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HARRINGTON’S PRINCIPLE IN HIGHER ORDER ARITHMETIC

Published online by Cambridge University Press:  22 April 2015

YONG CHENG
Affiliation:
INSTITUT FÜR MATHEMATISCHE LOGIK UND GRUNDLAGENFORSCHUNG UNIVERSITÄT MÜNSTER EINSTEINSTR. 6248149 MÜNSTER, GERMANYE-mail:world-cyr@hotmail.comE-mail:rds@math.uni-muenster.de
RALF SCHINDLER
Affiliation:
INSTITUT FÜR MATHEMATISCHE LOGIK UND GRUNDLAGENFORSCHUNG UNIVERSITÄT MÜNSTER EINSTEINSTR. 6248149 MÜNSTER, GERMANYE-mail:world-cyr@hotmail.comE-mail:rds@math.uni-muenster.de
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Abstract

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Let Z2, Z3, and Z4 denote 2nd, 3rd, and 4th order arithmetic, respectively. We let Harrington’s Principle, HP, denote the statement that there is a real x such that every x-admissible ordinal is a cardinal in L. The known proofs of Harrington’s theorem “$Det\left( {{\rm{\Sigma }}_1^1} \right)$ implies 0♯ exists” are done in two steps: first show that $Det\left( {{\rm{\Sigma }}_1^1} \right)$ implies HP, and then show that HP implies 0♯ exists. The first step is provable in Z2. In this paper we show that Z2 + HP is equiconsistent with ZFC and that Z3 + HP is equiconsistent with ZFC + there exists a remarkable cardinal. As a corollary, Z3 + HP does not imply 0♯ exists, whereas Z4 + HP does. We also study strengthenings of Harrington’s Principle over 2nd and 3rd order arithmetic.

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Copyright © The Association for Symbolic Logic 2015