Quantum isometry groups of dual of finitely generated discrete groups and quantum groups

Abstract

We study quantum isometry groups, denoted by [Formula: see text], of spectral triples on [Formula: see text] for a finitely generated discrete group [Formula: see text] coming from the word-length metric with respect to a symmetric generating set [Formula: see text]. We first prove a few general results about [Formula: see text] including: • For a group [Formula: see text] with polynomial growth property, the dual of [Formula: see text] has polynomial growth property provided the action of [Formula: see text] on [Formula: see text] has full spectrum. •[Formula: see text] for any discrete abelian group [Formula: see text], where [Formula: see text] is a suitable metric on the dual compact abelian group [Formula: see text]. We then carry out explicit computations of [Formula: see text] for several classes of examples including free and direct product of cyclic groups, Baumslag–Solitar group, Coxeter groups etc. In particular, we have computed quantum isometry groups of all finitely generated abelian groups which do not have factors of the form [Formula: see text] or [Formula: see text] for some [Formula: see text] in the direct product decomposition into cyclic subgroups.