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Proceedings of the American Mathematical Society
Article . 1991 . Peer-reviewed
Data sources: Crossref
Proceedings of the American Mathematical Society
Article . 1991 . Peer-reviewed
Data sources: Crossref
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Partitioning Pairs of Countable Sets

Authors: Stevo Todorčević;

Partitioning Pairs of Countable Sets

Abstract

We make the translations of our partitions from [3] in the context of all countable subsets of a fixed uncountable set. A different translation was obtained recently by Velleman [4]. The purpose of this paper is to define a two-cardinal version of one of our partitions from [3]. Theorem. For every uncountable set A there is a c :[[A]f0]]2 -* A such that, for every cofinal U C [AfQ' and a in A, there exist x c y in U such that c(x, y) = a. The proof will use straightforward generalization of one of the partitions from [3]. We shall assume that A is equal to some initial ordinal 0, and we shall fix an r: [0]8--+ {O, 1}w such that rx $ ry for x c y. [Identifying w1 with a subset of {O, 1}t, let rx (including finite x) be the standard code of (tp x, qx), where qx is defined recursively on sup x as follows assuming that, for each ordinal a of cofinality co, we have a fixed increasing sequence {ai} converging to a: If x has a maximal element 4 set qx(0) = 1 and qx(i + 1) = ry(i), where y = x nf . If a = sup x is a limit ordinal, let qx(O) = 0 and qx(2'(2j + 1)) = rx (j), where xi =x n ai.] Moreover, we shall fix a one-to-one ex : x -c o for each x in [0*"0 . For an integer n and x in [0]Ro, we set x(n) = 4E x: ex(4) < n}. For x c y in [Qfo0, let A(X , Y) = /\(rx, ry), i.e., the minimal place where the reals rx and ry disagree. Finally, for x c y in [6]fO and an ordinal A < 0, we set cA(x, y) = min(y(A(x, y)) \ sup(x n A)), Received by the editors October 10, 1989 and, in revised form, February 9, 1990. 1980 Mathematics Subject Classification (1985 Revision). Primary 03E05, 04A20. Research at MSRI supported in part by NSF Grant DMS-8505550. i) 1991 American Mathematical Society 0002-9939/91 $1.00 + $.25 per page

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
12
Average
Top 10%
Average
bronze