Growth in infinite groups of infinite subsets

Abstract

Given an infinite group G, we consider the finitely additive measure defined on finite unions of cosets of finite index subgroups. We show that this shares many properties with the size of subsets of a finite group, for instance we can obtain equivalent results on Ruzsa distance and product free sets. In particular if G has infinitely many finite index subgroups then it has subsets S of measure arbitrarily close to 1/2 with the square of S having measure less than 1.