Graph minors. III. Planar tree-width.

*(English)*Zbl 0548.05025[For part I see ibid. 35, 39-61 (1983; Zbl 0521.05062.]

This paper continues the important work by the authors on graph minors. A graph H is a minor of a graph G if H can be obtained from a subgraph of G by contraction. The tree decomposition of a graph G is a pair (T,X) where T is a tree and X is a family of subsets of V(G): \(X=\{X_ t:\quad t\in V(T)\}\) such that (i) the union of the \(X_ t\) is V(G); (ii) for every edge e of G, there exists an \(X_ t\) containing both ends of e; and (iii) for t, t’, t”\(\in V(T)\), if t’ is on the path of T between t and t”, then \(X_ t\cap X_{t''}\subseteq X_{t'}.\) The width of the tree decomposition is the maximum of \(| X_ t| -1.\) The graph G has tree-width w if w is the minimum width of any tree decomposition of G. The author’s main result is: For every planar graph H, there is a number w such that every planar graph with no minor isomorphic to H has tree- width \(\leq w\).

This paper continues the important work by the authors on graph minors. A graph H is a minor of a graph G if H can be obtained from a subgraph of G by contraction. The tree decomposition of a graph G is a pair (T,X) where T is a tree and X is a family of subsets of V(G): \(X=\{X_ t:\quad t\in V(T)\}\) such that (i) the union of the \(X_ t\) is V(G); (ii) for every edge e of G, there exists an \(X_ t\) containing both ends of e; and (iii) for t, t’, t”\(\in V(T)\), if t’ is on the path of T between t and t”, then \(X_ t\cap X_{t''}\subseteq X_{t'}.\) The width of the tree decomposition is the maximum of \(| X_ t| -1.\) The graph G has tree-width w if w is the minimum width of any tree decomposition of G. The author’s main result is: For every planar graph H, there is a number w such that every planar graph with no minor isomorphic to H has tree- width \(\leq w\).

Reviewer: A.Tucker

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\textit{N. Robertson} and \textit{P. D. Seymour}, J. Comb. Theory, Ser. B 36, 49--64 (1984; Zbl 0548.05025)

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##### References:

[1] | Robertson, N; Seymour, P.D, Graph minors. I. excluding a forest, J. combin. theory ser. B, 35, 39-61, (1983) · Zbl 0521.05062 |

[2] | \scN. Robertson and P. D. Seymour, Graph minors. II. Algorithmic aspects of tree-width, submitted. · Zbl 0611.05017 |

[3] | \scN. Robertson and P. D. Seymour, Graph minors. IV. Tree-width and well-quasiordering, submitted. · Zbl 0719.05032 |

[4] | \scN. Robertson and P. D. Seymour, Graph minors. V. Excluding a planar graph, submitted. · Zbl 1023.05040 |

[5] | \scN. Robertson and P. D. Seymour, Graph minors. VI. Disjoint paths on a surface, in preparation. · Zbl 0658.05044 |

[6] | \scN. Robertson and P. D. Seymour, Graph minors. VII. Disjoint paths on a surface, in preparation. · Zbl 0658.05044 |

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