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Low Level Nondelegability Results: Domination and Recursive Enumeration

  • Mingzhong Cai (a1) and Richard A. Shore (a2)


We study low level nondefinability in the Turing degrees. We prove a variety of results, including, for example, that being array nonrecursive is not definable by a Σ1 or Π1 formula in the language (≤, REA) where REA stands for the “r.e. in and above” predicate. In contrast, this property is definable by a Π2 formula in this language. We also show that the Σ1-theory of (, ≤, REA) is decidable.



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