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Finite level Borel games and a problem concerning the jump hierarchy

Published online by Cambridge University Press:  12 March 2014

Harold T. Hodes*
Affiliation:
Cornell University, Ithaca, New York 14853

Extract

The jump hierarchy of Turing degrees assigns to each ξ < (ℵ1)L the degree 0(ξ); we presuppose familiarity with its definition and with the basic terminology of [5]. Let λ be a limit ordinal, λ < (ℵ1)L. The central result of [5] concerns the relation between 0(λ) and exact pairs on Iλ = {0(ξ)ξ < λ}. In [6] this question is raised: Where a is an upper bound on Iλ, how far apart are a and 0(λ)? It is there shown that if λ is locally countable and admissible, they may be very far apart: 0(λ) = the least member of {a(Ind(λ))∣, a is an upper bound on Iλ}; this is rather pathological, for Ind(λ) may be larger than λ. If λ is locally countable but neither admissible nor a limit of admissibles, we are essentially in the case of λ < ; by results of Sacks [12] and Enderton and Putnam [2], 0(λ) = the least member of {a(2)a is an upper bound on Iλ}. If λ is not locally countable, Ind(λ) is neither admissible nor a limit of admissibles, so we are again in a case like that of λ < . But what if λ is locally countable and nonadmissible, but is a limit of admissibles? For the rest of this paper let λ be such an ordinal. The central result of this paper answers this question for some such λ.

Type
Research Article
Copyright
Copyright © Association for Symbolic Logic 1984

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References

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