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  1. (1 other version)Cupping and noncupping in the enumeration degrees of ∑20 sets.S. Barry Cooper, Andrea Sorbi & Xiaoding Yi - 1996 - Annals of Pure and Applied Logic 82 (3):317-342.
    We prove the following three theorems on the enumeration degrees of ∑20 sets. Theorem A: There exists a nonzero noncuppable ∑20 enumeration degree. Theorem B: Every nonzero Δ20enumeration degree is cuppable to 0′e by an incomplete total enumeration degree. Theorem C: There exists a nonzero low Δ20 enumeration degree with the anticupping property.
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