Quantum Automorphism Group of Direct Sum of Cuntz Algebras (2402.06241v2)

Abstract: In 1998, Wang constructed an ergodic action of the compact quantum group $U_n^+$ (free unitary quantum group) on the Cuntz algebra $\mathcal{O}n$. Later, in 2018-2019, S. Joardar and A. Mandal showed that the quantum automorphism group of the Cuntz algebra $\mathcal{O}{n}$ (as a graph $C^*$-algebra) is $U_n^+$ in the category introduced by them. In this article, we explore the quantum symmetry of the direct sum of Cuntz algebras viewing them as a graph $C^*$-algebra in the category as mentioned before. It has been shown that the quantum automorphism group of the direct sum of non-isomorphic Cuntz algebras ${\mathcal{O}{n_i}}{i=1}^{m}$ is ${U}{n_1}^{+}*{U}{n_2}^{+}* \cdots {U}{n_m}^{+}$ for distinct $n_i$'s, i.e. if $ L{n_i}$ (the graph contains $n_i$ loops based at a single vertex) is the underlying graph of $\mathcal{O}_{n_i}$, then \begin{equation} Q_{\tau}^{Lin}(\sqcup_{i=1}^{m} ~ L_{n_i}) \cong {i=1}^{m} ~~ Q{\tau}^{Lin}(L_{n_i}) \cong {U}_{n_1}^{+}{U}{n_2}^{+}* \cdots *{U}{n_m}^{+}. \end{equation*} Moreover, the quantum symmetry of the direct sum of $m$ copies of isomorphic Cuntz algebra $\mathcal{O}n$ (whose underlying graph is $L_n$) is $U_n^+ \wr* S_m^+,$ i.e. \begin{equation*} Q_{\tau}^{Lin}(\sqcup_{i=1}^{m} ~ L_n) \cong Q_{\tau}^{Lin}(L_n) \wr_* S_m^+ \cong U_n^+ \wr_* S_m^+. \end{equation*} On the other hand, it is known that the quantum automorphism group of $m$ disjoint copies of a simple, connected graph $\Gamma$ is isomorphic to the free wreath product of the quantum automorphism group of $\Gamma$ with $S_m^+$. Though an analoguous result is true for $\mathcal{O}_{n}$ (as a graph $C^*$-algebra), we have provided a counter-example to show that this result is not in general true for an arbitrary graph $C^*$-algebra.