Let $\mathcal{G}$ be compact quantum group in the sense of S. L. Woronowicz. As is well-known, every compact quantum group contains a dense Hopf algebra, called the **polynomial Hopf algebra** Pol$(\mathcal{G})$.
For example, consider the famous $C(SU\_q(2))$, the $q$-deformation of $SU(2)$, with generators $\alpha$ and $\beta$ :


<https://en.wikipedia.org/wiki/Compact_quantum_group>


The dense Hopf algebra is now the polynomial $\*$-algebra generated by $\alpha$ and $\beta$. A well-known fact about Pol($SU\_q(2)$) is that it has no zero-divisors, that is, it is a domain. What is a good example of a compact quantum group $\mathcal{G}$ such that Pol$(\mathcal{G})$ **has** zero divisors? On the other hand, is there any abstract characterization of the compact quantum groups such that the polynomial Hopf algebra is a domain?



Zero divisors in compact quantum groups
Concerning the first question, consider the quantum permutation group $S\_N^+$. Its polynomial algebra is generated by an $N\times N$ matrix $u=(u\_{ij})$ of self-adjoint projections which sum up to $1$ on each row and column. As a consequence, it has lots of zero divisors.


As for a general criterion, note that for any discrete group $\Gamma$, there is a "dual" compact quantum group with polynomial algebra $\mathbb{C}[\Gamma]$. Characterizing when this is a domain is an open problem. For instance, Kaplansky's zero divisor conjecture asserts that if $\Gamma$ is torsion-free then $k[\Gamma]$ is a domain for any field $k$ and even for $k = \mathbb{C}$ this is, as far as I know, still open in full generality.


