Invariant functionals and the uniqueness of invariant norms

Abstract

Let τ be a representation of a compact group G on a Banach space ( X,||·||). The question we address is whether X carries a unique invariant norm in the sense that ||·|| is the unique norm on X for which τ is a representation. We characterize the uniqueness of norm in terms of the automatic continuity of the invariant functionals in the case when X is a dual Banach space and τ is a σ(X,X ∗) -continuous representation of G on X such that τ( G) consists of σ(X,X ∗) -continuous operators. We illustrate the usefulness of this characterization by studying the uniqueness of the norm on the spaces L p(Ω) , where Ω is a locally compact Hausdorff space equipped with a positive Radon measure and G acts on Ω as a group of continuous invertible measure-preserving transformations.