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Relating Thompson's group V to graphs of groups and Hecke algebras

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Freeland, Richard 


This thesis is in two main sections, both of which feature Thompson's group V, relating it to classical constructions involving automorphism groups on trees or to representations of symmetric groups. In the first section, we take G to be a graph of groups, which acts on its universal cover, the Bass-Serre tree, by tree automorphisms. Brownlowe, Mundey, Pask, Spielberg and Thomas constructed a C-algebra for a graph of groups, writtten C∗(G), which bears many similarities to the C-algebra of a directed graph G. Inspired by the fact that directed graph C-algebras C∗(G) have algebraic analogues in Leavitt path algebras LK(G), we define a Leavitt graph-of-groups algebra LK(G) for G. We extend Leavitt path algebra results to LK(G), including uniqueness theorems describing homomorphisms out of LK(G), and establish a wider context for the algebras by showing they are Steinberg algebras of a particular '{e}tale groupoid. Finally we show that certain unitaries in LK(G) form a group we can understand as a variant of Thompson's V, combining features of both Nekrashevych-R"{o}ver groups and Matui's topological full groups of one-sided shifts. We prove finiteness and simplicity results for these Thompson variants. The latter section of this thesis turns to representation theory. We briefly state some results about representations of V (due to Dudko and Grigorchuk) which we generalize to the new family of Thompson groups, including a discussion of representations of finite factor type and Koopman representations. Then, we describe how one would try to construct a Hecke algebra for V, built from copies of the Iwahori-Hecke algebra of Sn in a way inspired by how V can be constructed from copies of the symmetric group. We survey attempts to construct this and demonstrate what we believe to be the closest possible analogue to the Sn theory. We discuss how this construction could prove useful for understanding further representation theory.





Brookes, Chris


Algebra, group theory, representation theory, Thompson's groups, Hecke algebras


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
I was sponsored by the research council EPSRC for the first 3.5 years of my thesis.