Strong Pseudo Transitivity and Intersection Graphs
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A directed graph G=(V,E) is strongly pseudo transitive if there is a partition {A,E-A} of E so that graphs G_1=(V,A) and G_2=(V,E-A) are transitive, and additionally, if ab∈ A and bc∈ E implies that ac∈ E. A strongly pseudo transitive graph G=(V,E) is strongly pseudo transitive of the first type, if ab∈ A and bc∈ E implies ac∈ A. An undirected graph is co-strongly pseudo transitive (co-strongly pseudo transitive of the first type) if its complement has an orientation which is strongly pseudo transitive (co-strongly pseudo transitive of the first type). Our purpose is show that the results in computational geometry CFP, Lu and intersection graph theory Ga2, ES can be unified and extended, using the notion of strong pseudo transitivity. As a consequence the general algorithmic framework in Sh is applicable to solve the maximum independent set in O(n^3) time in a variety of problems, thereby, avoiding case by case lengthily arguments for each problem. We show that the intersection graphs of axis parallel rectangles intersecting a diagonal line from bottom, and half segments are co-strongly pseudo transitive. In addition, we show that the class of the interval filament graphs is co-strongly transitive of the first type, and hence the class of polygon circle graphs which is contained in the class of interval filament graphs (but contains the classes of chordal graphs, circular arc, circle, and outer planar graphs), and the class of incomparability graphs are strongly transitive of the first type. For class of chordal graphs we give two different proofs, using two different characterizations, verifying that they are co-strongly transitive of the first type. We present some containment results.
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