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REFLECTION RANKS AND ORDINAL ANALYSIS

Part of: Proof theory and constructive mathematics

Published online by Cambridge University Press:  10 July 2020

FEDOR PAKHOMOV  and
JAMES WALSH
FEDOR PAKHOMOV
Affiliation:
STEKLOV MATHEMATICAL INSTITUTE OF RUSSIAN ACADEMY OF SCIENCES 8, GUBKINA STREET, MOSCOW119991, RUSSIAN FEDERATION and INSTITUTE OF MATHEMATICS OF THE CZECH ACADEMY OF SCIENCES, ŽITNÁ 25, 115 67 PRAHA 1, CZECH REPUBLIC E-mail: pakhfn@mi-ras.ruE-mail: pakhomov@math.cas.cz
JAMES WALSH
Affiliation:
DEPARTMENT OF PHILOSOPHY CORNELL UNIVERSITYITHACA, NY14853, USAE-mail: jameswalsh@cornell.edu
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Abstract

It is well-known that natural axiomatic theories are well-ordered by consistency strength. However, it is possible to construct descending chains of artificial theories with respect to consistency strength. We provide an explanation of this well-orderedness phenomenon by studying a coarsening of the consistency strength order, namely, the $\Pi ^1_1$ reflection strength order. We prove that there are no descending sequences of $\Pi ^1_1$ sound extensions of $\mathsf {ACA}_0$ in this ordering. Accordingly, we can attach a rank in this order, which we call reflection rank, to any $\Pi ^1_1$ sound extension of $\mathsf {ACA}_0$ . We prove that for any $\Pi ^1_1$ sound theory T extending $\mathsf {ACA}_0^+$ , the reflection rank of T equals the $\Pi ^1_1$ proof-theoretic ordinal of T. We also prove that the $\Pi ^1_1$ proof-theoretic ordinal of $\alpha $ iterated $\Pi ^1_1$ reflection is $\varepsilon _\alpha $ . Finally, we use our results to provide straightforward well-foundedness proofs of ordinal notation systems based on reflection principles.

Keywords

reflection principlesordinal analysissecond-order arithmeticfirst-order arithmetic

MSC classification

Primary: 03F15: Recursive ordinals and ordinal notations
Secondary: 03F35: Second- and higher-order arithmetic and fragments
Type
Article
Information
The Journal of Symbolic Logic , Volume 86 , Issue 4 , December 2021 , pp. 1350 - 1384
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Copyright
© Association for Symbolic Logic 2020

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