg is also given. Other appendices include a chart representing the convention seemingly adopted by the translator for lettering the drawings, a listing of inconsistent grammatical usage found in the manuscript, parallel passages from the Greek text, the text of the present edition, the versions of al-Maghribi and al-Tusi, and a privately owned manuscript, and finally a list of interlinear sigla found on the first few folios of the manuscript the purpose of which is unclear.The life and work of Prof. George Chrystal (1851-1911)
https://hdl.handle.net/10023/13379
This thesis is principally concerned with George Chrystal's life and his work, mainly in three directions viz., as an experimentalist, a mathematician, and an educationist. The main object is to bring to light the work of a personality who is representative of many more who are always forgotten. The majority of historians of science consider the works of the giants in science, ignoring totally the contributions made by the less prominent people like Prof. George Chrystal. In fact their contributions serve as one of the most important factors in propagation of scientific knowledge. His main contributions: verification of Ohm's Law experimentally; Non-Euclidean geometry; differential equations; text books on algebra; theory of seiches; institution of leaving certificate examination in Scottish education and many more have been discussed in detail. A survey of Chrystal's general thought is given in so far as it may be gathered from his scattered remarks. The references are mentioned by numerals in the superscript, details of which are given at the end of each chapter. The main text consists of six chapters. There are three appendices at the end,' Appendix 'A' consists of his correspondence with different scientists, most of which is still unpublished. Appendix 'B' contains a bibliography of his contributions in chronological order, and Appendix 'C contains his three Promoter's addresses. Tables and figures are attached at their proper places, including some rarely available photographs.
Mon, 01 Jan 1990 00:00:00 GMThttps://hdl.handle.net/10023/133791990-01-01T00:00:00ZYousuf, MohammadThis thesis is principally concerned with George Chrystal's life and his work, mainly in three directions viz., as an experimentalist, a mathematician, and an educationist. The main object is to bring to light the work of a personality who is representative of many more who are always forgotten. The majority of historians of science consider the works of the giants in science, ignoring totally the contributions made by the less prominent people like Prof. George Chrystal. In fact their contributions serve as one of the most important factors in propagation of scientific knowledge. His main contributions: verification of Ohm's Law experimentally; Non-Euclidean geometry; differential equations; text books on algebra; theory of seiches; institution of leaving certificate examination in Scottish education and many more have been discussed in detail. A survey of Chrystal's general thought is given in so far as it may be gathered from his scattered remarks. The references are mentioned by numerals in the superscript, details of which are given at the end of each chapter. The main text consists of six chapters. There are three appendices at the end,' Appendix 'A' consists of his correspondence with different scientists, most of which is still unpublished. Appendix 'B' contains a bibliography of his contributions in chronological order, and Appendix 'C contains his three Promoter's addresses. Tables and figures are attached at their proper places, including some rarely available photographs.Normalisation techniques in proof theory and category theory
https://hdl.handle.net/10023/13371
The word problem for the free categories with some structure generated by a category X can be solved using proof-theoretical means. These free categories give a semantics in which derivations of GENTZEN's propositional sequent calculus can be interpreted by means of arrows of those categories. In this thesis we describe, implement and document the cut-elimination and the normalization techniques in proof theory as outlined in SZABO [1978]: we show how these are used in order to solve, mechanically, the word problem for the free categories with structure of : cartesian, bicartesian, distributive bicartesian, cartesian closed, and bicartesian closed. This implementation is extended by a procedure to interpret intuitionistic propositional sequent derivations as arrows of the above categories. Implementation of those techniques has forced us to modify the techniques in various inessential ways. The description and the representation in the syntax of our implementation of the above categories is contained in chapters 1 - 5, where each chapter describes one theory and concludes with examples of the system In use to represent concepts and solve simple word problems from category theory ( of various typos ). Appendix 1 contains some apparent printing errors we have observed in the work done by SZABO. The algorithms used in the proof of the cut-elimination theorems and normalization through chapters 1 - 5 are collected in appendices 2 - 4. Appendices 5 - 8 concern the implementation and its user manual.
Wed, 01 Jan 1986 00:00:00 GMThttps://hdl.handle.net/10023/133711986-01-01T00:00:00ZHamza, Taher Tawfik AhmedThe word problem for the free categories with some structure generated by a category X can be solved using proof-theoretical means. These free categories give a semantics in which derivations of GENTZEN's propositional sequent calculus can be interpreted by means of arrows of those categories. In this thesis we describe, implement and document the cut-elimination and the normalization techniques in proof theory as outlined in SZABO [1978]: we show how these are used in order to solve, mechanically, the word problem for the free categories with structure of : cartesian, bicartesian, distributive bicartesian, cartesian closed, and bicartesian closed. This implementation is extended by a procedure to interpret intuitionistic propositional sequent derivations as arrows of the above categories. Implementation of those techniques has forced us to modify the techniques in various inessential ways. The description and the representation in the syntax of our implementation of the above categories is contained in chapters 1 - 5, where each chapter describes one theory and concludes with examples of the system In use to represent concepts and solve simple word problems from category theory ( of various typos ). Appendix 1 contains some apparent printing errors we have observed in the work done by SZABO. The algorithms used in the proof of the cut-elimination theorems and normalization through chapters 1 - 5 are collected in appendices 2 - 4. Appendices 5 - 8 concern the implementation and its user manual.Proof search issues in some non-classical logics
https://hdl.handle.net/10023/13362
This thesis develops techniques and ideas on proof search. Proof search is used with one of two meanings. Proof search can be thought of either as the search for a yes/no answer to a query (theorem proving), or as the search for all proofs of a formula (proof enumeration). This thesis is an investigation into issues in proof search in both these senses for some non-classical logics. Gentzen systems are well suited for use in proof search in both senses. The rules of Gentzen sequent calculi are such that implementations can be directed by the top level syntax of sequents, unlike other logical calculi such as natural deduction. All the calculi for proof search in this thesis are Gentzen sequent calculi. In Chapter 2, permutation of inference rules for Intuitionistic Linear Logic is studied. A focusing calculus, ILLF, in the style of Andreoli ([And92]) is developed. This calculus allows only one proof in each equivalence class of proofs equivalent up to permutations of inferences. The issue here is both theorem proving and proof enumeration. For certain logics, normal natural deductions provide a proof-theoretic semantics. Proof enumeration is then the enumeration of all these deductions. Herbelin's cut- free LJT ([Her95], here called MJ) is a Gentzen system for intuitionistic logic allowing derivations that correspond in a 1-1 way to the normal natural deductions of intuitionistic logic. This calculus is therefore well suited to proof enumeration. Such calculi are called 'permutation-free' calculi. In Chapter 3, MJ is extended to a calculus for an intuitionistic modal logic (due to Curry) called Lax Logic. We call this calculus PFLAX. The proof theory of MJ is extended to PFLAX. Chapter 4 presents work on theorem proving for propositional logics using a history mechanism for loop-checking. This mechanism is a refinement of one developed by Heuerding et al ([HSZ96]). It is applied to two calculi for intuitionistic logic and also to two modal logics; Lax Logic and intuitionistic S4. The calculi for intuitionistic logic are compared both theoretically and experimentally with other decision procedures for the logic. Chapter 5 is a short investigation of embedding intuitionistic logic in Intuitionistic Linear Logic. A new embedding of intuitionistic logic in Intuitionistic Linear Logic is given. For the hereditary Harrop fragment of intuitionistic logic, this embedding induces the calculus MJ for intuitionistic logic. In Chapter 6 a 'permutation-free' calculus is given for Intuitionistic Linear Logic. Again, its proof-theoretic properties are investigated. The calculus is proved to be sound and complete with respect to a proof-theoretic semantics and (weak) cut- elimination is proved. Logic programming can be thought of as proof enumeration in constructive logics. All the proof enumeration calculi in this thesis have been developed with logic programming in mind. We discuss at the appropriate points the relationship between the calculi developed here and logic programming. Appendix A contains presentations of the logical calculi used and Appendix B contains the sets of benchmark formulae used in Chapter 4.
