Extending Cooper’s theorem to Δ 3 0 Turing degrees
Victor Selivanov, Mars M. Yamaleev · Computability · 2018
In 1971 B. Cooper proved that there exists a 2-c.e. Turing degree which doesn’t contain a c.e. set. Thus, he showed that the second level of the Ershov hierarchy is proper. In this paper we investigate proper levels of some extensions of the Ershov h