Periodic higher rank graphs revisited
  Let $P$ be a finitely generated cancellative abelian monoid. A $P$-graph $\Lambda$ is a natural generalization of a higher rank graph. A pullback of $\Lambda$ is constructed by pulling it back over a given monoid morphism to $P$, while a pushout of $\Lambda$ is obtained by modding out its periodicity $\Per\Lambda$, which is deduced from a natural equivalence relation on $\Lambda$. One of our main results in this paper shows that, for a class of higher rank graphs $\Lambda$, $\Lambda$ is isomorphic to the pullback of its pushout via a natural quotient map, and that its graph C*-algebra can be embedded into the tensor product of the graph C*-algebra of its pushout and $\ca(\Per\Lambda)$. As a consequence, its cycline C*-algebra generated by the standard generators with equivalent pairs is an abelian core (particularly a MASA). Along the way, we give an in-depth study on periodicity of $P$-graphs. 
Dilian Yang
math.OA math.FA