Local definitions in degree structures: the Turing jump, hyperdegrees and beyond. (English) Zbl 1131.03018

\(D\) is the language of the Turing degrees with \(\leq\), \(\wedge\), and \(\vee\). Its (hyper)jump ideals are downward closed subsets of \(D\) closed also under (hyper)jump and join. The results given improve and extend work of Slaman and Woodin.
Using purely degree-theoretic methods, Shore shows how jump and double jump can be defined in any jump ideal \(I\) that contains \({\mathbf 0}^{(\omega)}\). The relation \({\mathbf x}''\leq {\mathbf y}''\) is defined in \(I\) by a \(\Pi_5\) formula, \({\mathbf w}= {\mathbf x}''\) by a \(\Pi_6\) and a \(\Sigma_6\), and \({\mathbf w}= {\mathbf x}'\) by a \(\Pi_8\) formula. Then every automorphism of \(I\) leaves fixed every degree \(\geq {\mathbf 0}''\). And for relations on \(I\) that are invariant under interchange of degrees with the same double jump or the same join with \({\mathbf 0}''\), those definable over \(I\) are exactly those definable in second-order arithmetic with set quantifiers ranging over sets whose degrees are in \(I\). Analagous conclusions are drawn for the hyperdegrees and their hyperjump ideals. Among open questions are whether the base \({\mathbf 0}''\) in the fixed cone of the result above can be lowered, and whether any jump ideal of \(D\) has uncountably many automorphisms.


03D28 Other Turing degree structures
03D30 Other degrees and reducibilities in computability and recursion theory
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