Betwixt and between 2-factor Hamiltonian and Perfect-Matching-Hamiltonian graphs

TL;DR

This paper explores the properties of 2-factor Hamiltonian and Perfect-Matching-Hamiltonian graphs, introducing malleable vertices and star products to characterize and construct such graphs, and proving new theorems related to their structure.

Contribution

It introduces the concept of malleable vertices, establishes their relation to the PMH-property, and proves a characterization of 2FH graphs via malleable vertices, extending previous conjectures.

Findings

Malleable vertices imply the PMH-property but not 2FH.

A cubic graph is 2FH iff all vertices are malleable.

Equivalence of a conjecture relating 2FH graphs to specific cubic graphs.

Abstract

A Hamiltonian graph is 2-factor Hamiltonian (2FH) if each of its 2-factors is a Hamiltonian cycle. A similar, but weaker, property is the Perfect-Matching-Hamiltonian property (PMH-property): a graph admitting a perfect matching is said to have this property if each one of its perfect matchings (1-factors) can be extended to a Hamiltonian cycle. It was shown that the star product operation between two bipartite 2FH-graphs is necessary and sufficient for a bipartite graph admitting a 3-edge-cut to be 2FH. The same cannot be said when dealing with the PMH-property, and in this work we discuss how one can use star products to obtain graphs (which are not necessarily bipartite, regular and 2FH) admitting the PMH-property with the help of malleable vertices, which we introduce here. We show that the presence of a malleable vertex in a graph implies that the graph has the PMH-property, but does not necessarily imply that it is 2FH. It was also conjectured that if a graph is a bipartite cubic 2FH-graph, then it can only be obtained from the complete bipartite graph $K_{3,3}$ and the Heawood graph by using star products. Here, we show that a cubic graph (not necessarily bipartite) is 2FH if and only if all of its vertices are malleable. We also prove that the above conjecture is equivalent to saying that, apart from the Heawood graph, every bipartite cyclically 4-edge-connected cubic graph with girth at least 6 having the PMH-property admits a perfect matching which can be extended to a Hamiltonian cycle in exactly one way. Finally, we also give two necessary and sufficient conditions for a graph admitting a 2-edge-cut to be: (i) 2FH, and (ii) PMH.