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Precoloring extension of co-Meyniel graphs. (English) Zbl 1123.05038
Summary: The precoloring extension problem consists, given a graph $$G$$ and a set of nodes to which some colors are already assigned, in finding a coloring of $$G$$ with the minimum number of colors which respects the precoloring assignment. This can be reduced to the usual coloring problem on a certain contracted graph. We prove that precoloring extension is polynomial for complements of Meyniel graphs. We answer a question of Hujter and Tuza by showing that “PrExt perfect” graphs are exactly the co-Meyniel graphs, which also generalizes results of M. Hujter and Zs. Tuza [Precoloring extensions I (1992; Zbl 0766.05026), II (1993; Zbl 0821.05026), III (1996; Zbl 0846.05034)] and of A. Hertz [Graphs Comb. 5, 149–157 (1989; Zbl 0677.05065)]. Moreover we show that, given a co-Meyniel graph, the corresponding contracted graph belongs to a restricted class of perfect graphs (“co-Artemis” graphs, which are “co-perfectly contractile” graphs), whose perfectness is easier to establish than the strong perfect graph theorem. However, the polynomiality of our algorithm still depends on the ellipsoid method for coloring perfect graphs.

##### MSC:
 05C15 Coloring of graphs and hypergraphs 05C17 Perfect graphs
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##### References:
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