Published online by Cambridge University Press: 20 April 2015
Let $P$ be a finitely generated cancellative abelian monoid. A
$P$-graph
${\rm\Lambda}$ is a natural generalization of a
$k$-graph. A pullback of
${\rm\Lambda}$ is constructed by pulling it back over a given monoid morphism to
$P$, while a pushout of
${\rm\Lambda}$ is obtained by modding out its periodicity, which is deduced from a natural equivalence relation on
${\rm\Lambda}$. One of our main results in this paper shows that, for some
$k$-graphs
${\rm\Lambda}$,
${\rm\Lambda}$ is isomorphic to the pullback of its pushout via a natural quotient map, and that its graph
$\text{C}^{\ast }$-algebra can be embedded into the tensor product of the graph
$\text{C}^{\ast }$-algebra of its pushout and
$\text{C}^{\ast }(\text{Per}\,{\rm\Lambda})$. As a consequence, in this case, the cycline algebra generated by the standard generators corresponding to equivalent pairs is a maximal abelian subalgebra, and there is a faithful conditional expectation from the graph
$\text{C}^{\ast }$-algebra onto it.