arXiv2015
Let $E \subset \mathbb{Z}^N$ be a set of positive upper Banach density and let $Γ< \operatorname{GL}_N(\mathbb{Z})$ be a finitely generated, strongly irreducible subgroup whose Zariski closure in $\operatorname{GL}_N(\mathbb{R})$ is a Zariski connected semisimple group with no compact factors. Let $Y$ be any set and suppose that $Ψ: \mathbb{Z}^N \rightarrow Y$ is a $Γ$-invariant function. We prove that for every positive integer $m$, there exists a positive integer $k$ with the property that for every finite set $F \subset \mathbb{Z}^N$ with $|F| = m$, we have \[ Ψ(kF) \subset Ψ(E-b) \quad \textrm{for some $b \in E$}. \] Furthermore, if $E$ is an aperiodic Bohr$_o$-set, we can choose $k = 1$ and $b = 0$. As one of many applications of this result, we show that if $E_o \subset \mathbb{Z}$ has positive upper Banach density, then, for any integer $m$, there exists an integer $k$ with the property for \emph{every} finite set $F \subset \mathbb{Z}$, we can find $x,y,z \in E_o$ such that \[ k^2 F \subset \big\{ (u-x)^2 + (v-y)^2 - (w-z)^2 \, : \, u,v,w \in E_o \big\}. \] In particular, if $E_o \subset \mathbb{Z}$ is an aperiodic Bohr$_o$-set, then every integer can be written on the form $u^2 + v^2 - w^2$ for some $u,v,w \in E_o$. Our techniques use recent results by Benoist-Quint and Bourgain-Furman-Lindenstrauss-Mozes on equidistribution of random walks on automorphism groups of tori.