On two graph isomorphism problems

Abstract: In 2015, Bogdanowicz gave a necessary and sufficient condition for a 4-regular circulant graph to be isomorphic to the Cartesian product of two cycles. Accordion graphs, denoted by $A[n,k]$, are 4-regular graphs on two parameters $n$ and $k$ which were recently introduced by the authors and studied with regards to Hamiltonicity and matchings. These graphs can be obtained by a slight modification in some of the edges of the Cartesian product of two cycles. Motivated by the work of Bogdanowicz, the authors also determined for which values of $n$ and $k$ the accordion graph $A[n,k]$ is circulant. In this work we investigate what parameters a 4-regular circulant graph must have in order to be isomorphic to an accordion graph, thus providing a complete characterisation similar to that given by Bogdanowicz. We also give a necessary and sufficient condition for two accordion graphs with distinct parameters to be isomorphic.