Let G be a graph of order n, σ k = min{ ϵ i=1 k d( ν i ): { ν 1,…, ν k } is an independent set of vertices in G}, NC = min{| N( u)∪ N( ν)|: uν∉ E( G)} and NC2 = min{| N( u)∪ N( ν)|: d( u, ν)=2}. Ore proved that G is hamiltonian if σ 2⩾ n⩾3, while Faudree et al. proved that G is hamiltonian if G is 2-connected and NC⩾ 1 3 (2n−1) . It is shown that both results are generalized by a recent result of Bauer et al. Various other existing results in hamiltonian graph theory involving degree-sums or cardinalities of neighborhood unions are also compared in terms of generality. Furthermore, some new results are proved. In particular, it is shown that the bound 1 3 (2n−1) on NC in the result of Faudree et al. can be lowered to 1 3 (2n−1) , which is best possible. Also, G is shown to have a cycle of length at least min{ n, 2(NC2)} if G is 2-connected and σ 3⩾ n+2. A D λ -cycle ( D λ -path) of G is a cycle (path) C such that every component of G− V( C) has order smaller than λ. Sufficient conditions of Lindquester for the existence of Hamilton cycles and paths involving NC2 are extended to D λ -cycles and D λ -paths.
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