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A hierarchy for the plus cupping Turing degrees

Published online by Cambridge University Press:  12 March 2014

Yong Wang  and
Angsheng Li
Yong Wang
Affiliation:
Institute of Software, Chinese Academy of Sciences, P.O. Box 8718, Beijing 100080, P. R. China, E-mail: wangyong@ios.ac.cn
Angsheng Li
Affiliation:
Institute of Software, Chinese Academy of Sciences, P.O. Box 8718, Beijing 100080, P. R. China School of Information Science, Beijing Normal University, Beijing, P. R. China, E-mail: angsheng@ios.ac.cn
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Abstract

We say that a computably enumerable (c. e.) degree a is plus-cupping, if for every c.e. degree x with 0 < x ≤ a, there is a c. e. degree y ≠ 0′ such that x ∨ y = 0′. We say that a is n-plus-cupping, if for every c. e. degree x, if 0 < x ≤ a, then there is a lown c. e. degree I such that x ∨ I = 0′. Let PC and PCn be the set of all plus-cupping, and n-plus-cupping c. e. degrees respectively. Then PC1 ⊆ PC2 ⊆ PC3 = PC. In this paper we show that PC1 ⊂ PC2, so giving a nontrivial hierarchy for the plus cupping degrees. The theorem also extends the result of Li, Wu and Zhang [14] showing that LC1 ⊂ LC2, as well as extending the Harrington plus-cupping theorem [8].

Type
Research Article
Information
The Journal of Symbolic Logic , Volume 68 , Issue 3 , September 2003 , pp. 972 - 988
Copyright
Copyright © Association for Symbolic Logic 2003

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References

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