文摘
We consider the problem of partitioning the vertex-set of a graph into four non-empty sets such that every vertex of is adjacent to every vertex of and every vertex of is adjacent to every vertex of . The complexity of deciding if a graph admits such a partition is unknown. We show that it can be solved in polynomial time for several classes of graphs: -free graphs, diamond-free graphs, planar graphs, graphs with bounded treewidth, claw-free graphs, -free graphs and graphs with few ¡¯s.