Fri, 01 Jan 1999 00:00:00 GMThttps://hdl.handle.net/10023/133621999-01-01T00:00:00ZHowe, Jacob M.This thesis develops techniques and ideas on proof search. Proof search is used with one of two meanings. Proof search can be thought of either as the search for a yes/no answer to a query (theorem proving), or as the search for all proofs of a formula (proof enumeration). This thesis is an investigation into issues in proof search in both these senses for some non-classical logics. Gentzen systems are well suited for use in proof search in both senses. The rules of Gentzen sequent calculi are such that implementations can be directed by the top level syntax of sequents, unlike other logical calculi such as natural deduction. All the calculi for proof search in this thesis are Gentzen sequent calculi. In Chapter 2, permutation of inference rules for Intuitionistic Linear Logic is studied. A focusing calculus, ILLF, in the style of Andreoli ([And92]) is developed. This calculus allows only one proof in each equivalence class of proofs equivalent up to permutations of inferences. The issue here is both theorem proving and proof enumeration. For certain logics, normal natural deductions provide a proof-theoretic semantics. Proof enumeration is then the enumeration of all these deductions. Herbelin's cut- free LJT ([Her95], here called MJ) is a Gentzen system for intuitionistic logic allowing derivations that correspond in a 1-1 way to the normal natural deductions of intuitionistic logic. This calculus is therefore well suited to proof enumeration. Such calculi are called 'permutation-free' calculi. In Chapter 3, MJ is extended to a calculus for an intuitionistic modal logic (due to Curry) called Lax Logic. We call this calculus PFLAX. The proof theory of MJ is extended to PFLAX. Chapter 4 presents work on theorem proving for propositional logics using a history mechanism for loop-checking. This mechanism is a refinement of one developed by Heuerding et al ([HSZ96]). It is applied to two calculi for intuitionistic logic and also to two modal logics; Lax Logic and intuitionistic S4. The calculi for intuitionistic logic are compared both theoretically and experimentally with other decision procedures for the logic. Chapter 5 is a short investigation of embedding intuitionistic logic in Intuitionistic Linear Logic. A new embedding of intuitionistic logic in Intuitionistic Linear Logic is given. For the hereditary Harrop fragment of intuitionistic logic, this embedding induces the calculus MJ for intuitionistic logic. In Chapter 6 a 'permutation-free' calculus is given for Intuitionistic Linear Logic. Again, its proof-theoretic properties are investigated. The calculus is proved to be sound and complete with respect to a proof-theoretic semantics and (weak) cut- elimination is proved. Logic programming can be thought of as proof enumeration in constructive logics. All the proof enumeration calculi in this thesis have been developed with logic programming in mind. We discuss at the appropriate points the relationship between the calculi developed here and logic programming. Appendix A contains presentations of the logical calculi used and Appendix B contains the sets of benchmark formulae used in Chapter 4.Theory and observations of the magnetic field in the solar corona
https://hdl.handle.net/10023/12948
Although the solar corona is one of the most studied areas in solar physics, its activity, such as flares, prominence eruptions and CMEs, is far from understood. Since the solar corona is a low-ß plasma, its structure and dynamics are driven by the magnetic field. The aim of this PhD thesis to study the magnetic field in the solar corona. Unfortunately, high quality direct measurements of the coronal magnetic field are not available and theoretical extrapolation using the observed photospheric magnetic field is required. The thesis is mainly divided in two parts. The first part deals with the comparison between theoretical models of magnetic fields and observed structures in the corona. For any theoretical model, a quantitative method to fit magnetic field lines to observed coronal loops is introduced. This method provides a quantity C that measures how closely a theoretical model can reproduce the observed coronal structures. Using linear force-free field extrapolation, the above field line fitting method is used to study the evolution of an active region. The method is also illustrated when the theoretical magnetic field depends on more than one parameter. The second part of the thesis focuses on the linear force-free field assumption using two different geometric configurations. Firstly a vertical rigid magnetic flux tube is considered. The analytical expression of the magnetic field is obtained as an expansion in terms of Bessel functions. The main properties of this system are discussed and compared with two cylindrically symmetric twist profiles. For the second system, the photosphere is assumed to be an infinite plane. Using translational geometry, the analytical expression of the linear force-free magnetic field that matches a prescribed line of sight magnetic field component is obtained. This solution is compared with the non-linear solution obtained by Roumeliotis (1993).
Sat, 01 Jan 2005 00:00:00 GMThttps://hdl.handle.net/10023/129482005-01-01T00:00:00ZCarcedo, LauraAlthough the solar corona is one of the most studied areas in solar physics, its activity, such as flares, prominence eruptions and CMEs, is far from understood. Since the solar corona is a low-ß plasma, its structure and dynamics are driven by the magnetic field. The aim of this PhD thesis to study the magnetic field in the solar corona. Unfortunately, high quality direct measurements of the coronal magnetic field are not available and theoretical extrapolation using the observed photospheric magnetic field is required. The thesis is mainly divided in two parts. The first part deals with the comparison between theoretical models of magnetic fields and observed structures in the corona. For any theoretical model, a quantitative method to fit magnetic field lines to observed coronal loops is introduced. This method provides a quantity C that measures how closely a theoretical model can reproduce the observed coronal structures. Using linear force-free field extrapolation, the above field line fitting method is used to study the evolution of an active region. The method is also illustrated when the theoretical magnetic field depends on more than one parameter. The second part of the thesis focuses on the linear force-free field assumption using two different geometric configurations. Firstly a vertical rigid magnetic flux tube is considered. The analytical expression of the magnetic field is obtained as an expansion in terms of Bessel functions. The main properties of this system are discussed and compared with two cylindrically symmetric twist profiles. For the second system, the photosphere is assumed to be an infinite plane. Using translational geometry, the analytical expression of the linear force-free magnetic field that matches a prescribed line of sight magnetic field component is obtained. This solution is compared with the non-linear solution obtained by Roumeliotis (1993).Loop oscillations in the corona
https://hdl.handle.net/10023/12947
Magnetic loops in the Sun's corona have been discovered to oscillate in a variety of modes. The oscillations are observed to exhibit strong damping. A number of theories have been put forward to explain the damping, including resonant absorption and phase mixing. Here we consider the modelling of loop oscillations, paying particular attention to two effects: gravity, and the addition of a chromospheric layer below the corona. We develop an acoustic model of coronal loop oscillations and consider two ways of describing the effects of the gravitational stratification and the chromospheric layers, considering either two media separated by a discontinuous interface or a single medium with a sound speed that varies along the loop. A dispersion relation for the two-layer isothermal atmosphere case is obtained and investigated numerically using a bisection code. On comparison with roots obtained for a single isothermal atmosphere, it was found that the effect of chromospheric footpoints on the period of a mode is slight. However, the effect of gravity was found to be more notable, rising up to a twenty percent change in period when considering the longer observed loops. This result is of especial interest since gravity is often ignored by authors discussing loop oscillations. The case of a linear sound speed has been investigated analytically, obtaining a dispersion relation in terms of Bessel functions. Our results show that the Bessel equation is a possible solution for describing the wave modes.
