Grundy number and products of graphs

International audienceThe Grundy number of a graph G, denoted by Gamma(G), is the largest k such that G has a greedy k-colouring, that is a colouring with k colours obtained by applying the greedy algorithm according to some ordering of the vertices of G. In this paper, we study the Grundy number of the lexicographic and cartesian products of two graphs in terms of the Grundy numbers of these graphs.Regarding the lexicographic product, we show that Gamma(G) x Gamma(H) = 1, given a graph G, it is CoNP-Complete to decide if Gamma(G) <= c x chi (G) and it is CoNP-Complete to decide if Gamma(G) <= c x omega(G).Regarding the cartesian product, we show that there is no upper bound of Gamma(G square H) as a function of Gamma(G) and Gamma(H). Nevertheless, we prove that Gamma(G square H) <= Delta(G).2 Gamma((H)-1) + Gamma(H)