Nested Locally Hamiltonian Graphs and the Oberly-Sumner Conjecture

Abstract A graph G is locally 𝒫, abbreviated L𝒫, if for every vertex v in G the open neighbourhood N(v) of v is non-empty and induces a graph with property 𝒫. Specifically, a graph G without isolated vertices is locally connected (LC) if N(v) induces a connected graph for each v ∈ V (G), and locally hamiltonian (LH) if N(v) induces a hamiltonian graph for each v ∈ V (G). A graph G is locally locally 𝒫 (abbreviated L2𝒫) if N(v) is non-empty and induces a locally 𝒫 graph for every v ∈ V (G). This concept is generalized to an arbitrary degree of nesting. For any k 0 we call a graph locally k-nested-hamiltonian if it is LmC for m = 0, 1, . . ., k and LkH (with L0C and L0H meaning connected and hamiltonian, respectively). The class of locally k-nested-hamiltonian graphs contains important subclasses. For example, Skupień had already observed in 1963 that the class of connected LH graphs (which is the class of locally 1-nested-hamiltonian graphs) contains all triangulations of closed surfaces. We show that for any k ≥ 1 the class of locally k-nested-hamiltonian graphs contains all simple-clique (k + 2)-trees. In 1979 Oberly and Sumner proved that every connected K1,3-free graph that is locally connected is hamiltonian. They conjectured that for k ≥ 1, every connected K1,k+3-free graph that is locally (k + 1)-connected is hamiltonian. We show that locally k-nested-hamiltonian graphs are locally (k + 1)-connected and consider the weaker conjecture that every K1,k+3-free graph that is locally k-nested-hamiltonian is hamiltonian. We show that if our conjecture is true, it would be “best possible” in the sense that for every k ≥ 1 there exist K1,k+4-free locally k-nested-hamiltonian graphs that are non-hamiltonian. We also attempt to determine the minimum order of non-hamiltonian locally k-nested-hamiltonian graphs and investigate the complexity of the Hamilton Cycle Problem for locally k-nested-hamiltonian graphs with restricted maximum degree.