Let Formula: see text is an independent set of Formula: see textFormula: see text. According to a theorem of Dirac, which states that if Formula: see text is a Formula: see text-connected graph on Formula: see text vertices and minimum degree Formula: see text, then the circumference Formula: see text, Cai, Li and Sun proved that if Formula: see text is a Formula: see text-connected graph on Formula: see text vertices, then Formula: see text. In this paper, we prove that if Formula: see text is a Formula: see text-connected graph on Formula: see text vertices, then Formula: see text.
Li et al. (Fri,) studied this question.