Thu, 01 Jan 2004 00:00:00 GMThttps://hdl.handle.net/10023/129472004-01-01T00:00:00ZJames, LornaMagnetic loops in the Sun's corona have been discovered to oscillate in a variety of modes. The oscillations are observed to exhibit strong damping. A number of theories have been put forward to explain the damping, including resonant absorption and phase mixing. Here we consider the modelling of loop oscillations, paying particular attention to two effects: gravity, and the addition of a chromospheric layer below the corona. We develop an acoustic model of coronal loop oscillations and consider two ways of describing the effects of the gravitational stratification and the chromospheric layers, considering either two media separated by a discontinuous interface or a single medium with a sound speed that varies along the loop. A dispersion relation for the two-layer isothermal atmosphere case is obtained and investigated numerically using a bisection code. On comparison with roots obtained for a single isothermal atmosphere, it was found that the effect of chromospheric footpoints on the period of a mode is slight. However, the effect of gravity was found to be more notable, rising up to a twenty percent change in period when considering the longer observed loops. This result is of especial interest since gravity is often ignored by authors discussing loop oscillations. The case of a linear sound speed has been investigated analytically, obtaining a dispersion relation in terms of Bessel functions. Our results show that the Bessel equation is a possible solution for describing the wave modes.Magnetic annihilation, null collapse and coronal heating
https://hdl.handle.net/10023/12946
The problem of how the Sun's corona is heated is of central importance to solar physics research. In this thesis we model three main areas. The first, annihilation, is a feature of non-ideal MHD and focusses on how magnetic field of opposite polarity meets at a null point and annihilates, after having been advected with plasma toward a stagnation point in the plasma flow. Generally, the null point of the field and the stagnation point of the flow are coincident at the origin, but in chapter 2 a simple extension is considered where an asymmetry in the boundary conditions of the field moves the null point away from the origin. Chapter 3 presents a model of reconnective annihilation in three dimensions. It represents flux being advected through the fan plane of a 3D null, and diffusing through a thin diffusion region before being annihilated at the spine line, and uses the method of matched asymptotic expansions to find the solution for small values of the resistivity. The second area of the thesis covers null collapse. This is when the magnetic field in close proximity to a null point is disturbed, causing the field to fold up on itself and collapse. This is a feature of ideal MHD, and causes a strong current to build up, allowing non-ideal effects to become important. When using linearised equations for the collapse problem, we are in fact looking at a linear instability. If this instability initiates a collapse, this is only a valid model until non-linear effects become important. By talking about collapse in chapters 4 and 5 (as it is talked about in the literature), we mean that the linear instability initiates collapse, which in principle, non-linear effects could later stop. Chapter 4 introduces a two-dimensional model for collapse, using the ideal, compressible, linearised MHD equations. It is a general solution in which all spatially linear nulls and their supporting plasma flows and pressure gradients can be checked for susceptibility to collapse under open boundary conditions. Chapter 5 uses the model introduced in chapter 4 to investigate the collapse of three-dimensional, potential nulls (again, spatially linear) for all possible supporting plasma flows and pressure gradients. Using this model, all nulls under consideration are found to collapse and produce large currents, except for a group of 2D O-type nulls supported by highly super-Alfvenic plasma flows. The third area of this thesis involves numerically simulating a model of heating by coronal tectonics (Priest et al, 2002). A simple magnetic field is created and the boundary is driven, also in a simple manner. Current sheets which scale with grid resolution are seen to build up on the quasi-separatrix layers, and there is some evidence of magnetic reconnection.
Thu, 01 Jan 2004 00:00:00 GMThttps://hdl.handle.net/10023/129462004-01-01T00:00:00ZMellor, ChristopherThe problem of how the Sun's corona is heated is of central importance to solar physics research. In this thesis we model three main areas. The first, annihilation, is a feature of non-ideal MHD and focusses on how magnetic field of opposite polarity meets at a null point and annihilates, after having been advected with plasma toward a stagnation point in the plasma flow. Generally, the null point of the field and the stagnation point of the flow are coincident at the origin, but in chapter 2 a simple extension is considered where an asymmetry in the boundary conditions of the field moves the null point away from the origin. Chapter 3 presents a model of reconnective annihilation in three dimensions. It represents flux being advected through the fan plane of a 3D null, and diffusing through a thin diffusion region before being annihilated at the spine line, and uses the method of matched asymptotic expansions to find the solution for small values of the resistivity. The second area of the thesis covers null collapse. This is when the magnetic field in close proximity to a null point is disturbed, causing the field to fold up on itself and collapse. This is a feature of ideal MHD, and causes a strong current to build up, allowing non-ideal effects to become important. When using linearised equations for the collapse problem, we are in fact looking at a linear instability. If this instability initiates a collapse, this is only a valid model until non-linear effects become important. By talking about collapse in chapters 4 and 5 (as it is talked about in the literature), we mean that the linear instability initiates collapse, which in principle, non-linear effects could later stop. Chapter 4 introduces a two-dimensional model for collapse, using the ideal, compressible, linearised MHD equations. It is a general solution in which all spatially linear nulls and their supporting plasma flows and pressure gradients can be checked for susceptibility to collapse under open boundary conditions. Chapter 5 uses the model introduced in chapter 4 to investigate the collapse of three-dimensional, potential nulls (again, spatially linear) for all possible supporting plasma flows and pressure gradients. Using this model, all nulls under consideration are found to collapse and produce large currents, except for a group of 2D O-type nulls supported by highly super-Alfvenic plasma flows. The third area of this thesis involves numerically simulating a model of heating by coronal tectonics (Priest et al, 2002). A simple magnetic field is created and the boundary is driven, also in a simple manner. Current sheets which scale with grid resolution are seen to build up on the quasi-separatrix layers, and there is some evidence of magnetic reconnection.Sir Arthur Eddington and the foundations of modern physics
https://hdl.handle.net/10023/12933
In this dissertation I analyze Sir Arthur Eddington's statistical theory as developed in the first six chapters of his posthumously published Fundamental Theory. In particular I look at the mathematical structure, philosophical implications, and relevancy to modern physics. This analysis is the only one of Fundamental Theory that compares it to modern quantum field theory and is the most comprehensive look at his statistical theory in four decades. Several major insights have been made in this analysis including the fact that he was able to derive Pauli's Exclusion Principle in part from Heisenberg's Uncertainty Principle. In addition the most profound general conclusion of this research is that Fundamental Theory is, in fact, an early quantum field theory, something that has never before been suggested. Contrary to the majority of historical reports and some comments by his contemporaries, this analysis shows that Eddington's later work is neither mystical nor was it that far from mainstream when it was published. My research reveals numerous profoundly deep ideas that were ahead of their time when Fundamental Theory was developed, but that have significant applicability at present. As such this analysis presents several important questions to be considered by modern philosophers of science, physicists, mathematicians, and historians. In addition it sheds new light on Eddington as a scientist and mathematician, in part indicating that his marginalization has been largely unwarranted.
Sat, 01 Jan 2005 00:00:00 GMThttps://hdl.handle.net/10023/129332005-01-01T00:00:00ZDurham, Ian T.In this dissertation I analyze Sir Arthur Eddington's statistical theory as developed in the first six chapters of his posthumously published Fundamental Theory. In particular I look at the mathematical structure, philosophical implications, and relevancy to modern physics. This analysis is the only one of Fundamental Theory that compares it to modern quantum field theory and is the most comprehensive look at his statistical theory in four decades. Several major insights have been made in this analysis including the fact that he was able to derive Pauli's Exclusion Principle in part from Heisenberg's Uncertainty Principle. In addition the most profound general conclusion of this research is that Fundamental Theory is, in fact, an early quantum field theory, something that has never before been suggested. Contrary to the majority of historical reports and some comments by his contemporaries, this analysis shows that Eddington's later work is neither mystical nor was it that far from mainstream when it was published. My research reveals numerous profoundly deep ideas that were ahead of their time when Fundamental Theory was developed, but that have significant applicability at present. As such this analysis presents several important questions to be considered by modern philosophers of science, physicists, mathematicians, and historians. In addition it sheds new light on Eddington as a scientist and mathematician, in part indicating that his marginalization has been largely unwarranted.The construction of finite soluble factor groups of finitely presented groups and its application
https://hdl.handle.net/10023/12600
Computational group theory deals with the design, analysis and computer implementation of algorithms for solving computational problems involving groups, and with the applications of the programs produced to interesting questions in group theory, in other branches of mathematics, and in other areas of science. This thesis describes an implementation of a proposal for a Soluble Quotient Algorithm, i.e. a description of the algorithms used and a report on the findings of an empirical study of the behaviour of the programs, and gives an account of an application of the programs. The programs were used for the construction of soluble groups with interesting properties, e.g. for the construction of soluble groups of large derived length which seem to be candidates for groups having efficient presentations. New finite soluble groups of derived length six with trivial Schur multiplier and efficient presentations are described. The methods for finding efficient presentations proved to be only practicable for groups of moderate order. Therefore, for a given derived length soluble groups of small order are of interest. The minimal soluble groups of derived length less than or equal to six are classified.
