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UNIVERSAL ROSSER PREDICATES

Published online by Cambridge University Press:  21 March 2017

MAKOTO KIKUCHI
Affiliation:
GRADUATE SCHOOL OF SYSTEM INFORMATICS KOBE UNIVERSITY, 1-1 ROKKODAI NADA, KOBE 657-8501, JAPAN E-mail: mkikuchi@kobe-u.ac.jp
TAISHI KURAHASHI
Affiliation:
DEPARTMENT OF NATURAL SCIENCES NATIONAL INSTITUTE OF TECHNOLOGY KISARAZU COLLEGE, 2-11-1 KIYOMIDAI-HIGASHI KISARAZU, CHIBA 292-0041, JAPAN E-mail: kurahashi@nebula.n.kisarazu.ac.jp
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Abstract

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Gödel introduced the original provability predicate in the proofs of Gödel’s incompleteness theorems, and Rosser defined a new one. They are equivalent in the standard model ${\mathbb N}$ of arithmetic or any nonstandard model of ${\rm PA} + {\rm Con_{PA}} $ , but the behavior of Rosser’s provability predicate is different from the original one in nonstandard models of ${\rm PA} + \neg {\rm Con_{PA}} $ . In this paper, we investigate several properties of the derivability conditions for Rosser provability predicates, and prove the existence of a Rosser provability predicate with which we can define any consistent complete extension of ${\rm PA}$ in some nonstandard model of ${\rm PA} + \neg {\rm Con_{PA}} $ . We call it a universal Rosser predicate. It follows from the theorem that the true arithmetic ${\rm TA}$ can be defined as the set of theorems of ${\rm PA}$ in terms of a universal Rosser predicate in some nonstandard model of ${\rm PA} + \neg {\rm Con_{PA}} $ . By using this theorem, we also give a new proof of a theorem that there is a nonstandard model M of ${\rm PA} + \neg {\rm Con_{PA}} $ such that if N is an initial segment of M which is a model of ${\rm PA} + {\rm Con_{PA}} $ then every theorem of ${\rm PA}$ in N is a theorem of $\rm PA$ in ${\mathbb N}$ . In addition, we prove that there is a Rosser provability predicate such that the set of theorems of $\rm PA$ in terms of the Rosser provability predicate is inconsistent in any nonstandard model of ${\rm PA} + \neg {\rm Con_{PA}} $ .

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Copyright © The Association for Symbolic Logic 2017