Nordhaus–Gaddum-Type Theorem for Rainbow Connection Number of Graphs

An edge-colored graph G is rainbow connected if every two vertices of G are connected by a path whose edges have distinct colors. The rainbow connection number of G, denoted by rc(G), is the minimum number of colors that are needed to make G rainbow connected. In this paper we give a Nordhaus–Gaddum-type result for the rainbow connection number. We prove that if G and ${\overline{G}}$ are both connected, then ${4\leq rc(G)+rc(\overline{G})\leq n+2}$ . Examples are given to show that the upper bound is sharp for n ≥ 4, and the lower bound is sharp for n ≥ 8. Sharp lower bounds are also given for n = 4, 5, 6, 7, respectively.