Wed, 01 Jan 1992 00:00:00 GMThttps://hdl.handle.net/10023/126001992-01-01T00:00:00ZWegner, AlexanderComputational group theory deals with the design, analysis and computer implementation of algorithms for solving computational problems involving groups, and with the applications of the programs produced to interesting questions in group theory, in other branches of mathematics, and in other areas of science. This thesis describes an implementation of a proposal for a Soluble Quotient Algorithm, i.e. a description of the algorithms used and a report on the findings of an empirical study of the behaviour of the programs, and gives an account of an application of the programs. The programs were used for the construction of soluble groups with interesting properties, e.g. for the construction of soluble groups of large derived length which seem to be candidates for groups having efficient presentations. New finite soluble groups of derived length six with trivial Schur multiplier and efficient presentations are described. The methods for finding efficient presentations proved to be only practicable for groups of moderate order. Therefore, for a given derived length soluble groups of small order are of interest. The minimal soluble groups of derived length less than or equal to six are classified.Relaxation methods in compressible flow
https://hdl.handle.net/10023/12298
Sat, 01 Jan 1949 00:00:00 GMThttps://hdl.handle.net/10023/122981949-01-01T00:00:00ZMitchell, A. R. (Andrew R.)Modelling the spatial dynamics of non-state terrorism : world study, 2002-2013
https://hdl.handle.net/10023/12067
To this day, terrorism perpetrated by non-state actors persists as a worldwide threat, as exemplified by the recent lethal attacks in Paris, London, Brussels, and the ongoing massacres perpetrated by the Islamic State in Iraq, Syria and neighbouring countries. In response, states deploy various counterterrorism policies, the costs of which could be reduced through more efficient preventive measures. The literature has not applied statistical models able to account for complex spatio-temporal dependencies, despite their potential for explaining and preventing non-state terrorism at the sub-national level. In an effort to address this shortcoming, this thesis employs Bayesian hierarchical models, where the spatial random field is represented by a stochastic partial differential equation. The results show that lethal terrorist attacks perpetrated by non-state actors tend to be concentrated in areas located within failed states from which they may diffuse locally, towards neighbouring areas. At the sub-national level, the propensity of attacks to be lethal and the frequency of lethal attacks appear to be driven by antagonistic mechanisms. Attacks are more likely to be lethal far away from large cities, at higher altitudes, in less economically developed areas, and in locations with higher ethnic diversity. In contrast, the frequency of lethal attacks tends to be higher in more economically developed areas, close to large cities, and within democratic countries.
Thu, 07 Dec 2017 00:00:00 GMThttps://hdl.handle.net/10023/120672017-12-07T00:00:00ZPython, AndréTo this day, terrorism perpetrated by non-state actors persists as a worldwide threat, as exemplified by the recent lethal attacks in Paris, London, Brussels, and the ongoing massacres perpetrated by the Islamic State in Iraq, Syria and neighbouring countries. In response, states deploy various counterterrorism policies, the costs of which could be reduced through more efficient preventive measures. The literature has not applied statistical models able to account for complex spatio-temporal dependencies, despite their potential for explaining and preventing non-state terrorism at the sub-national level. In an effort to address this shortcoming, this thesis employs Bayesian hierarchical models, where the spatial random field is represented by a stochastic partial differential equation. The results show that lethal terrorist attacks perpetrated by non-state actors tend to be concentrated in areas located within failed states from which they may diffuse locally, towards neighbouring areas. At the sub-national level, the propensity of attacks to be lethal and the frequency of lethal attacks appear to be driven by antagonistic mechanisms. Attacks are more likely to be lethal far away from large cities, at higher altitudes, in less economically developed areas, and in locations with higher ethnic diversity. In contrast, the frequency of lethal attacks tends to be higher in more economically developed areas, close to large cities, and within democratic countries.Counting subwords and other results related to the generalised star-height problem for regular languages
https://hdl.handle.net/10023/12024
The Generalised Star-Height Problem is an open question in the field of formal language theory that concerns a measure of complexity on the class of regular languages; specifically, it asks whether or not there exists an algorithm to determine the generalised star-height of a given regular language. Rather surprisingly, it is not yet known whether there exists a regular language of generalised star-height greater than one.
Motivated by a theorem of Thérien, we first take a combinatorial approach to the problem and consider the languages in which every word features a fixed contiguous subword an exact number of times. We show that these languages are all of generalised star-height zero. Similarly, we consider the languages in which every word features a fixed contiguous subword a prescribed number of times modulo a fixed number and show that these languages are all of generalised star-height at most one.
Using these combinatorial results, we initiate work on identifying the generalised star-height of the languages that are recognised by finite semigroups. To do this, we establish the generalised star-height of languages recognised by Rees zero-matrix semigroups over nilpotent groups of classes zero and one before considering Rees zero-matrix semigroups over monogenic semigroups.
Finally, we explore the generalised star-height of languages recognised by finite groups of a given order. We do this through the use of finite state automata and 'count arrows' to examine semidirect products of the form 𝐴 ⋊ ℤ[sub]𝑟, where 𝐴 is an abelian group and ℤ[sub]𝑟 is the cyclic group of order 𝑟.
Thu, 07 Dec 2017 00:00:00 GMThttps://hdl.handle.net/10023/120242017-12-07T00:00:00ZBourne, ThomasThe Generalised Star-Height Problem is an open question in the field of formal language theory that concerns a measure of complexity on the class of regular languages; specifically, it asks whether or not there exists an algorithm to determine the generalised star-height of a given regular language. Rather surprisingly, it is not yet known whether there exists a regular language of generalised star-height greater than one.
Motivated by a theorem of Thérien, we first take a combinatorial approach to the problem and consider the languages in which every word features a fixed contiguous subword an exact number of times. We show that these languages are all of generalised star-height zero. Similarly, we consider the languages in which every word features a fixed contiguous subword a prescribed number of times modulo a fixed number and show that these languages are all of generalised star-height at most one.
Using these combinatorial results, we initiate work on identifying the generalised star-height of the languages that are recognised by finite semigroups. To do this, we establish the generalised star-height of languages recognised by Rees zero-matrix semigroups over nilpotent groups of classes zero and one before considering Rees zero-matrix semigroups over monogenic semigroups.
Finally, we explore the generalised star-height of languages recognised by finite groups of a given order. We do this through the use of finite state automata and 'count arrows' to examine semidirect products of the form 𝐴 ⋊ ℤ[sub]𝑟, where 𝐴 is an abelian group and ℤ[sub]𝑟 is the cyclic group of order 𝑟.Modelling complex dependencies inherent in spatial and spatio-temporal point pattern data
https://hdl.handle.net/10023/12009
Point processes are mechanisms that beget point patterns. Realisations of point processes are observed in many contexts, for example, locations of stars in the sky, or locations of trees in a forest. Inferring the mechanisms that drive point processes relies on the development of models that appropriately account for the dependencies inherent in the data. Fitting models that adequately capture the complex dependency structures in either space, time, or both is often problematic. This is commonly due to—but not restricted to—the intractability of the likelihood function, or computational burden of the required numerical operations.
This thesis primarily focuses on developing point process models with some hierarchical structure, and specifically where this is a latent structure that may be considered as one of the following: (i) some unobserved construct assumed to be generating the observed structure, or (ii) some stochastic process describing the structure of the point pattern. Model fitting procedures utilised in this thesis include either (i) approximate-likelihood techniques to circumvent intractable likelihoods, (ii) stochastic partial differential equations to model continuous spatial latent structures, or (iii) improving computational speed in numerical approximations by exploiting automatic differentiation.
Moreover, this thesis extends classic point process models by considering multivariate dependencies. This is achieved through considering a general class of joint point process model, which utilise shared stochastic structures. These structures account for the dependencies inherent in multivariate point process data. These models are applied to data originating from various scientific fields; in particular, applications are considered in ecology, medicine, and geology. In addition, point process models that account for the second order behaviour of these assumed stochastic structures are also considered.
Fri, 23 Jun 2017 00:00:00 GMThttps://hdl.handle.net/10023/120092017-06-23T00:00:00ZJones-Todd, Charlotte M.Point processes are mechanisms that beget point patterns. Realisations of point processes are observed in many contexts, for example, locations of stars in the sky, or locations of trees in a forest. Inferring the mechanisms that drive point processes relies on the development of models that appropriately account for the dependencies inherent in the data. Fitting models that adequately capture the complex dependency structures in either space, time, or both is often problematic. This is commonly due to—but not restricted to—the intractability of the likelihood function, or computational burden of the required numerical operations.
This thesis primarily focuses on developing point process models with some hierarchical structure, and specifically where this is a latent structure that may be considered as one of the following: (i) some unobserved construct assumed to be generating the observed structure, or (ii) some stochastic process describing the structure of the point pattern. Model fitting procedures utilised in this thesis include either (i) approximate-likelihood techniques to circumvent intractable likelihoods, (ii) stochastic partial differential equations to model continuous spatial latent structures, or (iii) improving computational speed in numerical approximations by exploiting automatic differentiation.
