On quantitative orbit equivalence for lamplighter-like groups

TL;DR

This paper investigates the quantitative orbit equivalence of lamplighter-like groups, establishing stability results and optimality conditions, and quantifying geometric differences via isoperimetric profiles.

Contribution

It introduces a new notion of orbit equivalence for pairs, extends stability results to permutational halo products, and quantifies geometric differences between groups.

Findings

Most of these constructions are quantitatively optimal.

Shuffler groups are L^p orbit equivalent iff p<k/(k+ell).

Established a method to build orbit equivalence couplings using F{46}lner tilings.

Abstract

We focus on halo products, a class of groups introduced by Genevois and Tessera, and whose geometry mimics lamplighters. Famous examples are lampshufflers. Motivated by their work on the classifications up to quasi-isometry of these groups, we initiate a more quantitative study of their geometry. Indeed, it follows from the work of Delabie, Koivisto, Le Ma\^itre and Tessera that quantitative orbit equivalence between amenable groups is closely related to their large scale geometry, such a connection being justified by the use, in their main results, of a well-known quasi-isometry invariant: the isoperimetric profile. Inspired by their work on quantitative orbit equivalence between lamplighters, we prove a stability result for orbit equivalence of permutational halo products, going beyond the framework of standard halo products, using a new notion of orbit equivalence of pairs. Combined with our asymptotics of isoperimetric profiles obtained in an earlier article, we prove that most of these constructions are quantitatively optimal. For instance, we show that $\mathsf{Shuffler}(\mathbb{Z}^{k+\ell})$ and $\mathsf{Shuffler}(\mathbb{Z}^{k})$ are $\mathrm{L}^p$ orbit equivalent if and only if $p<\frac{k}{k+\ell}$, thus quantifying how much the geometries of these non-quasi-isometric groups differ. We finally build orbit equivalence couplings using the notion of F{\o}lner tiling sequences.