Canonical double covers of generalized Petersen graphs, and double generalized Petersen graphs

TL;DR

This paper proves Wilson's conjecture on the instability of generalized Petersen graphs and fully characterizes their automorphism groups and isomorphisms with their canonical double covers.

Contribution

It confirms Wilson's conjecture on nontrivially unstable generalized Petersen graphs and classifies all isomorphisms and automorphism groups of their double covers.

Findings

Wilson's conjecture is proven true.

Complete classification of automorphism groups of double covers.

All isomorphisms among generalized Petersen graphs and their double covers identified.

Abstract

The canonical double cover $\D(\Gamma)$ of a graph $\Gamma$ is the direct product of $\Gamma$ and $K_2$. If $\Aut(\D(\Gamma))\cong\Aut(\Gamma)\times\ZZ_2$ then $\Gamma$ is called stable; otherwise $\Gamma$ is called unstable. An unstable graph is said to be nontrivially unstable if it is connected, non-bipartite and no two vertices have the same neighborhood. In 2008 Wilson conjectured that, if the generalized Petersen graph $\GP(n,k)$ is nontrivially unstable, then both $n$ and $k$ are even, and either $n/2$ is odd and $k^2\equiv\pm 1 \pmod{n/2}$, or $n=4k$. In this note we prove that this conjecture is true. At the same time we determine all possible isomorphisms among the generalized Petersen graphs, the canonical double covers of the generalized Petersen graphs, and the double generalized Petersen graphs. Based on these we completely determine the full automorphism group of the canonical double cover of $\GP(n,k)$ for any pair of integers $n, k$ with $1 \leqslant k < n/2$.