Topological Boundaries of Representations and Coideals

Abstract

For a locally compact quantum group $\mathbb{G}$, a (left) coideal is a (left) $\mathbb{G}$-invariant von Neumann subalgebra of $L^\infty(\mathbb{G})$. We introduce and analyze various generalizations of amenability and coamenability to coideals of discrete and compact quantum groups. We focus on a particular class of coideals found in the category of compact quantum groups, which are associated with a compact quasi-subgroup. This class includes all coideals of the quotient type. We also introduce the notion of a Furstenberg-Hamana boundary for representations of discrete quantum groups and use it to study amenability and coamenability properties of coideals. We then prove that a coideal of a compact quantum group that is associated with a compact quasi-subgroup is coamenable if and only if its codual coideal is $\mathbb{G}$-injective. If $\mathbb{G}$ is a unimodular or an exact discrete quantum group, we can replace $\mathbb{G}$-injectivity in the latter statement with the weaker condition of relative amenability. This result leads to a complete characterization of the unique trace property. Specifically, a unimodular discrete quantum group $\mathbb{G}$ has the unique trace property if and only if the action of $\mathbb{G}$ on its noncommutative Furstenberg boundary is faithful. We also demonstrated that if a unimodular discrete quantum group $\mathbb{G}$ is $C^*$-simple then it has the unique trace property. These findings are the quantum analogs of the groundbreaking results of Breuillard, Kalantar, Kennedy, and Ozawa and they provide answers to questions posted by Kalantar, Kasprzak, Skalski, and Vergnioux.