Measurable splittings and the measured group theoretic structure of wreath products

TL;DR

This paper investigates the measure-theoretic and orbit equivalence properties of wreath products involving groups with measurable splittings, revealing new invariance and non-equivalence results in group theory.

Contribution

It establishes measure and orbit equivalence between various wreath products and demonstrates non-equivalence under certain rigidity conditions.

Findings

Wreath products with measure equivalent groups are orbit equivalent.

Wreath products with groups B and B×Z are orbit equivalent.

Certain wreath product actions are not stably orbit equivalent under rigidity conditions.

Abstract

Let $\Gamma$ be a countable group that admits an essential measurable splitting (for instance, any group measure equivalent to a free product of nontrivial groups). We show: (1) for any two nontrivial countable groups $B$ and $C$ that are measure equivalent, the wreath product groups $B\wr\Gamma$ and $C\wr\Gamma$ are measure equivalent (in fact, orbit equivalent) -- this is interesting even in the case when the groups $B$ and $C$ are finite; and (2) the groups $B\wr \Gamma$ and $(B\times\mathbf{Z})\wr\Gamma$ are measure equivalent (in fact, orbit equivalent) for every nontrivial countable group $B$. On the other hand, we show that certain wreath product actions are not even stably orbit equivalent if $\Gamma$ is instead assumed to be a sofic icc group that is Bernoulli superrigid, and $B$ and $C$ have different cardinalities.