Schreier sets in Ramsey theory |
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Authors: | V Farmaki S Negrepontis |
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Institution: | Department of Mathematics, Athens University, Panepistemiopolis, Athens 157 84, Greece ; Department of Mathematics, Athens University, Panepistemiopolis, Athens 157 84, Greece |
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Abstract: | We show that Ramsey theory, a domain presently conceived to guarantee the existence of large homogeneous sets for partitions on -tuples of words (for every natural number ) over a finite alphabet, can be extended to one for partitions on Schreier-type sets of words (of every countable ordinal). Indeed, we establish an extension of the partition theorem of Carlson about words and of the (more general) partition theorem of Furstenberg-Katznelson about combinatorial subspaces of the set of words (generated from -tuples of words for any fixed natural number ) into a partition theorem about combinatorial subspaces (generated from Schreier-type sets of words of order any fixed countable ordinal). Furthermore, as a result we obtain a strengthening of Carlson's infinitary Nash-Williams type (and Ellentuck type) partition theorem about infinite sequences of variable words into a theorem, in which an infinite sequence of variable words and a binary partition of all the finite sequences of words, one of whose components is, in addition, a tree, are assumed, concluding that all the Schreier-type finite reductions of an infinite reduction of the given sequence have a behavior determined by the Cantor-Bendixson ordinal index of the tree-component of the partition, falling in the tree-component above that index and in its complement below it. |
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Keywords: | Ramsey theory Schreier sets words |
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