Skip to main content
2 of 4
added 3 characters in body
Joel David Hamkins
  • 236.5k
  • 44
  • 777
  • 1.4k

Yes. Let $Q$ be the set of sequences that separate a fixed computably inseparable pair $A$ and $B$, such as the set $A$ of programs that halt with output $0$ and the set $B$ of programs that halt with output $1$. There is no computable separation of $A$ and $B$, and so all elements of $Q$ are noncomputable, but $Q$ is effectively closed, because a finite sequence has no extension in $Q$ just in case it places a $0$ for an element of $A$ or a $1$ for an element of $B$, and this can be recognized in finite time.

Joel David Hamkins
  • 236.5k
  • 44
  • 777
  • 1.4k