Moreover, this thesis extends classic point process models by considering multivariate dependencies. This is achieved through considering a general class of joint point process model, which utilise shared stochastic structures. These structures account for the dependencies inherent in multivariate point process data. These models are applied to data originating from various scientific fields; in particular, applications are considered in ecology, medicine, and geology. In addition, point process models that account for the second order behaviour of these assumed stochastic structures are also considered.New aspects of particle acceleration in collapsing magnetic traps
https://hdl.handle.net/10023/11954
Collapsing magnetic traps (CMTs) have been suggested as one of the mechanisms that could contribute to particle energisation in solar flares. The basic idea behind CMTs is that charged particles will be trapped on the magnetic field lines below the reconnection region of a flare. This thesis discusses a number of important new aspects in particle energisation processes in CMTs, based on the model by Giuliani et al. (2005). In particular, we extend previous studies of particle acceleration in this CMT model to the relativistic regime and compare our results obtained using relativistic guiding centre theory with results obtained using the non-relativistic guiding centre theory. The similarities and differences found are discussed. We then present a detailed study of the question, what leads to the trapping or escape of particle orbits from CMTs. The answer to this question is investigated by using results from the non-relativistic orbit calculations with guiding centre theory and a number of simple models for particle energy gain in CMTs. We find that there is a critical pitch angle dividing trapped particle orbits from the escaping particle orbits and that this critical pitch angle does not coincide with the initial loss cone angle. Furthermore, we also present a calculation of the time evolution of an anisotropic pressure tensor and of the plasma density under the assumptions that they evolve in line with our kinematic MHD CMT model and that the pressure tensor satisfies the double-adiabatic Chew-Goldburger-Low (CGL) theory.
Finally, we make a first step to introduce Coulomb scattering by a Maxwellian background plasma into our guiding centre equations by changing them into a set of stochastic differential equations. We study the influence of a static background plasma onto selected particle orbits by pitch angle scattering and energy losses, and look at its effect on the particle energy and the trapping conditions.
Wed, 01 Jan 2014 00:00:00 GMThttps://hdl.handle.net/10023/119542014-01-01T00:00:00ZEradat Oskoui, SolmazCollapsing magnetic traps (CMTs) have been suggested as one of the mechanisms that could contribute to particle energisation in solar flares. The basic idea behind CMTs is that charged particles will be trapped on the magnetic field lines below the reconnection region of a flare. This thesis discusses a number of important new aspects in particle energisation processes in CMTs, based on the model by Giuliani et al. (2005). In particular, we extend previous studies of particle acceleration in this CMT model to the relativistic regime and compare our results obtained using relativistic guiding centre theory with results obtained using the non-relativistic guiding centre theory. The similarities and differences found are discussed. We then present a detailed study of the question, what leads to the trapping or escape of particle orbits from CMTs. The answer to this question is investigated by using results from the non-relativistic orbit calculations with guiding centre theory and a number of simple models for particle energy gain in CMTs. We find that there is a critical pitch angle dividing trapped particle orbits from the escaping particle orbits and that this critical pitch angle does not coincide with the initial loss cone angle. Furthermore, we also present a calculation of the time evolution of an anisotropic pressure tensor and of the plasma density under the assumptions that they evolve in line with our kinematic MHD CMT model and that the pressure tensor satisfies the double-adiabatic Chew-Goldburger-Low (CGL) theory.
Finally, we make a first step to introduce Coulomb scattering by a Maxwellian background plasma into our guiding centre equations by changing them into a set of stochastic differential equations. We study the influence of a static background plasma onto selected particle orbits by pitch angle scattering and energy losses, and look at its effect on the particle energy and the trapping conditions.Theory of one-dimensional Vlasov-Maxwell equilibria: with applications to collisionless current sheets and flux tubes
https://hdl.handle.net/10023/11916
Vlasov-Maxwell equilibria are characterised by the self-consistent descriptions of the steady-states of collisionless plasmas in particle phase-space, and balanced macroscopic forces. We study the theory of Vlasov-Maxwell equilibria in one spatial dimension, as well as its application to current sheet and flux tube models.
The ‘inverse problem’ is that of determining a Vlasov-Maxwell equilibrium distribution function self-consistent with a given magnetic field. We develop the theory of inversion using expansions in Hermite polynomial functions of the canonical momenta. Sufficient conditions for the convergence of a Hermite expansion are found, given a pressure tensor. For large classes of DFs, we prove that non-negativity of the distribution function is contingent on the magnetisation
of the plasma, and make conjectures for all classes.
The inverse problem is considered for nonlinear ‘force-free Harris sheets’. By applying the Hermite method, we construct new models that can describe sub-unity values of the plasma beta (βpl) for the first time. Whilst analytical convergence is proven for all βpl, numerical convergence is attained for βpl = 0.85, and then βpl = 0.05 after a ‘re-gauging’ process.
We consider the properties that a pressure tensor must satisfy to be consistent with ‘asymmetric Harris sheets’, and construct new examples. It is possible to analytically solve the inverse problem in some cases, but others must be tackled numerically. We present new exact Vlasov-Maxwell equilibria for asymmetric current sheets, which can be written as a sum of shifted Maxwellian distributions. This is ideal for implementations in particle-in-cell simulations.
We study the correspondence between the microscopic and macroscopic descriptions of equilibrium in cylindrical geometry, and then attempt to find Vlasov-Maxwell equilibria for the nonlinear force-free ‘Gold-Hoyle’ model. However, it is necessary to include a background field, which can be arbitrarily weak if desired. The equilibrium can be electrically non-neutral, depending on the bulk flows.
Thu, 07 Dec 2017 00:00:00 GMThttps://hdl.handle.net/10023/119162017-12-07T00:00:00ZAllanson, Oliver DouglasVlasov-Maxwell equilibria are characterised by the self-consistent descriptions of the steady-states of collisionless plasmas in particle phase-space, and balanced macroscopic forces. We study the theory of Vlasov-Maxwell equilibria in one spatial dimension, as well as its application to current sheet and flux tube models.
The ‘inverse problem’ is that of determining a Vlasov-Maxwell equilibrium distribution function self-consistent with a given magnetic field. We develop the theory of inversion using expansions in Hermite polynomial functions of the canonical momenta. Sufficient conditions for the convergence of a Hermite expansion are found, given a pressure tensor. For large classes of DFs, we prove that non-negativity of the distribution function is contingent on the magnetisation
of the plasma, and make conjectures for all classes.
The inverse problem is considered for nonlinear ‘force-free Harris sheets’. By applying the Hermite method, we construct new models that can describe sub-unity values of the plasma beta (βpl) for the first time. Whilst analytical convergence is proven for all βpl, numerical convergence is attained for βpl = 0.85, and then βpl = 0.05 after a ‘re-gauging’ process.
We consider the properties that a pressure tensor must satisfy to be consistent with ‘asymmetric Harris sheets’, and construct new examples. It is possible to analytically solve the inverse problem in some cases, but others must be tackled numerically. We present new exact Vlasov-Maxwell equilibria for asymmetric current sheets, which can be written as a sum of shifted Maxwellian distributions. This is ideal for implementations in particle-in-cell simulations.
We study the correspondence between the microscopic and macroscopic descriptions of equilibrium in cylindrical geometry, and then attempt to find Vlasov-Maxwell equilibria for the nonlinear force-free ‘Gold-Hoyle’ model. However, it is necessary to include a background field, which can be arbitrarily weak if desired. The equilibrium can be electrically non-neutral, depending on the bulk flows.Two variants of the froidure-pin algorithm for finite semigroups
https://hdl.handle.net/10023/11879
In this paper, we present two algorithms based on the Froidure-Pin Algorithm for computing the structure of a finite semigroup from a generating set. As was the case with the original algorithm of Froidure and Pin, the algorithms presented here produce the left and right Cayley graphs, a confluent terminating rewriting system, and a reduced word of the rewriting system for every element of the semigroup. If U is any semigroup, and A is a subset of U, then we denote by <A> the least subsemigroup of U containing A. If B is any other subset of U, then, roughly speaking, the first algorithm we present describes how to use any information about <A>, that has been found using the Froidure-Pin Algorithm, to compute the semigroup <A∪B>. More precisely, we describe the data structure for a finite semigroup S given by Froidure and Pin, and how to obtain such a data structure for <A∪B> from that for <A>. The second algorithm is a lock-free concurrent version of the Froidure-Pin Algorithm.
