On the stabilizers of finite sets of numbers in the R. Thompson group $F$

TL;DR

This paper investigates the algebraic structure of stabilizer subgroups of finite sets in the Thompson group $F$, revealing conditions for finite generation, isomorphisms, and conjugacy properties, with implications for non-amenability.

Contribution

It characterizes when stabilizer subgroups are finitely generated and establishes isomorphism conditions, introducing a new conjugacy subgroup within the completion of $F$.

Findings

Stabilizers are finitely generated iff the set consists of rational numbers.

Stabilizers of rational sets with non-binary fractions are often isomorphic.

The subgroup $ar F$ is non-amenable.

Abstract

We study subgroups $H_U$ of the R. Thompson group $F$ which are stabilizers of finite sets $U$ of numbers in the interval $(0,1)$. We describe the algebraic structure of $H_U$ and prove that the stabilizer $H_U$ is finitely generated if and only if $U$ consists of rational numbers. We also show that such subgroups are isomorphic surprisingly often. In particular, we prove that if finite sets $U\subset [0,1]$ and $V\subset [0,1]$ consist of rational numbers which are not finite binary fractions, and $|U|=|V|$, then the stabilizers of $U$ and $V$ are isomorphic. In fact these subgroups are conjugate inside a subgroup $\bar F<\Homeo([0,1])$ which is the completion of $F$ with respect to what we call the Hamming metric on $F$. Moreover the conjugator can be found in a certain subgroup $\F < \bar F$ which consists of possibly infinite tree-diagrams with finitely many infinite branches. We also show that the group $\F$ is non-amenable.