Polynomial permutation stability, soficity, and universal polynomial groups

Abstract

We introduce polynomial permutation stability, extending classical permutation stability from homomorphisms to polynomial maps of groups. The universal polynomial group ${\mathrm{Pol}}_s(G)$ provides a natural framework for this theory, and we prove that polynomial stability of degree $s$ is equivalent to permutation stability of ${\mathrm{Pol}}_s(G)$. This yields, in particular, polynomial stability of all finite groups in degree two. We develop a corresponding theory of polynomial sofic approximations and show that, for countable groups, it does not give a new notion of soficity. Nevertheless, the associated universal polynomial groups contain new structural information: in degree two they embed into a wreath product, which allows us to characterize their soficity in terms of the original group. We also formulate and study weak polynomial stability through the weak stability theory of universal polynomial groups. For finite groups, we investigate the structure of higher-degree universal polynomial groups. We obtain general obstructions to amenability and prove largeness results for a family of finite groups with perfect derived subgroup and abelianization of order two.

19 pages