Conservation theorems for the Cohesiveness Principle
We prove that the Cohesiveness Principle (COH) is $Π^1_1$ conservative over $RCA_0 + IΣ^0_n$ and over $RCA_0 + BΣ^0_n$ for all $n \geq 2$ by recursion-theoretic means. We first characterize COH over $RCA_0 + BΣ^0_2$ as a `jumped' version of Weak König's Lemma (WKL) and develop suitable machinery including a version of the Friedberg jump-inversion theorem. The main theorem is obtained when we combine these with known results about WKL. In an appendix we give a proof of the $Π^1_1$ conservativity of WKL over $RCA_0$ by way of the Superlow Basis Theorem and a new proof of a recent jump-inversion theorem of Towsner.
math.LO↗