Thu, 08 Feb 2018 00:00:00 GMThttps://hdl.handle.net/10023/118792018-02-08T00:00:00ZJonusas, JuliusMitchell, J. D.Pfeiffer, M.In this paper, we present two algorithms based on the Froidure-Pin Algorithm for computing the structure of a finite semigroup from a generating set. As was the case with the original algorithm of Froidure and Pin, the algorithms presented here produce the left and right Cayley graphs, a confluent terminating rewriting system, and a reduced word of the rewriting system for every element of the semigroup. If U is any semigroup, and A is a subset of U, then we denote by <A> the least subsemigroup of U containing A. If B is any other subset of U, then, roughly speaking, the first algorithm we present describes how to use any information about <A>, that has been found using the Froidure-Pin Algorithm, to compute the semigroup <A∪B>. More precisely, we describe the data structure for a finite semigroup S given by Froidure and Pin, and how to obtain such a data structure for <A∪B> from that for <A>. The second algorithm is a lock-free concurrent version of the Froidure-Pin Algorithm.Parameter redundancy in log-linear models
https://hdl.handle.net/10023/11739
Log-linear models are widely used to analyse categorical variables arranged in a contingency
table. Sampling zero entries in the table can cause the problem of large standard
errors for some model parameter estimates. This thesis focuses on the reason of this
problem and suggests a solution by utilising the parameter redundancy approach. This
approach detects whether a model is non-identifiable and parameter redundant, and
specifies a smaller set of parameters or combinations of them that all are estimable. The
parameter redundancy method is adapted here for Poisson log-linear models which are
parameter redundant because of the number and pattern of the zero observations in the
contingency table. Furthermore, it is shown that in some parameter redundant log-linear
models, the presence of constraints referred to as esoteric constraints can make more
parameters estimable. It is proven in a theorem that for a saturated Poisson log-linear
model fitted to an lm table with one zero cell count, which model parameters are not
estimable. Three examples of real data in sparse contingency tables are presented to
demonstrate the process of identifying the estimable parameters and reducing the model.
An alternative approach is the existence of the MLE method that checks for the
existence of the maximum likelihood estimates of the cell means in the log-linear
model after observing the zero entries. The method considers the log-linear model as
a polyhedral cone and provides conditions to detect the estimability of the cell means.
This method is compared here with the parameter redundancy approach and their
similarities and differences are explained and illustrated using examples. In parameter
redundant models with existent MLE, it is observed that the presence of the esoteric
constraints makes all the parameters estimable.
Thu, 07 Dec 2017 00:00:00 GMThttps://hdl.handle.net/10023/117392017-12-07T00:00:00ZSharifi Far, ServehLog-linear models are widely used to analyse categorical variables arranged in a contingency
table. Sampling zero entries in the table can cause the problem of large standard
errors for some model parameter estimates. This thesis focuses on the reason of this
problem and suggests a solution by utilising the parameter redundancy approach. This
approach detects whether a model is non-identifiable and parameter redundant, and
specifies a smaller set of parameters or combinations of them that all are estimable. The
parameter redundancy method is adapted here for Poisson log-linear models which are
parameter redundant because of the number and pattern of the zero observations in the
contingency table. Furthermore, it is shown that in some parameter redundant log-linear
models, the presence of constraints referred to as esoteric constraints can make more
parameters estimable. It is proven in a theorem that for a saturated Poisson log-linear
model fitted to an lm table with one zero cell count, which model parameters are not
estimable. Three examples of real data in sparse contingency tables are presented to
demonstrate the process of identifying the estimable parameters and reducing the model.
An alternative approach is the existence of the MLE method that checks for the
existence of the maximum likelihood estimates of the cell means in the log-linear
model after observing the zero entries. The method considers the log-linear model as
a polyhedral cone and provides conditions to detect the estimability of the cell means.
This method is compared here with the parameter redundancy approach and their
similarities and differences are explained and illustrated using examples. In parameter
redundant models with existent MLE, it is observed that the presence of the esoteric
constraints makes all the parameters estimable.Wave of chaos in a spatial eco-epidemiological system : generating realistic patterns of patchiness in rabbit-lynx dynamics
https://hdl.handle.net/10023/11666
In the present paper, we propose and analyse an eco-epidemiological model with diffusion to study the dynamics of rabbit populations which are consumed by lynx populations. Existence, boundedness, stability and bifurcation analyses of solutions for the proposed rabbit-lynx model are performed. Results show that in the presence of diffusion the model has the potential of exhibiting Turing instability. Numerical results (finite difference and finite element methods) reveal the existence of the wave of chaos and this appears to be a dominant mode of disease dispersal. We also show the mechanism of spatiotemporal pattern formation resulting from the Hopf bifurcation analysis, which can be a potential candidate for understanding the complex spatiotemporal dynamics of eco-epidemiological systems. Implications of the asymptotic transmission rate on disease eradication among rabbit population which in turn enhances the survival of Iberian lynx are discussed.
AM and CV would like to acknowledge support from the Engineering and Physical Sciences Research Council grant (EP/J016780/1) and the Leverhulme Trust Research Project Grant (RPG-2014-149).
Tue, 01 Nov 2016 00:00:00 GMThttps://hdl.handle.net/10023/116662016-11-01T00:00:00ZUpadhyay, RanjitRoy, ParimitaVenkataraman, C.Madzvamuse, AnotidaIn the present paper, we propose and analyse an eco-epidemiological model with diffusion to study the dynamics of rabbit populations which are consumed by lynx populations. Existence, boundedness, stability and bifurcation analyses of solutions for the proposed rabbit-lynx model are performed. Results show that in the presence of diffusion the model has the potential of exhibiting Turing instability. Numerical results (finite difference and finite element methods) reveal the existence of the wave of chaos and this appears to be a dominant mode of disease dispersal. We also show the mechanism of spatiotemporal pattern formation resulting from the Hopf bifurcation analysis, which can be a potential candidate for understanding the complex spatiotemporal dynamics of eco-epidemiological systems. Implications of the asymptotic transmission rate on disease eradication among rabbit population which in turn enhances the survival of Iberian lynx are discussed.Spatial variation in boundary conditions can govern selection and location of eyespots in butterfly wings
https://hdl.handle.net/10023/11618
Despite being the subject of widespread study, many aspects of the development of eyespot patterns in butterfly wings remain poorly understood. In this work, we examine, through numerical simulations, a mathematical model for eyespot focus point formation in which a reaction-diffusion system is assumed to play the role of the patterning mechanism. In the model, changes in the boundary conditions at the veins at the proximal boundary alone are capable of determining whether or not an eyespot focus forms in a given wing cell and the eventual position of focus points within the wing cell. Furthermore, an auxiliary surface reaction diffusion system posed along the entire proximal boundary of the wing cells is proposed as the mechanism that generates the necessary changes in the proximal boundary profiles. In order to illustrate the robustness of the model, we perform simulations on a curved wing geometry that is somewhat closer to a biological realistic domain than the rectangular wing cells previously considered, and we also illustrate the ability of the model to reproduce experimental results on artificial selection of eyespots.
Sun, 01 Jan 2017 00:00:00 GMThttps://hdl.handle.net/10023/116182017-01-01T00:00:00ZVenkataraman, ChandrasekharSekimura, ToshioDespite being the subject of widespread study, many aspects of the development of eyespot patterns in butterfly wings remain poorly understood. In this work, we examine, through numerical simulations, a mathematical model for eyespot focus point formation in which a reaction-diffusion system is assumed to play the role of the patterning mechanism. In the model, changes in the boundary conditions at the veins at the proximal boundary alone are capable of determining whether or not an eyespot focus forms in a given wing cell and the eventual position of focus points within the wing cell. Furthermore, an auxiliary surface reaction diffusion system posed along the entire proximal boundary of the wing cells is proposed as the mechanism that generates the necessary changes in the proximal boundary profiles. In order to illustrate the robustness of the model, we perform simulations on a curved wing geometry that is somewhat closer to a biological realistic domain than the rectangular wing cells previously considered, and we also illustrate the ability of the model to reproduce experimental results on artificial selection of eyespots.Erwin Schrödinger and quantum wave mechanics
https://hdl.handle.net/10023/11543
The fathers of matrix quantum mechanics believed that the quantum particles are unanschaulich (unvisualizable) and that quantum particles pop into existence only when we measure them. Challenging the orthodoxy, in 1926 Erwin Schrödinger developed his wave equation that describes the quantum particles as a packet of quantum probability amplitudes evolving in space and time. Thus, Schrödinger visualized the unvisualizable and lifted the veil that has been obscuring the wonders of the quantum world.
