Uniform non-amenability

Abstract

For any finitely generated group G an invariant Føl G ⩾ 0 is introduced which measures the “amount of non-amenability” of G. If G is amenable, then Føl G = 0 . If Føl G > 0 , we call G uniformly non-amenable. We study the basic properties of this invariant; for example, its behaviour when passing to subgroups and quotients of G. We prove that the following classes of groups are uniformly non-amenable: non-abelian free groups, non-elementary word-hyperbolic groups, large groups, free Burnside groups of large enough odd exponent, and groups acting acylindrically on a tree. Uniform non-amenability implies uniform exponential growth. We also exhibit a family of non-amenable groups (in particular including all non-solvable Baumslag–Solitar groups) which are not uniformly non-amenable, that is, they satisfy Føl G = 0 . Finally, we derive a relation between our uniform Følner constant and the uniform Kazhdan constant with respect to the left regular representation of G.