Complexity among the finitely generated subgroups of Thompson's group

TL;DR

This paper constructs a hierarchy of finitely generated subgroups of Thompson's group F, revealing their embeddability order and elementary amenability properties, and providing explicit examples with complex embedding relations.

Contribution

It introduces a well-ordered family of finitely generated subgroups of F with explicit descriptions, extending known groups and analyzing their embeddability and amenability classes.

Findings

Existence of a strictly well-ordered family of subgroups by embeddability

Construction of elementary amenable subgroups with EA-class up to 

Examples of subgroups with non-embeddable pairs

Abstract

We demonstrate the existence of a family of finitely generated subgroups of Richard Thompson's group $F$ which is strictly well-ordered by the embeddability relation in type $\epsilon_0 +1$. All except the maximum element of this family (which is $F$ itself) are elementary amenable groups. In fact we also obtain, for each $\alpha < \epsilon_0$, a finitely generated elementary amenable subgroup of $F$ whose EA-class is $\alpha + 2$. These groups all have simple, explicit descriptions and can be viewed as a natural continuation of the progression which starts with $\mathbf{Z} + \mathbf{Z}$, $\mathbf{Z} \wr \mathbf{Z}$, and the Brin-Navas group $B$. We also give an example of a pair of finitely generated elementary amenable subgroups of $F$ with the property that neither is embeddable into the other.