On context-free subgroups and R. Thompson's group $V$

Abstract

Consider $V_*$, the subgroup of R. Thompson's group $V$ which stabilises $0^ω$ under the natural action upon the Cantor set, $\{0, 1\}^ω$. Let $G_*$ be any subgroup of a finitely generated group $G$. We show that $G_*$ is a context-free subgroup of $G$ if and only if $G_*$ is a pullback of $V_*$ under a homomorphism $G \rightarrow V$. In particular, this shows the existence of a hardest context-free membership problem. As a consequence, we prove that a group $G$ embeds into $V$ if and only if it is the transition group of a finite union of context-free automata, or equivalently, if there exist finitely many context-free subgroups of $G$ whose cores intersect trivially.