Fragments of Arithmetic and true sentences

摘要

By a theorem of R. Kaye, J. Paris and C. Dimitracopoulos, the class of the Π_(n+1)-sentences true in the standard model is the only (up to deductive equivalence) consistent Π_(n+1)-theory which extends the scheme of induction for parameter free Π_(n+1)-formulas. Motivated by this result, we present a systematic study of extensions of bounded quantifier complexity of fragments of first-order Peano Arithmetic. Here, we improve that result and show that this property describes a general phenomenon valid for parameter free schemes. As a consequence, we obtain results on the quantifier complexity, (non)finite axiomatizability and relative strength of schemes for Δ_(n+1)-formulas.