It is an open question whether compositional truth with the principle of propositional soundness: “All arithmetical sentences which are propositional tautologies are true” is conservative over Peano Arithmetic. In this article, we show that the principle of propositional soundness imposes some saturation-like properties on the truth predicate, thus showing significant limitations to the possible conservativity proof.
We develop a framework for nonstandard analysis that gives foundations to the interplay between external and internal iterations of the star map, and we present a few examples to show the strength and flexibility of such a nonstandard technique for applications in combinatorial number theory.
Feferman [9] defines an impredicative system of explicit mathematics, which is proof-theoretically equivalent to the subsystem of second-order arithmetic. In this paper, we propose several systems of Frege structure with the same proof-theoretic strength as . To be precise, we first consider the Kripke–Feferman theory, which is one of the most famous truth theories, and we extend it by two kinds of induction principles inspired by [22]. In addition, we give similar results for the system based on Aczel's original Frege structure [1]. Finally, we equip Cantini's supervaluation-style theory with the notion of universes, the strength of which was an open problem in [24].