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WELL ORDERING PRINCIPLES AND ${\Pi }^{1}_{4}$-STATEMENTS: A PILOT STUDY

Part of: Proof theory and constructive mathematics Computability and recursion theory General logic

Published online by Cambridge University Press:  16 February 2021

ANTON FREUND
ANTON FREUND*
Affiliation:
FACHBEREICH MATHEMATIK TECHNISCHE UNIVERSITÄT DARMSTADT SCHLOSSGARTENSTR. 7, 64289DARMSTADT, GERMANYE-mail:[email protected]
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Abstract

In previous work, the author has shown that $\Pi ^1_1$ -induction along $\mathbb N$ is equivalent to a suitable formalization of the statement that every normal function on the ordinals has a fixed point. More precisely, this was proved for a representation of normal functions in terms of Girard’s dilators, which are particularly uniform transformations of well orders. The present paper works on the next type level and considers uniform transformations of dilators, which are called 2-ptykes. We show that $\Pi ^1_2$ -induction along $\mathbb N$ is equivalent to the existence of fixed points for all 2-ptykes that satisfy a certain normality condition. Beyond this specific result, the paper paves the way for the analysis of further $\Pi ^1_4$ -statements in terms of well ordering principles.

Keywords

well ordering principlesreverse mathematicsΠ14-statementsdilatorsptykesΠ12-induction

MSC classification

Primary: 03B30: Foundations of classical theories (including reverse mathematics)
Secondary: 03D65: Higher-type and set recursion theory 03F15: Recursive ordinals and ordinal notations 03F35: Second- and higher-order arithmetic and fragments
Type
Article
Information
The Journal of Symbolic Logic , Volume 86 , Issue 2 , June 2021 , pp. 709 - 745
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Copyright
© The Association for Symbolic Logic 2021

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