Title: Isomorphism and Morita equivalence classes for crossed products of irrational rotation algebras by cyclic subgroups of $SL_2(\mathbb{Z})$,
Abstract: Let $\theta, \theta'$ be irrational numbers and $A, B$ be matrices in
$SL_2(\mathbb{Z})$ of infinite order. We compute the $K$-theory of the crossed
product $\mathcal{A}_{\theta}\rtimes_A \mathbb{Z}$ and show that
$\mathcal{A}_{\theta} \rtimes_A\mathbb{Z}$ and $\mathcal{A}_{\theta'} \rtimes_B
\mathbb{Z}$ are $*$-isomorphic if and only if $\theta = \pm\theta'
\pmod{\mathbb{Z}}$ and $I-A^{-1}$ is matrix equivalent to $I-B^{-1}$. Combining
this result and an explicit construction of equivariant bimodules, we show that
$\mathcal{A}_{\theta} \rtimes_A\mathbb{Z}$ and $\mathcal{A}_{\theta'} \rtimes_B
\mathbb{Z}$ are Morita equivalent if and only if $\theta$ and $\theta'$ are in
the same $GL_2(\mathbb{Z})$ orbit and $I-A^{-1}$ is matrix equivalent to
$I-B^{-1}$. Finally, we determine the Morita equivalence class of
$\mathcal{A}_{\theta} \rtimes F$ for any finite subgroup $F$ of
$SL_2(\mathbb{Z})$.