It has been asked by Duffus et al [ MR 92e:06009] whether the clique hypergraph of perfect graphs is colorable with a constant number of colors. Several results followed showing that for some classes of perfect graphs this constant is actually 2 or 3 :
They proved it for comparability and cocomparability graphs, Bacsó et al proved the same for some more perfect graphs, and noticed that 'almost all' perfect graphs are 3-clique-colorable (applying Prömel and Steger's result (Probability and Computation, 1, 1992)); they ask whether this bound holds for all perfect graphs:
Can the vertex-set of any perfect graph be partitioned into three classes, so that no (inclusionwise) maximal clique (of size ) is included in any of these ? Is the same true already for odd hole free graphs ?
If the clique-hypergraph of a graph and of all of its subgraphs can be colored with colors, that is, there exists a partition of the vertex-set into parts so that none of them contains an (inclusionwise) maximal clique, then Hoàng and McDiarmid [ MR 2002j:05110] say the graph is strongly -divisible. This is indeed a sharpening of -divisibility where `maximal' is replaced by `maximum' (cardinality). In these terms the above conjecture states that perfect graphs are strongly -divisible. We formulate another problem in this language:
Can strong -divisibility be decided in polytime ?
A major difficulty with the coloration of the maximal clique hypergraph is that it is NP-hard to decide whether a partition of the vertices is a clique-coloration, and even in very particular classes of perfect graphs.
Contributed by Myriam Preissmann and András Sebo
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