Invariant mean on weakly almost periodic functionals over a Hopf-von Neumann algebra

Abstract

Let \(M\) be a Hopf--von Neumann algebra with predual \(M_*\). Using minimal weak*-closed left ideals, we obtain an idempotent state \(φ\in\operatorname{WAP}(M_*)^*\) satisfying \(φ\star ω\starφ=φ\) for every state \(ω\) on \(\operatorname{WAP}(M_*)\). This state determines the largest closed introverted subspace \(X_φ\subseteq\operatorname{WAP}(M_*)\) containing the identity and admitting a two-sided invariant mean. We also characterize the existence of a unique two-sided invariant mean on introverted operator systems in \(\operatorname{WAP}(M_*)\) by a symmetry condition on their left and right state orbits. The criterion recovers the classical results for locally compact groups and their Fourier algebras, and gives a unified treatment of cocommutative Hopf--von Neumann algebras, compact quantum groups, unital \(C^*\)-Eberlein algebras, and \(\operatorname{WAP}(M(G))\) for every locally compact group \(G\).