Higher-dimensional digraphs from cube complexes and their spectral theory

Abstract

We define $k$-dimensional digraphs and initiate a study of their spectral theory. The $k$-dimensional digraphs can be viewed as generating graphs for small categories called $k$-graphs. Guided by geometric insight, we obtain several new series of $k$-graphs using cube complexes covered by Cartesian products of trees, for $k\ge 2$. These $k$-graphs can not be presented as virtual products and constitute novel models of such small categories. The constructions yield rank-$k$ Cuntz–Krieger algebras for all $k\ge 2$. We introduce Ramanujan $k$-graphs satisfying optimal spectral gap property and show explicitly how to construct the underlying $k$-digraphs.