Fair amenability for semigroups

Abstract

A new flavour of amenability for discrete semigroups is proposed that generalises group amenability and follows from a \Folner-type condition. Some examples are explored, to argue that this new notion better captures some essential ideas of amenability. A semigroup $S$ is left fairly amenable if, and only if, it supports a mean $m\in\ell^\infty(S)^*$ satisfying $m(f) = m(s\ast f)$ whenever $s\ast f\in\ell^\infty(S)$, thus justifying the nomenclature "fairly amenable''.