Stavros D. Nikolopoulos Leonidas Palios Algorithms for P<sub>4</sub>-Comparability Graph Recognition and Acyclic P<sub>4</sub>-Transitive Orientation. 95-126 2004 39 Algorithmica 2 http://dx.doi.org/10.1007/s00453-003-1075-9 db/journals/algorithmica/algorithmica39.html#NikolopoulosP04