All Minimal Prime Extensions of Hereditary Classes of Graphs

The substitution composition of two disjoint graphs G1 and G2 is obtained by first removing a vertex x from G2 and then making every vertex in G1 adjacent to all neighbours of x in G2. Let F be a family of graphs defined by a set Z* of forbidden configurations. Giakoumakis [V. Giakoumakis, On the closure of graphs under substitution, Discrete Mathematics 177 (1997) 83–97] proved that F∗, the closure under substitution of F, can be characterized by a set Z∗ of forbidden configurations — the minimal prime extensions of Z. He also showed that Z∗ is not necessarily a finite set. Since substitution preserves many of the properties of the composed graphs, an important problem is the following: find necessary and sufficient conditions for the finiteness of Z∗. Giakoumakis [V. Giakoumakis, On the closure of graphs under substitution, Discrete Mathematics 177 (1997) 83–97] presented a sufficient condition for the finiteness of Z∗ and a simple method for enumerating all its elements. Since then, many other researchers have studied various classes of graphs for which the substitution closure can be characterized by a finite set of forbidden configurations. The main contribution of this paper is to completely solve the above problem by characterizing all classes of graphs having a finite number of minimal prime extensions. We then go on to point out a simple way for generating an infinite number of minimal prime extensions for all the other classes of F∗

Basic perfect graphs and their extensions

In this article, we present a characterization of basic graphs in terms of forbidden induced subgraphs. This class of graphs was introduced by Conforti et al. (Square-free perfect graphs, J. Combin. Theory Ser. B, 90 (2) (2004) 257-307), and it plays an essential role in the announced proof of the Strong Perfect Graph Conjecture by Chudnovsky et al. (http://arxiv.org/PS_cache/math/ pdf/0212/0212070.pdf). Let G and H be graphs. A substitution of H in G replacing a vertex v∈V(G) is the graph G(v→H) consisting of disjoint union of H and G-v with the additional edge-set {xy:x∈V(H),y∈NG(v)}. For a class of graphs P, its substitutional closure P* consists of all graphs that can be obtained from graphs of P by repeated substitutions. We apply the reducing pseudopath method (Discrete Appl. Math. 128 (2-3) (2003) 487-509) to characterize the substitutional closure of the class of basic graphs in terms of forbidden induced subgraphs. © 2005 Elsevier B.V. All rights reserved