Ramsey's theorem for singletons and strong computable reducibility
Abstract
We answer a question posed by Hirschfeldt and Jockusch by showing that whenever k > , Ramsey's theorem for singletons and k-colorings, RT 1 k , is not strongly computably reducible to the stable Ramsey's theorem for-colorings, SRT 2. Our proof actually establishes the following considerably stronger fact: given k > , there is a coloring c : ω → k such that for every stable coloring d : [ω] 2 → (computable from c or not), there is an infinite homogeneous set H for d that computes no infinite homogeneous set for c. This also answers a separate question of Dzhafarov, as it follows that the cohesive principle, COH, is not strongly computably reducible to the stable Ramsey's theorem for all colorings, SRT 2 <∞. The latter is the strongest partial result to date in the direction of giving a negative answer to the longstanding open question of whether COH is implied by the stable Ramsey's theorem in ω-models of RCA 0 .
Domains
Logic [math.LO]Origin | Files produced by the author(s) |
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