Abstract

IΣ n andBΣ n are well known fragments of first-order arithmetic with induction and collection forΣ n formulas respectively;IΣ n 0 andBΣ n 0 are their second-order counterparts. RCA0 is the well known fragment of second-order arithmetic with recursive comprehension;WKL 0 isRCA 0 plus weak König's lemma. We first strengthen Harrington's conservation result by showing thatWKL 0 +BΣ n 0 is Π 1 1 -conservative overRCA 0 +BΣ n 0. Then we develop some model theory inWKL 0 and illustrate the use of formalized model theory by giving a relatively simple proof of the fact thatIΣ 1 provesBΣ n+1 to be Π n+2-conservative overIΣ n. Finally, we present a proof-theoretic proof of the stronger fact that theΠ n+2 conservation result is provable already inIΔ 0 + superexp. ThusIΣ n+1 proves 1-Con (BΣ n+1) andIΔ 0 +superexp proves Con (IΣ n )↔Con(BΣ n+1)