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The Gamma question for many-one degrees

Logic 2017-09-29 v2

Abstract

A set AA is coarsely computable with density r[0,1]r \in [0,1] if there is an algorithm for deciding membership in AA which always gives a (possibly incorrect) answer, and which gives a correct answer with density at least rr. To any Turing degree a\mathbf{a} we can assign a value ΓT(a)\Gamma_T(\mathbf{a}): the minimum, over all sets AA in a\mathbf{a}, of the highest density at which AA is coarsely computable. The closer ΓT(a)\Gamma_T(\mathbf{a}) is to 11, the closer a\mathbf{a} is to being computable. Andrews, Cai, Diamondstone, Jockush, and Lempp noted that ΓT\Gamma_T can take on the values 00, 1/21/2, and 11, but not any values in strictly between 1/21/2 and 11. They asked whether the value of ΓT\Gamma_T can be strictly between 00 and 1/21/2. This is the Gamma question. Replacing Turing degrees by many-one degrees, we get an analogous question, and the same arguments show that Γm\Gamma_m can take on the values 00, 1/21/2, and 11, but not any values strictly between 1/21/2 and 11. We will show that for any r[0,1/2]r \in [0,1/2], there is an mm-degree a\mathbf{a} with Γm(a)=r\Gamma_m(\mathbf{a}) = r. Thus the range of Γm\Gamma_m is [0,1/2]{1}[0,1/2] \cup \{1\}. Benoit Monin has recently announced a solution to the Gamma question for Turing degrees. Interestingly, his solution gives the opposite answer: the only possible values of ΓT\Gamma_T are 00, 1/21/2, and 11.

Keywords

Cite

@article{arxiv.1606.05701,
  title  = {The Gamma question for many-one degrees},
  author = {Matthew Harrison-Trainor},
  journal= {arXiv preprint arXiv:1606.05701},
  year   = {2017}
}

Comments

9 pages

R2 v1 2026-06-22T14:28:22.578Z