On asymptotic approximate groups in nilpotent groups

Abstract

Let $G$ be a group and let $A\subseteq G$ be non-empty. We call $A$ an asymptotic $(r,l)$-approximate group if, for a fixed dilation factor $r$, the larger product sets $A^{hr}$ can, for all sufficiently large $h$, be covered by a bounded number of left translates of $A^h$, with the bound $l$ independent of $h$. We show that, in virtually nilpotent groups, finite sets whose powers contain a symmetric word ball of radius comparable to $h$ are asymptotic approximate groups. We also prove a nonabelian semilinear-set analogue for certain infinite sets in these groups.