Quasi-isometric rigidity for lamplighters with lamps of polynomial growth

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 $N\wr G⟶M\wr H$ 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 $\{1\}$. 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 $n=m$.

24 pages, comments welcome! v2: Final version, implementing comments of the referee. To appear in Annales de l'Institut Fourier