On the von Neumann algebra of class functions on a compact quantum group

Abstract

We study analogues of the radial subalgebras in free group factors (called the algebras of class functions) in the setting of compact quantum groups. For the free orthogonal quantum groups we show that they are not MASAs, as soon as we are in a non-Kac situation. The most important notion to our present work is that of a (quasi-)split inclusion. We prove that the inclusion of the algebra of class functions is quasi-split for some unitary quantum groups -in this case the subalgebra is non-abelian and we also obtain a result concerning its relative commutant. In the positive direction, we construct certain bicrossed products from the quantum group SUq(2) for which the algebra of class functions is a MASA. 2020 Mathematics Subject Classification. Primary 46L67, 20G42. Key words and phrases. Compact quantum group, Quasi-split inclusion, Maximal abelian subalgebra. 1 Note however that for G = Fn we do not have equality of R and C G . In fact, C G = L ∞ (G) holds for all abelian compact quantum groups.