Completeness of the primitive recursive $\omega$-rule

Abstract

Shoenfield's completeness theorem (1959) states that every true first order arithmetical sentence has a recursive $\omega$-proof encodable by using recursive applications of the $\omega$-rule. For a suitable encoding of Gentzen style $\omega$-proofs, we show that Shoenfield's completeness theorem applies to cut free $\omega$-proofs encodable by using primitive recursive applications of the $\omega$-rule. We also show that the set of codes of $\omega$-proofs, whether it is based on recursive or primitive recursive applications of the $\omega$-rule, is $\Pi^1_1$ complete. The same $\Pi^1_1$ completeness results apply to codes of cut free $\omega$-proofs.