Even if there is no general algorithm to decide if any program will halt, but there could be properties or meta-questions about the programs that is decidable. For example, given program $A$ and a program $B$ that is obtainable from program $A$ by adding in finite steps of calculations that halts. So if $A$ halts, $B$ definitely halts (and vice versa). A trivial example is $B = A$.

The question "Do $A$ and $B$ have the same behaviour?" is decidable (they either both halt or run forever), even if we do not know whether $A$ or $B$ halts or not.

My question: Is it true that, for any program $A$, there is always a non-trivial program $B$ such that there is a decidable meta-question about the duo $(A, B)$? (Non-trivial means $B ≠ A$ and $B$ is not obtainable from $A$ via adding extra finite steps that halts. In general, it means $B\neq f(A)$, where $f$ is a computable function.)

I wonder if there is a field of research on this type of meta-problems?

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    $\begingroup$ "Do A and B have the same behavior" is very much not decidable. If it were I could just set A to a known halting machine and then ask whether arbitrary program B halts by asking A = B? $\endgroup$ – orlp Oct 10 at 7:52
  • $\begingroup$ @orlp I have edited the question to mention that the finite extra steps halts. $\endgroup$ – Danny Oct 10 at 8:38
  • $\begingroup$ Your non-triviality requirement doesn't make sense: for any specific $A$ and $B$ (with any relationship at all) there is a computable function on Turing machine indices sending $A$ to $B$ - namely, just send everything to $B$. Precisely defining nontriviality in this context is going to be - appropriately enough - nontrivial. $\endgroup$ – Noah Schweber Oct 11 at 20:47
  • $\begingroup$ @NoahSchweber, the non-triviality is suppose to mean one can’t find out about the computability of $B$, given the computability of $A$. I think sending $A$ to $B$ does not achieve that. $\endgroup$ – Danny Oct 13 at 7:58

I'm not convinced by your setup. All programs are finite sequences of steps, so the equivalence of $A$ and $B$ in your first paragraph isn't decidable: they're equivalent iff $B$ halts on every input that $A$ halts on.

Your main question seems to be under-specified in a way that makes it trivial. The empty langauge is a decidable problem about pairs $(A,B)$. For any computable function $f$, the language $\{(A,B)\mid B=f(A)\}$ is decidable. But I suspect that any attempt to make the problem more precise will fail because of the issue I raised above.

  • $\begingroup$ Thanks. I think you have stated what I was trying to capture. Can I amend the question to: by non-trivial it means B is not a computable function of A. $\endgroup$ – Danny Oct 10 at 8:26

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