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Milliken's tree theorem : ウィキペディア英語版
Milliken's tree theorem
In mathematics, Milliken's tree theorem in combinatorics is a partition theorem generalizing Ramsey's theorem to infinite trees, objects with more structure than sets.
Let T be a finitely splitting rooted tree of height ω, n a positive integer, and \mathbb^n_T the collection of all strongly embedded subtrees of T of height n. In one of its simple forms, Milliken's tree theorem states that if \mathbb^n_T=C_1 \cup ... \cup C_r then for some strongly embedded infinite subtree R of T, \mathbb^n_R \subset C_i for some i ≤ r.
This immediately implies Ramsey's theorem; take the tree T to be a linear ordering on ω vertices.
Define \mathbb^n= \bigcup_T \mathbb^n_T where T ranges over finitely splitting rooted trees of height ω. Milliken's tree theorem says that not only is \mathbb^n partition regular for each n < ω, but that the homogeneous subtree R guaranteed by the theorem is strongly embedded in T.
== Strong embedding ==
Call T an α-tree if each branch of T has cardinality α. Define Succ(p, P)= \, and IS(p,P) to be the set of immediate successors of p in P. Suppose S is an α-tree and T is a β-tree, with 0 ≤ α ≤ β ≤ ω. S is ''strongly embedded'' in T if:
* S \subset T, and the partial order on S is induced from T,
* if s \in S is nonmaximal in S and t \in IS(s,T), then |Succ(t,T) \cap IS(s,S)|=1,
* there exists a strictly increasing function from \alpha to \beta, such that S(n) \subset T(f(n)).
Intuitively, for S to be strongly embedded in T,
* S must be a subset of T with the induced partial order
* S must preserve the branching structure of T; ''i.e.'', if a nonmaximal node in S has n immediate successors in T, then it has n immediate successors in S
* S preserves the level structure of T; all nodes on a common level of S must be on a common level in T.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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