Finiteness Properties of Locally Defined Groups

Daniel S. Farley, Bruce Hughes

Published 2020-10-15Version 1

Let $X$ be a set and let $S$ be an inverse semigroup of partial bijections of $X$. Thus, an element of $S$ is a bijection between two subsets of $X$, and the set $S$ is required to be closed under the operations of taking inverses and compositions of functions. We define $\Gamma_{S}$ to be the set of self-bijections of $X$ in which each $\gamma \in \Gamma_{S}$ is expressible as a union of finitely many members of $S$. This set is a group with respect to composition. The groups $\Gamma_{S}$ form a class containing numerous widely studied groups, such as Thompson's group $V$, the Nekrashevych-R\"{o}ver groups, Houghton's groups, and the Brin-Thompson groups $nV$, among many others. We offer a unified construction of geometric models for $\Gamma_{S}$ and a general framework for studying the finiteness properties of these groups.