# Milliken's tree theorem

Milliken's tree theorem This article relies too much on references to primary sources. Please improve this by adding secondary or tertiary sources. Find sources: "Milliken's tree theorem" – news · newspapers · books · scholar · JSTOR (July 2022) (Learn how and when to remove this template message) 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 {displaystyle mathbb {S} _{T}^{n}} 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 {displaystyle mathbb {S} _{T}^{n}=C_{1}cup ...cup C_{r}} then for some strongly embedded infinite subtree R of T, {displaystyle mathbb {S} _{R}^{n}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 {displaystyle mathbb {S} ^{n}=bigcup _{T}mathbb {S} _{T}^{n}} where T ranges over finitely splitting rooted trees of height ω. Milliken's tree theorem says that not only is {displaystyle mathbb {S} ^{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)= {displaystyle {qin P:qgeq p}} , and {displaystyle 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: {displaystyle Ssubset T} , and the partial order on S is induced from T, if {displaystyle sin S} is nonmaximal in S and {displaystyle tin IS(s,T)} , then {displaystyle |Succ(t,T)cap IS(s,S)|=1} , there exists a strictly increasing function from {displaystyle alpha } to {displaystyle beta } , such that {displaystyle 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. References Keith R. Milliken, A Ramsey Theorem for Trees J. Comb. Theory (Series A) 26 (1979), 215-237 Keith R. Milliken, A Partition Theorem for the Infinite Subtrees of a Tree, Trans. Amer. Math. Soc. 263 No.1 (1981), 137-148. Categories: Ramsey theoryTheorems in discrete mathematicsTrees (set theory)

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