Discrete quantum group coaction on the circle

Abstract: We prove by an explicit calculation that if any Kac-type discrete quantum group $\mathcal{Q}$ has a faithful coaction on $C(S^1)$ which is `linear' in the sense that it leaves the space spanned by ${ Z, \overline{Z} }$ invariant, then $\mathcal{Q}$ must be classical, i.e. isomorphic with $C_0(\Gamma)$ for some discrete group $\Gamma$. This parallels the well-known result of non-existence of genuine compact quantum group symmetry obtained by the first author and his collaborators ([32] and the references therein).