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

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.

30 pages; v2: removed a problem and a statement about conjugacy om Homeo(R), 29 pages; v3: Added Section 7 where we prove that if U and V satisfy the conditions of Theorem 4.1, then $H_U$ and $H_V$ are conjugate inside the completion $\bar{F}<Homeo([0,1])$ of $F$ with respect to certain natural metric