Quantifying metric approximations of discrete groups

Abstract

We introduce and systematically study a profile function whose asymptotic behavior quantifies the dimension or the size of a metric approximation of a finitely generated group $G$ by a family of groups $\mathcal{F}=\{(G_{\alpha}, d_{\alpha}, k_{\alpha}, \varepsilon_{\alpha })\}_{\alpha\in I, }$ where each group $G_\alpha$ is equipped with a bi-invariant metric $d_{\alpha}$ and a dimension $k_{\alpha}$, for strictly positive real numbers $\varepsilon_{\alpha }$ such that $\inf_{\alpha }\varepsilon_{\alpha }>0$. Through the notion of a residually amenable profile that we introduce, our approach generalizes classical isoperimetric (aka Folner) profiles of amenable groups and recently introduced functions quantifying residually finite groups. Our viewpoint is much more general and covers hyperlinear and sofic approximations as well as many other metric approximations such as weakly sofic, weakly hyperlinear, and linear sofic approximations.