Ergodic actions of the compact quantum group $O_{-1}(2)$

Abstract: Among the ergodic actions of a compact quantum group $\mathbb{G}$ on possibly non-commutative spaces, those that are {\it embeddable} are the natural analogues of actions of a compact group on its homogeneous spaces. These can be realized as {\it coideal subalgebras} of the function algebra $\mathcal{O}(\mathbb{G})$ attached to the compact quantum group. We classify the embeddable ergodic actions of the compact quantum group $O_{-1}(2)$, basing our analysis on the bijective correspondence between all ergodic actions of the classical group $O(2)$ and those of its quantum twist resulting from the monoidal equivalence between their respective tensor categories of unitary representations. In the last section we give counterexamples showing that in general we cannot expect a bijective correspondence between embeddable ergodic actions of two monoidally equivalent compact quantum groups.