We prove that a partially ordered set of all computably enumerable (c. e.) degrees that are the least upper bounds of two superlow c. e. degrees is an upper semilattice not elementary equivalent to the semilattice of all ...
In this paper we prove the following theorem: For every notation of a constructive ordinal there exists a low 2-computably enumerable degree that is not splittable into two lower 2-computably enumerable degrees whose jumps ...
We look at infinite levels of the Ershov hierarchy in the natural system of notation, which are proper for jumps of sets. It is proved that proper infinite levels for jumps are confined to δa -1-levels, where a stands for ...