# Minor of a graph

Let be a graph (possibly with loops and multiple edges). A minor of is any graph obtained by a succession of the following operations:

i) deletion of a single edge;

ii) contraction of a single edge;

iii) removal of an isolated vertex.

The graph minor theorem of N. Robertson and P.D. Seymour says the following. Given an infinite sequence of finite graphs, there are indices such that is (isomorphic to) a minor of . This was conjectured by K. Wagner (Wagner's well-quasi-ordering conjecture).

Let be any property of a graph which is closed under taking minors. For instance, the property of being imbeddable in a compact Riemann surface. Let be the set of all minor-minimal isomorphism classes of graphs not possessing property . Then the graph minor theorem says that is finite. This implies that property is characterized by saying that a graph has property if and only if it does not contain a minor from the finite set . In the case of the property "planar" the set is given by the two graphs and , cf. Kuratowski graph.

The finite forms of the graph minor theorem are as follows:

a) For all there is an such that for -tuples of graphs with there is a pair with isomorphic to a minor of . Here denotes the sum of the number of vertices and the number of edges of .

b) For all there is an such that for all -tuples of graphs with there are such that is isomorphic to a minor of for all .

Wagner's conjecture does not hold, in general, for infinite graphs, in the sense that there is a sequence of infinite graphs such that for all , is not a minor of . The conjecture is open (1988) for countable graphs.

A consequence (application) is that each graph property that is closed under taking minors can be tested in polynomial time. So, for instance, planarity can be tested in polynomial time. Further, there exists a polynomial-time algorithm for the following problem: Given a graph and vertices of , find paths so that connects and for all and such that the are vertex disjoint. Such problems arise in VLSI design.

#### References

[a1] | H. Friedman, N. Robertson, P.D. Seymour, "The metamathematics of the graph minor theorem" Logic and combinatorics (ed. S.G. Simpson) Contemporary Math. 65, Amer. Math. Soc. (1987) pp. 229–261 Zbl 0635.03060 |

[a2] | N. Robertson, P.D. Seymour, "Graph minors - a survey" I. Anderson (ed.) , Surveys in Combinatorics 1985 , Cambridge Univ. Press (1985) pp. 153–171
Zbl 0568.05025 |

[a3] | R. Thomas, "A counter-example to "Wagner's conjecture" for infinite graphs" Math. Proc. Cambridge Philos. Soc. , 103 (1988) pp. 55–57 Zbl 0647.05054 |

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Minor of a graph.

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