Approximate amenability of semigroup algebras and Segal algebras

Abstract

In recent years, there have been several studies of various `approximate' versions of the key notion of amenability, which is defined for all Banach algebras; these studies began with work of Ghahramani and Loy in 2004. The present memoir continues such work: we shall define various notions of approximate amenability, and we shall discuss and extend the known background, which considers the relationships between different versions of approximate amenability. There are a number of open questions on these relationships; these will be considered. In Chapter 1, we shall give all the relevant definitions and a number of basic results, partly surveying existing work; we shall concentrate on the case of Banach function algebras. In Chapter 2, we shall discuss these properties for the semigroup algebra ℓ¹(S) of a semigroup S. In the case where S has only finitely many idempotents, ℓ¹(S) is approximately amenable if and only if it is amenable. In Chapter 3, we shall consider the class of weighted semigroup algebras of the form ℓ¹(\N∧,ω), where ω:\Z→[1,∞) is an arbitrary function. We shall determine necessary and sufficient conditions on ω for these Banach sequence algebras to have each of the various approximate amenability properties that interest us. In this way we shall illuminate the implications between these properties. In Chapter 4, we shall discuss Segal algebras on \T and on \R. It is a conjecture that every proper Segal algebra on \T fails to be approximately amenable; we shall establish this conjecture for a wide class of Segal algebras.