Cohesive sets: $A\subset \omega$, for each recursively enumerable set $W_e$, either $A\cap W_e$ is finite or $A\cap(\omega\setminus W_e)$ is finite.

Non-high degrees: Degree $a$ such that $a'\not\geq 0''$.

I'm wondering if it is possible to construct a cohesive set using some non-high 1-generic degree as an oracle? i.e. are there $A$ cohesive, and $B$ non-high 1-generic such that $A\leq_T B$? Thanks in advance!


1 Answer 1


The answer to this question is in Jockusch and Stephan's 1993 paper 'A cohesive set which is not high.' http://www.comp.nus.edu.sg/~fstephan/coh.ps

Take $A$ and $B$ such that $A \le_T B$, $A$ is Cohesive and $B' \not\ge_T 0''$. This implies that $A' \not \ge_T 0''$ and so by the paper mentioned above $A$ computes a diagonally-not-computable function. But no 1-generic can compute such a function hence $B$ cannot be 1-generic.

  • $\begingroup$ @Adam: Thank you! I would take a look at that paper! Is it also mentioned there why no 1-generic set could compute a DNC function? $\endgroup$
    – Jing Zhang
    Feb 9, 2013 at 19:13
  • $\begingroup$ No but this is not so difficult. Fix a functional $\Phi$, define $W$ by adding any string $\sigma$ such that for some $n$, $\varphi_n(n)\downarrow$ and $\Phi^\sigma(n) \ne \varphi_n(n)$. If $G$ is 1-generic and $G$ meets $W$ the $\Phi^G$ is not DNC, if $G$ avoids $W$ (say at $\sigma$) then $\Phi^G$ cannot be total (otherwise build a computable DNC by looking at convergences compatible with $\sigma$). $\endgroup$
    – Adam Day
    Feb 10, 2013 at 16:27
  • $\begingroup$ @Adam: Thanks for the note. But I believe in your definition of $W$ it should be $\Phi^\sigma(n)=\varphi_n(n)$ $\endgroup$
    – Jing Zhang
    Feb 11, 2013 at 8:26

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