Authors: Raghavan, Dilip
Todorčević, Stevo 
Title: Combinatorial dichotomies and cardinal invariants
Journal: Mathematical Research Letters
Volume: 21
Issue: 2
First page: 379
Last page: 401
Issue Date: 1-Jan-2014
Rank: M22
ISSN: 1073-2780
DOI: 10.4310/MRL.2014.v21.n2.a13
Assuming the P-ideal dichotomy, we attempt to isolate those cardinal characteristics of the continuum that are correlated with two well-known consequences of the proper forcing axiom. We find a cardinal invariant x such that the statement that x < ω1 is equivalent to the statement that 1, ω, ω1, ω × ω1, and [ω1]<ω are the only cofinal types of directed sets of size at most N1. We investigate the corresponding problem for the partition relation ω1 → (ω1, α)2 for all α < ω1. To this effect, we investigate partition relations for pairs of comparable elements of a coherent Suslin tree S. We show that a positive partition relation for such pairs follows from the maximal amount of the proper forcing axiom compatible with the existence of S. As a consequence, we conclude that after forcing with the coherent Suslin tree S over a ground model satisfying this relativization of the proper forcing axiom, ω1 → (ω1, α)2 for all α < ω1. We prove that this positive partition relation for S cannot be improved by showing in ZFC that S → (N1, ω + 2)2.
Keywords: Cardinal invariants | Coherent Suslin tree | Combinatorial dichotomies | Laver property | P-ideal dichotomy | Partition relation
Publisher: International Press

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