✍ Scribed by Petr Hliněný
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Contact graphs are a special kind of intersection graphs of geometrical objects in which the objects are not allowed to cross but only to touch each other. Contact graphs of simple curves, and line segments as a special case, in the plane are considered. Various classes of contact graphs are introduced and the inclusions between them are described, also the recognition of the contact graphs is studied. As one of the main results, it is proved that the recognition of 3-contact graphs is NP-complete for planar graphs, while the same question for planar triangulations is polynomial.
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In 1981, Chvatal defined the class of perfectly orderable graphs. This class of perfect graphs contains the comparability graphs and the triangulated graphs. In this paper, we introduce four classes of perfectly orderable graphs, including natural generalizations of the comparability and triangulate
A graph is called "perfectly orderable" if its vertices can be ordered in such a way that, for each induced subgraph F, a certain "greedy" coloring heuristic delivers an optimal coloring of F. No polynomial-time algorithm to recognize these graphs is known. We present four classes of perfectly order