Normalish amenable subgroups of the R. Thompson groups
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Results in C∗ algebras, of Matte Bon and Le Boudec, and of Haagerup and Olesen, apply to the R. Thompson groups F≤T≤V. These results together show that F is non-amenable if and only if T has a simple reduced C∗-algebra. In further investigations into the structure of C∗-algebras, Breuillard, Kalantar, Kennedy, and Ozawa introduce the notion of a normalish subgroup of a group G. They show that if a group G admits no non-trivial finite normal subgroups and no normalish amenable subgroups then it has a simple reduced C∗-algebra. Our chief result concerns the R. Thompson groups F<T<V; we show that there is an elementary amenable group E<F [where here, E≅…)≀Z)≀Z)≀Z] with E normalish in V. The proof given uses a natural partial action of the group V on a regular language determined by a synchronising automaton in order to verify a certain stability condition: once again highlighting the existence of interesting intersections of the theory of V with various forms of formal language theory.
Bleak , C P 2021 , ' Normalish amenable subgroups of the R. Thompson groups ' , International Journal of Foundations of Computer Science , vol. 32 , no. 06 , pp. 785-800 . https://doi.org/10.1142/s0129054121420089
International Journal of Foundations of Computer Science
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DescriptionFunding: UK Engineering and Physical Sciences Research Council (EPSRC) grant EP/R032866/1.
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