A Simple Extension of Dirac's Theorem on Hamiltonicity

Abstract

The classical Dirac theorem asserts that every graph $G$ on $n$ vertices with minimum degree $\delta (G)\ge \lceil n/2\rceil$ is Hamiltonian. The lower bound of $\lceil n/2\rceil$ on the minimum degree of a graph is tight. In this paper, we extend the classical Dirac theorem to the case where $\delta (G)\ge \lfloor n/2\rfloor$ by identifying the only non-Hamiltonian graph families in this case. We first present a short and simple proof. We then provide an alternative proof that is constructive and self-contained. Consequently, we provide a polynomial-time algorithm that constructs a Hamiltonian cycle, if exists, of a graph $G$ with $\delta (G)\ge \lfloor n/2\rfloor$, or determines that the graph is non-Hamiltonian. Finally, we present a self-contained proof for our algorithm which provides insight into the structure of Hamiltonian cycles when $\delta (G)\ge \lfloor n/2\rfloor$ and is promising for extending the results of this paper to the cases with smaller degree bounds.