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Bounding computably enumerable degrees in the Ershov hierarchy
2006
Annals of Pure and Applied Logic
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Lachlan observed that any nonzero d.c.e. degree bounds a nonzero c.e. degree. In this paper, we study the c.e. predecessors of d.c.e. degrees, and prove that given a nonzero d.c.e. degree a, there is a c.e. degree b below a and a high d.c.e. degree d > b such that b bounds all the c.e. degrees below d. This result gives a unified approach to some seemingly unrelated results. In particular, it has the following two known theorems as corollaries: ( 1 ) there is a low c.e. degree isolating a high
doi:10.1016/j.apal.2005.10.004
fatcat:as3psv42jjakrbrhdetdqetwt4