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Minor (Graph Theory)In graph theory, a graph H is called a minor of the graph G if H is isomorphic to a graph that results from a subgraph of G by zero or more edge contractions. Here, "contracting an edge" means removing the edge and identifying its two endpoints, keeping all other edges. For example, the graph * | *--*--* | * is a minor of * /| *-*--*-*-* |/ * (the outer edges are removed, the long middle edge is contracted). The relation "being a minor of" is a partial order on the isomorphism classes of graphs. Many classes of graphs can be characterized by "forbidden minors": a graph belongs to the class if and only if it does not have a minor from a certain specified list. The best-known example is Kuratowski's theorem for the characterization of planar graphs. The general situation is described by the Robertson-Seymour theorem. Another deep result by Robertson-Seymour states that if any infinite list G1, G2,... of finite graphs is given, then there always exists two indices i < j such that Gi is a minor of Gj.
In linear algebra, there is a different unrelated meaning of the word minor. See minor (linear algebra).
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