Minimum Cuts of Distance-Regular Digraphs

In this paper, we investigate the structure of minimum vertex and edge cuts of distance-regular digraphs. We show that each distance-regular digraph $\Gamma$, different from an undirected cycle, is super edge-connected, that is, any minimum edge cut of $\Gamma$ is the set of all edges going into (or coming out of) a single vertex. Moreover, we will show that except for undirected cycles, any distance regular-digraph $\Gamma$ with diameter $D=2$, degree $k\leq 3$ or $\lambda=0$ ($\lambda$ is the number of 2-paths from $u$ to $v$ for an edge $uv$ of $\Gamma$) is super vertex-connected, that is, any minimum vertex cut of $\Gamma$ is the set of all out-neighbors (or in-neighbors) of a single vertex in $\Gamma$. These results extend the same known results for the undirected case with quite different proofs.