Lower bounds of length of longest cycles in graphs involving neighborhood unions Original Research Article

Let G be a simple graph of order n. Let NC = min {|N(x)∪N(y)|: xy ∉ E(G)}. Denote by c(G), the circumference of G, the length of a longest cycle in G. In this paper we settle a conjecture of Faudree, Gould, Jacobson and Schelp [4], proving that (a) if G is 3-connected, then c(G) ⩾ min {n, 3(NC + 1)/2}; and (b) if G is 4-connected, then c(G) ⩾ min {n, 2NC}. Two examples are given to show that both bounds are sharp.