Stability of circulant graphs

Abstract

The canonical double cover $\mathrm{D}(Γ)$ of a graph $Γ$ is the direct product of $Γ$ and $K_2$. If $\mathrm{Aut}(\mathrm{D}(Γ))=\mathrm{Aut}(Γ)\times\mathbb{Z}_2$ then $Γ$ is called stable; otherwise $Γ$ is called unstable. An unstable graph is nontrivially unstable if it is connected, non-bipartite and distinct vertices have different neighborhoods. In this paper we prove that every circulant graph of odd prime order is stable and there is no arc-transitive nontrivially unstable circulant graph. The latter answers a question of Wilson in 2008. We also give infinitely many counterexamples to a conjecture of Marušič, Scapellato and Zagaglia Salvi in 1989 by constructing a family of stable circulant graphs with compatible adjacency matrices.