Stability of pair graphs

Abstract

We start up the study of the stability of general graph pairs. This notion is a generalization of the concept of the stability of graphs. We say that a pair of graphs $(\Gamma ,\Sigma )$ is stable if $Aut(\Gamma \times \Sigma )\cong Aut(\Gamma )\times Aut(\Sigma )$ and unstable otherwise, where $\Gamma \times \Sigma$ is the direct product of $\Gamma$ and $\Sigma$. An unstable graph pair $(\Gamma ,\Sigma )$ is said to be a nontrivially unstable graph pair if $\Gamma$ and $\Sigma$ are connected coprime graphs, at least one of them is non-bipartite, and each of them has the property that different vertices have distinct neighbourhoods. We obtain necessary conditions for a pair of graphs to be stable. We also give a characterization of a pair of graphs $(\Gamma ,\Sigma )$ to be nontrivially unstable in the case when both graphs are connected and regular with coprime valencies and $\Sigma$ is vertex-transitive. This characterization is given in terms of the $\Sigma$-automorphisms of $\Gamma$, which are a new concept introduced in this paper as a generalization of both automorphisms and two-fold automorphisms of a graph.

22 pages