Cupping the Recursively Enumerable Degrees by D.R.E. Degrees
Angsheng Li, Xiaoding Yi · Proceedings of the London Mathematical Society · 1999
We prove that there are two incomplete d.r.e. degrees (the Turing degrees of differences of two recursively enumerable sets) such that every non-zero recursively enumerable degree cups at least one of them to 0′, the greatest recursively enumerable (Turing) degree. 1991 Mathematics Subject Classification: primary 03D25, 03D30; secondary 03D35.