K-theory of Rotation Algebra Crossed Products by Amalgamated Products of Finite Cyclic Groups

Abstract

The $K$-groups of the crossed product of the rotation C*-algebra $A_\theta$ by free and amalgamated products of the cyclic groups $\mathbb Z_n$, for $n=2,3,4,6$, are calculated. The actions here arise from the canonical actions of these groups on the rotation algebra under the flip, cubic, Fourier, and hexic automorphisms, respectively. An interesting feature in this study is that although the inclusion $A_\theta \to A_\theta \rtimes \mathbb Z_n$ induces injective maps on their $K_0$-groups, the same is not the case for the inclusions $A_\theta \rtimes \mathbb Z_d \to A_\theta \rtimes \mathbb Z_n$ for $2\le d < n \le 6$ and $d|n$, which we endeavor to calculate. Further, while for free products $K_1(A_\theta \rtimes [\mathbb Z_{m} \ast \mathbb Z_{n}]) = 0$, for amalgamated products $K_1(A_\theta \rtimes [\mathbb Z_{m} \ast_{\mathbb Z_{d}} \mathbb Z_{n}]) = \mathbb Z^k$ is non-vanishing ($k=1,2$).