Papillon graphs: perfect matchings, Hamiltonian cycles and edge-colourings in cubic graphs

Marién Abreu, John Baptist Gauci, Domenico Labbate, Federico Romaniello, Jean Paul Zerafa

Published 2021-06-01Version 1

A graph $G$ has the Perfect-Matching-Hamiltonian property (PMH-property) if for each one of its perfect matchings, there is another perfect matching of $G$ such that the union of the two perfect matchings yields a Hamiltonian cycle of $G$. The study of graphs that have the PMH-property, initiated in the 1970s by Las Vergnas and H\"{a}ggkvist, combines three well-studied properties of graphs, namely matchings, Hamiltonicity and edge-colourings. In this work, we study these concepts for cubic graphs in an attempt to characterise those cubic graphs for which every perfect matching corresponds to one of the colours of a proper 3-edge-colouring of the graph. We discuss that this is equivalent to saying that such graphs are even-2-factorable (E2F), that is, all 2-factors of the graph contain only even cycles. The case for bipartite cubic graphs is trivial, since if $G$ is bipartite then it is E2F. Thus, we restrict our attention to non-bipartite cubic graphs. A sufficient, but not necessary, condition for a cubic graph to be E2F is that it has the PMH-property. The aim of this work is to introduce two infinite families of non-bipartite cubic graphs, which we term papillon graphs and unbalanced papillon graphs, and determine the values of their respective parameters for which these graphs have the PMH-property or are just E2F.