Weaker cousins of Ramsey's theorem over a weak base theory

Abstract

The paper is devoted to a reverse-mathematical study of some well-known consequences of Ramsey's theorem for pairs, focused on the chain-antichain principle CAC, the ascending-descending sequence principle ADS, and the Cohesive Ramsey Theorem for pairs CRT 2 2 . We study these principles over the base theory RCA * 0 , which is weaker than the usual base theory RCA0 considered in reverse mathematics in that it allows only ∆ 0 1 -induction as opposed to Σ 0 1 -induction. In RCA * 0 , it may happen that an unbounded subset of N is not in bijective correspondence with N. Accordingly, Ramsey-theoretic principles split into at least two variants, "normal" and "long", depending on the sense in which the set witnessing the principle is required to be infinite.We prove that the normal versions of our principles, like that of Ramsey's theorem for pairs and two colours, are equivalent to their relativizations to proper Σ 0 1 -definable cuts. Because of this, they are all Π 0 3 -but not Π 1 1 -conservative over RCA * 0 , and, in any model of RCA * 0 + ¬RCA0, if they are true then they are computably true relative to some set. The long versions exhibit one of two behaviours: they either imply RCA0 over RCA * 0 or are Π 0 3 -conservative over RCA * 0 . The conservation results are obtained using a variant of the so-called grouping principle.We also show that the cohesion principle COH, a strengthening of CRT 2 2 , is never computably true in a model of RCA * 0 and, as a consequence, does not follow from RT 2 2 over RCA * 0 .