Banach algebras generated by an invertible isometry of an Lp-space

Gardella E., Thiel H.

Abstract

We provide a complete description of those Banach algebras that are generated by an invertible isometry of an Lp-space together with its inverse. Examples include the algebra PFp(Z) of p-pseudofunctions on Z, the commutative C*-algebra C(S1) and all of its quotients, as well as uncountably many 'exotic' Banach algebras.We associate to each isometry of an Lp-space a spectral invariant called 'spectral configuration', which contains considerably more information than its spectrum as an operator. It is shown that the spectral configuration describes the isometric isomorphism type of the Banach algebra that the isometry generates together with its inverse.It follows from our analysis that these algebras are semisimple. With the exception of PFp(Z), they are all closed under continuous functional calculus, and their Gelfand transform is an isomorphism.As an application of our results, we show that Banach algebras that act on L1-spaces are not closed under quotients. This answers the case p=1 of a question asked by Le Merdy 20 years ago.