THE STRENGTH OF RAMSEY'S THEOREM FOR COLORING RELATIVELY LARGE SETS

We characterize the effective content and the proof-theoretic strength of a Ramsey-type theorem for bi-colorings of so-called exactly large sets. An exactly large set is a set X subset of N such that card(X) = min(X) + 1. The theorem we analyze is as follows. For every infinite subset M of N, for every coloring C of the exactly large subsets of M in two colors, there exists and infinite subset L of M such that C is constant on all exactly large subsets of L. This theorem is essentially due to Pudlak and Rodl and independently to Farmaki. We prove that-over RCA(0)-this theorem is equivalent to closure under the omega th Turing jump (i.e., under arithmetical truth). Natural combinatorial theorems at this level of complexity are rare. In terms of Reverse Mathematics we give the first Ramsey-theoretic characterization of ACA(0)(+). Our results give a complete characterization of the theorem from the point of view of Computability Theory and of the Proof Theory of Arithmetic. This nicely extends the current knowledge about the strength of Ramsey's Theorem. We also show that analogous results hold for a related principle based on the Regressive Ramsey's Theorem. We conjecture that analogous results hold for larger ordinals.