Hurewicz and Dranishnikov-Smith theorems for asymptotic dimension of countable approximate groups

Abstract

We establish two main results for the asymptotic dimension of countable approximate groups. The first one is a Hurewicz type formula for a global morphism of countable approximate groups $f:(\Xi, \Xi^\infty) \to (\Lambda, \Lambda^\infty)$, stating that $\mathrm{asdim} \Xi \leq \mathrm{asdim} \Lambda +\mathrm{asdim} ([\mathrm{ker} f]_c)$. This is analogous to the Dranishnikov-Smith result for groups, and is relying on another Hurewicz type formula we prove, using a 6-local morphism instead of a global one. The second result is similar to the Dranishnikov-Smith theorem stating that, for a countable group $G$, $\mathrm{asdim} G$ is equal to the supremum of asymptotic dimensions of finitely generated subgroups of $G$. Our version states that, if $(\Lambda, \Lambda^\infty)$ is a countable approximate group, then $\mathrm{asdim} \Lambda$ is equal to the supremum of asymptotic dimensions of approximate subgroups of finitely generated subgroups of $\Lambda^\infty$, with these approximate subgroups contained in $\Lambda^2$.