On a measurable analogue of small topological full groups

TL;DR

This paper introduces $ ext{L}^1$ full groups as a measurable analogue of small topological full groups, analyzing their structure, simplicity, connectedness, and amenability, especially for ergodic $ ext{Z}$-actions.

Contribution

It defines and studies $ ext{L}^1$ full groups, establishing their topological properties, invariance under flip conjugacy, and connections to entropy and amenability.

Findings

Closure of the derived group is topologically simple for ergodic actions.

Closure of the derived group is connected.

For non-amenable groups, the groups are never amenable.

Abstract

We initiate the study of a measurable analogue of small topological full groups that we call $\mathrm L^1$ full groups. These groups are endowed with a Polish group topology which admits a natural complete right invariant metric. We mostly focus on $\mathrm L^1$ full groups of measure-preserving $\mathbb Z$-actions which are actually a complete invariant of flip conjugacy. We prove that for ergodic actions the closure of the derived group is topologically simple although it can fail to be simple. We also show that the closure of the derived group is connected, and that for measure-preserving free actions of non-amenable groups the closure of the derived group and the $\mathrm L^1$ full group itself are never amenable. In the case of a measure-preserving ergodic $\mathbb Z$-action, the closure of the derived group is shown to be the kernel of the index map. If such an action is moreover by homeomorphism on the Cantor space, we show that the topological full group is dense in the $\mathrm L^1$ full group. Using Juschenko-Monod and Matui's results on topological full groups, we conclude that $\mathrm L^1$ full groups of ergodic $\mathbb Z$-actions are amenable as topological groups, and that they are topologically finitely generated if and only if the $\mathbb Z$-action has finite entropy.