Quasi-isometric rigidity for lamplighters with lamps of polynomial growth

TL;DR

This paper proves a rigidity property for quasi-isometries between lamplighter groups with polynomial growth lamps, showing such maps are measure-scaling with a trivial scaling group, leading to classification results and examples of lamplighter rigidity.

Contribution

It establishes a measure-scaling rigidity for quasi-isometries between lamplighter groups with polynomial growth lamps, reducing their scaling groups to trivial and providing classification insights.

Findings

Quasi-isometries are measure-scaling with a specific factor.

The scaling group of these wreath products is trivial.

Examples of lamplighter-rigid groups are provided.

Abstract

A quasi-isometry between two connected graphs is measure-scaling if one can control precisely the sizes of pre-images of finite subsets. Such a notion is motivated by the work of Eskin-Fisher-Whyte on lamplighters over $\mathbb{Z}$ and the work of Dymarz on biLipschitz equivalences of amenable groups, and led Genevois and Tessera to introduce the scaling group $\text{Sc}(X)$ of an amenable bounded degree graph $X$. The main result of our article is a rigidity property for quasi-isometries between lamplighters with lamps of polynomial growth. Under assumptions on $G$ and $H$, any such quasi-isometry $N\wr G\longrightarrow M\wr H$ must be measure-scaling for some scaling factor depending on the growth degrees of $N$ and $M$. In particular, the scaling group of such wreath products is reduced to $\lbrace 1\rbrace$. As applications, we obtain additional examples of pairs of quasi-isometric groups that are not biLipschitz equivalent. We also give applications to the quasi-isometric classification of some iterated wreath products, and we exhibit the first example of an amenable finitely generated group $H$ which is lamplighter-rigid, in the sense that $\mathbb{Z}/n\mathbb{Z}\wr H$ and $\mathbb{Z}/m\mathbb{Z}\wr H$ are quasi-isometric if and only if $n=m$.