Glasner property for unipotently generated group actions on tori

Abstract

A theorem of Glasner from 1979 shows that if $A \subset \mathbb{T} = \mathbb{R}/\mathbb{Z}$ is infinite then for each $\epsilon > 0$ there exists an integer $n$ such that $nA$ is $\epsilon$-dense and Berend-Peres later showed that in fact one can take $n$ to be of the form $f(m)$ for any non-constant $f(x) \in \mathbb{Z}[x]$. Alon and Peres provided a general framework for this problem that has been used by Kelly-L\^{e} and Dong to show that the same property holds for various linear actions on $\mathbb{T}^d$. We complement the result of Kelly-L\^{e} on the $\epsilon$-dense images of integer polynomial matrices in some subtorus of $\mathbb{T}^d$ by classifying those integer polynomial matrices that have the Glasner property in the full torus $\mathbb{T}^d$. We also extend a recent result of Dong by showing that if $\Gamma \leq \operatorname{SL}_d(\mathbb{Z})$ is generated by finitely many unipotents and acts irreducibly on $\mathbb{R}^d$ then the action $\Gamma \curvearrowright \mathbb{T}^d$ has a uniform Glasner property.