Tue, 22 Aug 2017 00:00:00 GMThttps://hdl.handle.net/10023/115432017-08-22T00:00:00ZO'Connor, John J.Robertson, Edmund F.The fathers of matrix quantum mechanics believed that the quantum particles are unanschaulich (unvisualizable) and that quantum particles pop into existence only when we measure them. Challenging the orthodoxy, in 1926 Erwin Schrödinger developed his wave equation that describes the quantum particles as a packet of quantum probability amplitudes evolving in space and time. Thus, Schrödinger visualized the unvisualizable and lifted the veil that has been obscuring the wonders of the quantum world.Epigenetic and oncogenic regulation of SLC16A7 (MCT2) results in protein over-expression, impacting on signalling and cellular phenotypes in prostate cancer
https://hdl.handle.net/10023/11445
Monocarboxylate Transporter 2 (MCT2) is a major pyruvate transporter encoded by the SLC16A7 gene. Recent studies pointed to a consistent overexpression of MCT2 in prostate cancer (PCa) suggesting MCT2 as a putative biomarker and molecular target. Despite the importance of this observation the mechanisms involved in MCT2 regulation are unknown. Through an integrative analysis we have discovered that selective demethylation of an internal SLC16A7/MCT2 promoter is a recurrent event in independent PCa cohorts. This demethylation is associated with expression of isoforms differing only in 5'-UTR translational control motifs, providing one contributing mechanism for MCT2 protein overexpression in PCa. Genes co-expressed with SLC16A7/MCT2 also clustered in oncogenic-related pathways and effectors of these signalling pathways were found to bind at the SLC16A7/MCT2 gene locus. Finally, MCT2 knock-down attenuated the growth of PCa cells. The present study unveils an unexpected epigenetic regulation of SLC16A7/MCT2 isoforms and identifies a link between SLC16A7/MCT2, Androgen Receptor (AR), ETS-related gene (ERG) and other oncogenic pathways in PCa. These results underscore the importance of combining data from epigenetic, transcriptomic and protein level changes to allow more comprehensive insights into the mechanisms underlying protein expression, that in our case provide additional weight to MCT2 as a candidate biomarker and molecular target in PCa.
Felisbino S. received a fellowship from the Sao Paulo Research Foundation (FAPESP) ref. 2013/08830-2 and 2013/06802-1. Anne Y Warren research time funded by: Cambridge Biomedical Research Centre.
Tue, 02 Jun 2015 00:00:00 GMThttps://hdl.handle.net/10023/114452015-06-02T00:00:00ZPértega-Gomes, NelmaVizcaino, Jose R.Felisbino, SergioWarren, Anne Y.Shaw, GregKay, JonathanWhitaker, HayleyLynch, Andy G.Fryer, LeeNeal, David E.Massie, Charles E.Monocarboxylate Transporter 2 (MCT2) is a major pyruvate transporter encoded by the SLC16A7 gene. Recent studies pointed to a consistent overexpression of MCT2 in prostate cancer (PCa) suggesting MCT2 as a putative biomarker and molecular target. Despite the importance of this observation the mechanisms involved in MCT2 regulation are unknown. Through an integrative analysis we have discovered that selective demethylation of an internal SLC16A7/MCT2 promoter is a recurrent event in independent PCa cohorts. This demethylation is associated with expression of isoforms differing only in 5'-UTR translational control motifs, providing one contributing mechanism for MCT2 protein overexpression in PCa. Genes co-expressed with SLC16A7/MCT2 also clustered in oncogenic-related pathways and effectors of these signalling pathways were found to bind at the SLC16A7/MCT2 gene locus. Finally, MCT2 knock-down attenuated the growth of PCa cells. The present study unveils an unexpected epigenetic regulation of SLC16A7/MCT2 isoforms and identifies a link between SLC16A7/MCT2, Androgen Receptor (AR), ETS-related gene (ERG) and other oncogenic pathways in PCa. These results underscore the importance of combining data from epigenetic, transcriptomic and protein level changes to allow more comprehensive insights into the mechanisms underlying protein expression, that in our case provide additional weight to MCT2 as a candidate biomarker and molecular target in PCa.Fractal, group theoretic, and relational structures on Cantor space
https://hdl.handle.net/10023/11370
Cantor space, the set of infinite words over a finite alphabet, is a type of metric space
with a `self-similar' structure. This thesis explores three areas concerning Cantor space
with regard to fractal geometry, group theory, and topology.
We find first results on the dimension of intersections of fractal sets within the Cantor
space. More specifically, we examine the intersection of a subset E of the n-ary Cantor
space, C[sub]n with the image of another subset Funder a random isometry. We obtain
almost sure upper bounds for the Hausdorff and upper box-counting dimensions of the
intersection, and a lower bound for the essential supremum of the Hausdorff dimension.
We then consider a class of groups, denoted by V[sub]n(G), of homeomorphisms of the
Cantor space built from transducers. These groups can be seen as homeomorphisms
that respect the self-similar and symmetric structure of C[sub]n, and are supergroups of the
Higman-Thompson groups V[sub]n. We explore their isomorphism classes with our primary
result being that V[sub]n(G) is isomorphic to (and conjugate to) V[sub]n if and only if G is a
semiregular subgroup of the symmetric group on n points.
Lastly, we explore invariant relations on Cantor space, which have quotients homeomorphic to fractals in many different classes. We generalize a method of describing these
quotients by invariant relations as an inverse limit, before characterizing a specific class
of fractals known as Sierpiński relatives as invariant factors. We then compare relations
arising through edge replacement systems to invariant relations, detailing the conditions
under which they are the same.
Fri, 01 Jan 2016 00:00:00 GMThttps://hdl.handle.net/10023/113702016-01-01T00:00:00ZDonoven, Casey RyallCantor space, the set of infinite words over a finite alphabet, is a type of metric space
with a `self-similar' structure. This thesis explores three areas concerning Cantor space
with regard to fractal geometry, group theory, and topology.
We find first results on the dimension of intersections of fractal sets within the Cantor
space. More specifically, we examine the intersection of a subset E of the n-ary Cantor
space, C[sub]n with the image of another subset Funder a random isometry. We obtain
almost sure upper bounds for the Hausdorff and upper box-counting dimensions of the
intersection, and a lower bound for the essential supremum of the Hausdorff dimension.
We then consider a class of groups, denoted by V[sub]n(G), of homeomorphisms of the
Cantor space built from transducers. These groups can be seen as homeomorphisms
that respect the self-similar and symmetric structure of C[sub]n, and are supergroups of the
Higman-Thompson groups V[sub]n. We explore their isomorphism classes with our primary
result being that V[sub]n(G) is isomorphic to (and conjugate to) V[sub]n if and only if G is a
semiregular subgroup of the symmetric group on n points.
Lastly, we explore invariant relations on Cantor space, which have quotients homeomorphic to fractals in many different classes. We generalize a method of describing these
quotients by invariant relations as an inverse limit, before characterizing a specific class
of fractals known as Sierpiński relatives as invariant factors. We then compare relations
arising through edge replacement systems to invariant relations, detailing the conditions
under which they are the same.Constructing 2-generated subgroups of the group of homeomorphisms of Cantor space
https://hdl.handle.net/10023/11362
We study finite generation, 2-generation and simplicity of subgroups of H[sub]c, the
group of homeomorphisms of Cantor space.
In Chapter 1 and Chapter 2 we run through foundational concepts and notation. In Chapter 3 we study vigorous subgroups of H[sub]c. A subgroup G of H[sub]c is vigorous if for any non-empty clopen set A with proper non-empty clopen subsets B and C there exists g ∈ G with supp(g) ⊑ A and Bg ⊆ C. It is a corollary of the main theorem of Chapter 3 that all finitely generated simple vigorous subgroups of H[sub]c are in fact 2-generated. We show the family of finitely generated, simple, vigorous subgroups of H[sub]c is closed under several natural constructions.
In Chapter 4 we use a generalised notion of word and the tight completion construction from [13] to construct a family of subgroups of H[sub]c which generalise Thompson's group V . We give necessary conditions for these groups to be finitely generated and simple. Of these we show which are vigorous. Finally we give some examples.
