Stability of homomorphisms, coverings and cocycles II: Examples, applications and open problems

Abstract

Coboundary expansion (with $F_2$ coefficients), and variations on it, have been the focus of intensive research in the last two decades. It was used to study random complexes, property testing, and above all Gromov's topological overlapping property.

In part I of this paper, we extended the notion of coboundary expansion (and its variations) to cochains with permutation coefficients, equipped with the normalized Hamming distance. We showed that this gives a unified language for studying covering stability of complexes, as well as stability of group homomorphisms — a topic that drew a lot of attention in recent years.

In this part, we extend the theory to the permutation coefficients setting. This gives some new results, even for $F_2$ coefficients, opens several new directions of research, and suggests a pattern to proving the existence of non-sofic groups. Along the way, we solve the dimension 2 case of a problem of Gromov, exhibiting a family of bounded degree coboundary expanders with $F_2$ coefficients.