Cupping and noncupping in the enumeration degrees of ∑20 sets
S. Barry Cooper, Andrea Sorbi, Xiaoding Yi · Annals of Pure and Applied Logic · 1996
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.