Sun, 01 Jan 2017 00:00:00 GMThttps://hdl.handle.net/10023/113622017-01-01T00:00:00ZHyde, James ThomasWe study finite generation, 2-generation and simplicity of subgroups of H[sub]c, the
group of homeomorphisms of Cantor space.
In Chapter 1 and Chapter 2 we run through foundational concepts and notation. In Chapter 3 we study vigorous subgroups of H[sub]c. A subgroup G of H[sub]c is vigorous if for any non-empty clopen set A with proper non-empty clopen subsets B and C there exists g ∈ G with supp(g) ⊑ A and Bg ⊆ C. It is a corollary of the main theorem of Chapter 3 that all finitely generated simple vigorous subgroups of H[sub]c are in fact 2-generated. We show the family of finitely generated, simple, vigorous subgroups of H[sub]c is closed under several natural constructions.
In Chapter 4 we use a generalised notion of word and the tight completion construction from [13] to construct a family of subgroups of H[sub]c which generalise Thompson's group V . We give necessary conditions for these groups to be finitely generated and simple. Of these we show which are vigorous. Finally we give some examples.Extremal problems in combinatorial semigroup theory
https://hdl.handle.net/10023/11322
In this thesis we shall consider three types of extremal problems (i.e. problems involving maxima and minima) concerning semigroups. In the first chapter we show how to construct a minimal semigroup presentation that defines a group of non-negative deficiency given a minimal group presentation for that group. This demonstrates that the semigroup deficiency of a group of non-negative deficiency is equal to the group deficiency of that group. Given a finite monoid we find a necessary and sufficient condition for the monoid deficiency to equal the semigroup deficiency. We give a class of infinite monoids for which this equality also holds. The second type of problem we consider concerns infinite semigroups of relations and transformations. We find the relative rank of the full transformation semigroup, over an infinite set, modulo some standard subsets and subsemigroups, including the set of contraction maps and the set of order preserving maps (for some infinite ordered sets). We also find the relative rank of the semigroup of all binary relations (over an infinite set) modulo the partial transformation semigroup, the full transformation semigroup, the symmetric inverse semigroup, the symmetric group and the set of idempotent relations. Analogous results are also proven for the symmetric inverse semigroup. The third, and final, type of problem studied concerns generalising notions of independence from linear algebra to semigroups and groups. We determine the maximum cardinality of an independent set in finite abelian groups, Brandt semigroups, free nilpotent semigroups, and some examples of infinite groups.
Mon, 01 Jul 2002 00:00:00 GMThttps://hdl.handle.net/10023/113222002-07-01T00:00:00ZMitchell, James DavidIn this thesis we shall consider three types of extremal problems (i.e. problems involving maxima and minima) concerning semigroups. In the first chapter we show how to construct a minimal semigroup presentation that defines a group of non-negative deficiency given a minimal group presentation for that group. This demonstrates that the semigroup deficiency of a group of non-negative deficiency is equal to the group deficiency of that group. Given a finite monoid we find a necessary and sufficient condition for the monoid deficiency to equal the semigroup deficiency. We give a class of infinite monoids for which this equality also holds. The second type of problem we consider concerns infinite semigroups of relations and transformations. We find the relative rank of the full transformation semigroup, over an infinite set, modulo some standard subsets and subsemigroups, including the set of contraction maps and the set of order preserving maps (for some infinite ordered sets). We also find the relative rank of the semigroup of all binary relations (over an infinite set) modulo the partial transformation semigroup, the full transformation semigroup, the symmetric inverse semigroup, the symmetric group and the set of idempotent relations. Analogous results are also proven for the symmetric inverse semigroup. The third, and final, type of problem studied concerns generalising notions of independence from linear algebra to semigroups and groups. We determine the maximum cardinality of an independent set in finite abelian groups, Brandt semigroups, free nilpotent semigroups, and some examples of infinite groups.Optimized automated survey design in wildlife population assessment
https://hdl.handle.net/10023/11318
Increased pressure on the environment has placed numerous ecological populations under threat of extinction. Management schemes dedicated to the future conservation of wildlife populations rely on effective monitoring of the size of those populations. This requires that accurate and precise abundance estimates are obtained for the purposes of wildlife population assessment. The accuracy and precision of estimates are determined to a large extent by the survey design used to obtain population samples. Methods for optimizing the survey design process are detailed, with a particular- focus on automating the sui-vey designs using computer software. The technique of automated survey design is a simulation-based tool, which provides the means to assess the properties of any type of survey design, permits the evaluation of abundance estimates over sui-vey regions with assumed population densities, and from a practical standpoint facilitates the creation of a survey plan that can be implemented in the field. Survey design properties include the probability of a particular location being included in the sample, the spatial distribution of the sampling locations within the survey region, and the distances covered by observers to obtain the sample data. The design properties are directly linked to the accuracy and precision of estimates, as well as the efficiency, achieved by a type of design. A comparative study of a number of different survey designs that can be broadly classified as systematic or non-systematic is presented. The simulation results show their performance with regard to the above-mentioned properties and the abundance estimates obtained if the designs are applied to some known population densities. Due to the more even spatial distribution of the systematic designs the estimates they produce are potentially more precise and the distances covered by observers less variable as well. It is also shown how biased estimates can result if the probability of a particular location being included in the sample is assumed to be even over the entire survey region when it is not. The problems associated with surveying along the boundary of a survey region and within non-convex regions are addressed. The methods are illustrated with a number of survey design examples.
Tue, 01 May 2001 00:00:00 GMThttps://hdl.handle.net/10023/113182001-05-01T00:00:00ZStrindberg, SamanthaIncreased pressure on the environment has placed numerous ecological populations under threat of extinction. Management schemes dedicated to the future conservation of wildlife populations rely on effective monitoring of the size of those populations. This requires that accurate and precise abundance estimates are obtained for the purposes of wildlife population assessment. The accuracy and precision of estimates are determined to a large extent by the survey design used to obtain population samples. Methods for optimizing the survey design process are detailed, with a particular- focus on automating the sui-vey designs using computer software. The technique of automated survey design is a simulation-based tool, which provides the means to assess the properties of any type of survey design, permits the evaluation of abundance estimates over sui-vey regions with assumed population densities, and from a practical standpoint facilitates the creation of a survey plan that can be implemented in the field. Survey design properties include the probability of a particular location being included in the sample, the spatial distribution of the sampling locations within the survey region, and the distances covered by observers to obtain the sample data. The design properties are directly linked to the accuracy and precision of estimates, as well as the efficiency, achieved by a type of design. A comparative study of a number of different survey designs that can be broadly classified as systematic or non-systematic is presented. The simulation results show their performance with regard to the above-mentioned properties and the abundance estimates obtained if the designs are applied to some known population densities. Due to the more even spatial distribution of the systematic designs the estimates they produce are potentially more precise and the distances covered by observers less variable as well. It is also shown how biased estimates can result if the probability of a particular location being included in the sample is assumed to be even over the entire survey region when it is not. The problems associated with surveying along the boundary of a survey region and within non-convex regions are addressed. The methods are illustrated with a number of survey design examples.Helioseismology and diagnostics of internal magnetic layers
https://hdl.handle.net/10023/11315
Solar magnetic fields, as well as temperature changes, introduce pressure deviations that play a significant role in modulating the resonant frequencies of p-mode oscillations. Those pressure deviations occurring in the atmosphere or sub-surface of the Sun can explain the frequency shifts observed on the timescale of the solar activity cycle. A separate study of the contribution of internal magnetic layers can clarify the relative importance of surface effects. Results from helioseismology provide realistic constraints for choosing parameters suitable to represent the magnetic layers buried in the solar interior and available for modelling, i.e. at the base of the convection zone and in the sunspots’ anchoring zone. Diagnostics of the internal magnetic layers are obtained through a schematic model in which the Sun is plane-stratified. The influence of a buried magnetic field on p-modes is explored, and the nature of various waves and instabilities that can arise on such a buried magnetic field is assessed. By treating the effects of internal magnetic layers, this thesis contributes to the building of a bridge between theories and observations. On the one hand, the theoretical analysis is explored carefully in the course of its formulation, which generates new hypotheses that were not obvious so far. On the other hand, observations help to understand wh