A new closure concept preserving graph Hamiltonicity and based on neighborhood equivalence

A graph is Hamiltonian if it contains a cycle which goes through all vertices exactly once. Determining if a graph is Hamiltonian is known as an NP-complete problem, and no satisfactory characterization for these graphs has been found. 6, Bondy and Chvàtal introduced a way to get round the Hamiltonicity problem complexity by using a closure of the graph. This closure is a supergraph of G which is Hamiltonian iff G is. In particular, if the closure is the complete graph, then G is Hamiltonian. Since this seminal work, several closure concepts preserving Hamiltonicity have been introduced. In particular, in 1997, Ryjáček defined a closure concept for claw-free graphs based on local completion. ing a different approach, in 1974, Goodman and Hedetniemi gave a sufficient condition for Hamiltonicity based on the existence of a clique covering of the graph. This condition was recently generalized using the notion of Eulerian clique covering. In this context, closure concepts based on local completion are interesting since the closure of a graph contains more simplicial vertices than the graph itself, making the search for a clique covering easier. s article, we introduce a new closure concept based on local completion which preserves the Hamiltonicity for every graph. Note that, moreover, the closure may be claw free even when the graph is not.