Published online by Cambridge University Press: 22 September 2016
We investigate the $K$-theory of unital UCT Kirchberg algebras
${\mathcal{Q}}_{S}$ arising from families
$S$ of relatively prime numbers. It is shown that
$K_{\ast }({\mathcal{Q}}_{S})$ is the direct sum of a free abelian group and a torsion group, each of which is realized by another distinct
$C^{\ast }$-algebra naturally associated to
$S$. The
$C^{\ast }$-algebra representing the torsion part is identified with a natural subalgebra
${\mathcal{A}}_{S}$ of
${\mathcal{Q}}_{S}$. For the
$K$-theory of
${\mathcal{Q}}_{S}$, the cardinality of
$S$ determines the free part and is also relevant for the torsion part, for which the greatest common divisor
$g_{S}$ of
$\{p-1:p\in S\}$ plays a central role as well. In the case where
$|S|\leq 2$ or
$g_{S}=1$ we obtain a complete classification for
${\mathcal{Q}}_{S}$. Our results support the conjecture that
${\mathcal{A}}_{S}$ coincides with
$\otimes _{p\in S}{\mathcal{O}}_{p}$. This would lead to a complete classification of
${\mathcal{Q}}_{S}$, and is related to a conjecture about
$k$-graphs.