A graph G is called k-critical if it has chromatic number k; but every proper subgraph of G is ðk À 1Þ-colourable. We prove that every k-critical graph ðkX6Þ on nXk þ 2 vertices has at least
Recently, Kostochka and Stiebitz REF proved that for k ≥ 6, every k-critical graph other than K k has at least
A new lower bound on the number of edges in colour-critical graphs and hypergraphs
J. Comb. Theory, Ser. B
J. Comb. Theory, Ser. B
Mathematics Computer Science