I was reading this post about the DP completeness of the problem SAT-UNSAT (both are well defined in this post). The answer added a note at the end that states the class complexity DP differs from NP, unless NP = coNP.

I fail to see why.

I searched and I came across multiples posts such at this one and that one that prove that if SAT-UNSAT is in coNP, then NP = coNP. But unless the fact that SAT-UNSAT $\in NP \implies$ SAT-UNSAT $\in coNP$ (which I do not see), then those proofs are not exactly what would help me. The same goes for this question, I would need SAT-UNSAT $\in coNP$.

Question : Considering the first question (and the answer associated), if the problem SAT-UNSAT $\in NP$, why does NP = coNP.

My take : Well, I can see that the problem SAT-UNSAT is NP-hard and coNP-hard. If SAT-UNSAT $\in NP$, then SAT-UNSAT is NP-complete. This implies things such that the problem UNSAT (which is coNP-complete) is NP-hard since we can reduce UNSAT to SAT-UNSAT which is NP-complete. That's all I got and that doesn't really help.

I'd appreciate any clarification on the subject. Thanks to you all


1 Answer 1


Suppose that $X$ is a coNP problem which is NP-hard.

Let $A$ be any problem in NP. Since $X$ is NP-hard, there is a polytime reduction from $A$ to $X$. Since $X$ is in coNP, this shows that $A$ is in coNP.

We have shown that $\mathsf{NP} \subseteq \mathsf{coNP}$. Now suppose $B$ is in coNP. Then $\overline{B}$ is in NP, and so in coNP. Hence $B$ is in NP. This shows that also $\mathsf{coNP} \subseteq \mathsf{NP}$, and so $\mathsf{NP} = \mathsf{coNP}$.

  • $\begingroup$ Are you implying that if a problem DP is in NP, then is also is in coNP? Because my question is about SAT-UNSAT (or any DP-complete problem really), and nowhere does it say that it is in coNP. $\endgroup$
    – Exeloz
    Commented Sep 30, 2020 at 13:41
  • $\begingroup$ Let's suppose that a DP-complete problem is in NP, then why is it that the problem is in coNP? $\endgroup$
    – Exeloz
    Commented Sep 30, 2020 at 13:43
  • $\begingroup$ Switch NP and coNP on my answer everywhere. $\endgroup$ Commented Sep 30, 2020 at 14:08
  • $\begingroup$ Well, I could have figured this out on my own really... All this post wasn't really needed after all $\endgroup$
    – Exeloz
    Commented Sep 30, 2020 at 14:12

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