Theories of Frege structure equivalent to Feferman's system $\mathsf{T}_0$

Abstract

Feferman (1975) defines an impredicative system $\mathsf{T}_0$ of explicit mathematics, which is proof-theoretically equivalent to the subsystem $\Delta^1_2$-$\mathsf{CA} + \mathsf{BI}$ of second-order arithmetic. In this paper, we propose several systems of Frege structure with the same proof-theoretic strength as $\mathsf{T}_0$. 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 (J\"ager et al. 2001). In addition, we give similar results for the system based on Aczel's original Frege structure (Aczel 1980). Finally, we equip Cantini's supervaluation-style theory with the notion of universes, the strength of which was an open problem in (Kahle 2001).