Connected greedy colourings of perfect graphs and other classes: the good, the bad and the ugly

Abstract

The Grundy number of a graph is the maximum number of colours used by the "First-Fit" greedy colouring algorithm over all vertex orderings. Given a vertex ordering σ = v1, . . . , vn, the "First-Fit" greedy colouring algorithm colours the vertices in the order of σ by assigning to each vertex the smallest colour unused in its neighbourhood. By restricting this procedure to vertex orderings that are connected, we obtain connected greedy colourings. For some graphs, all connected greedy colourings use exactly χ(G) colours; they are called good graphs. On the opposite, some graphs do not admit any connected greedy colouring using only χ(G) colours; they are called ugly graphs. We show that no perfect graph is ugly. We also give simple proofs of this fact for subclasses of perfect graphs (block graphs, comparability graphs), and show that no K4-minor free graph is ugly.