Abstract
In 1960 Ore proved the following theorem: Let G be a graph of order n. If d(u)+d(v)⩾n for every pair of nonadjacent vertices u and v, then G is hamiltonian. In this note we strengthen Ore's theorem as follows: we determine the maximum number of pairs of nonadjacent vertices that can have degree sum less than n (i.e. violate Ore's condition) but still imply that the graph is hamiltonian. Some other sufficient conditions (i.e. Fan's condition) for hamiltonian graphs are strengthened as well.
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