On metric approximate subgroups

Abstract

Let $G$ be a group with a metric $\mathrm{d}$ invariant under left and right translations, and let ${\bar{\mathbb{D}}}_r$ be the ball of radius $r$ around the identity. A $(k,r)$-metric approximate subgroup is a symmetric subset $X$ of $G$ such that the pairwise product set $XX$ is covered by at most $k$ translates of $X{\bar{\mathbb{D}}}_r$. This notion was introduced in arXiv:math/0601431 along with the version for discrete groups (approximate subgroups). In arXiv:0909.2190, it was shown for the discrete case that, at the asymptotic limit of $X$ finite but large, the "approximateness" (or need for more than one translate) can be attributed to a canonically associated Lie group. Here we prove an analogous result in the metric setting, under a certain finite covering assumption on $X$ replacing finiteness. In particular, if $G$ has bounded exponent, we show that any $(k,r)$-metric approximate subgroup is close to a $(1,r^{\prime })$-metric approximate subgroup for an appropriate $r^{\prime }$.