Haven (graph theory)

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In graph theory, if G is a graph, and k \ge 0 is an integer, a haven of order k in G is a function assigning to every set X \subseteq V(G) with \big|X\big| < k a vertex set of a component of G \big\backslash X, \beta\big(X\big), such that if X \subseteq Y \subseteq V(G) and \big|Y\big| < k, then \beta(Y) \subseteq \beta(X).

The Min-max theorem for tree-width states that a graph has a haven of order k if and only if it has tree width at least k − 1.

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