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CALIBRATING DETERMINACY STRENGTH IN LEVELS OF THE BOREL HIERARCHY

Published online by Cambridge University Press:  19 June 2017

SHERWOOD HACHTMAN
SHERWOOD HACHTMAN*
Affiliation:
DEPARTMENT OF MATHEMATICS UNIVERSITY OF CALIFORNIA, LOS ANGELES LOS ANGELES, CA90024, USA E-mail: [email protected]
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Abstract

We analyze the set-theoretic strength of determinacy for levels of the Borel hierarchy of the form $\Sigma _{1 + \alpha + 3}^0 $, for α < ω1. Well-known results of H. Friedman and D.A. Martin have shown this determinacy to require α + 1 iterations of the Power Set Axiom, but we ask what additional ambient set theory is strictly necessary. To this end, we isolate a family of weak reflection principles, Π1-RAPα, whose consistency strength corresponds exactly to the logical strength of ${\rm{\Sigma }}_{1 + \alpha + 3}^0 $ determinacy, for $\alpha < \omega _1^{CK} $. This yields a characterization of the levels of L by or at which winning strategies in these games must be constructed. When α = 0, we have the following concise result: The least θ so that all winning strategies in ${\rm{\Sigma }}_4^0 $ games belong to Lθ+1 is the least so that $L_\theta \models {\rm{``}}{\cal P}\left( \omega \right)$ exists, and all wellfounded trees are ranked”.

Keywords

03F3503E60Borel determinacyadmissible setsreflectioninductive definability
Type
Articles
Information
The Journal of Symbolic Logic , Volume 82 , Issue 2 , June 2017 , pp. 510 - 548
Copyright
Copyright © The Association for Symbolic Logic 2017 

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