Some Central Digraphs and their Automorphisms

Abstract

We define a family of finite directed graphs $G(k,c,w)$ on vertex set $\mathbb{Z}/k\mathbb{Z}\times \mathbb{Z}/k\mathbb{Z}$, where $w\in {\mathbb{Z}/k\mathbb{Z}}^{\times }$ has multiplicative order $c$, which divides $k$. We show that each $G(k,c,w)$ is a central digraph (its adjacency matrix squares to the all-ones matrix). We deduce explicit descriptions of the graph automorphism groups for graphs of type $G(k,1,1)$; $G(k,2,-1)$ when $k$ is even; and $G(p^2,p,1+p)$, when $p$ is prime.

13 pages, no figures