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  1. A marriage of Brouwer’s intuitionism and Hilbert’s finitism I: Arithmetic.Takako Nemoto & Sato Kentaro - 2022 - Journal of Symbolic Logic 87 (2):437-497.
    We investigate which part of Brouwer’s Intuitionistic Mathematics is finitistically justifiable or guaranteed in Hilbert’s Finitism, in the same way as similar investigations on Classical Mathematics (i.e., which part is equiconsistent with$\textbf {PRA}$or consistent provably in$\textbf {PRA}$) already done quite extensively in proof theory and reverse mathematics. While we already knew a contrast from the classical situation concerning the continuity principle, more contrasts turn out: we show that several principles are finitistically justifiable or guaranteed which are classically not. Among them (...)
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  • Reflections on reflections in explicit mathematics.Gerhard Jäger & Thomas Strahm - 2005 - Annals of Pure and Applied Logic 136 (1-2):116-133.
    We give a broad discussion of reflection principles in explicit mathematics, thereby addressing various kinds of universe existence principles. The proof-theoretic strength of the relevant systems of explicit mathematics is couched in terms of suitable extensions of Kripke–Platek set theory.
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  • Variation on a theme of Schutte.D. Probst & G. Jager - 2004 - Mathematical Logic Quarterly 50 (3):258.
    Let ≺ be a primitive recursive well-ordering on the natural numbers and assume that its order-type is greater than or equal to the proof-theoretic ordinal of the theory T. We show that the proof-theoretic strength of T is not increased if we add the negation of the statement which formalizes transfinite induction along ≺.
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  • The Proof-Theoretic Analysis of Transfinitely Iterated Quasi Least Fixed Points.Dieter Probst - 2006 - Journal of Symbolic Logic 71 (3):721 - 746.
    The starting point of this article is an old question asked by Feferman in his paper on Hancock's conjecture [6] about the strength of ${\rm ID}_{1}^{\ast}$. This theory is obtained from the well-known theory ID₁ by restricting fixed point induction to formulas that contain fixed point constants only positively. The techniques used to perform the proof-theoretic analysis of ${\rm ID}_{1}^{\ast}$ also permit to analyze its transfinitely iterated variants ${\rm ID}_{\alpha}^{\ast}$. Thus, we eventually know that $|\widehat{{\rm ID}}_{\alpha}|=|{\rm ID}_{\alpha}^{\ast}|$.
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  • On the relationship between fixed points and iteration in admissible set theory without foundation.Dieter Probst - 2005 - Archive for Mathematical Logic 44 (5):561-580.
    In this article we show how to use the result in Jäger and Probst [7] to adapt the technique of pseudo-hierarchies and its use in Avigad [1] to subsystems of set theory without foundation. We prove that the theory KPi0 of admissible sets without foundation, extended by the principle (Σ-FP), asserting the existence of fixed points of monotone Σ operators, has the same proof-theoretic ordinal as KPi0 extended by the principle (Σ-TR), that allows to iterate Σ operations along ordinals. By (...)
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