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HARRINGTON’S PRINCIPLE IN HIGHER ORDER ARITHMETIC

  • YONG CHENG (a1) and RALF SCHINDLER (a1)

Abstract

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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References

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HARRINGTON’S PRINCIPLE IN HIGHER ORDER ARITHMETIC

  • YONG CHENG (a1) and RALF SCHINDLER (a1)

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