Can you please explain to me why the following is true?

Ιf 3SAT reduces to its complement then NP=coNP.

Thoughts: 3SAT is NP-complete so for every X in NP

$X \leq 3SAT$

$\overline {3SAT} $ is NP-complete so for every Y in coNP

$Y \leq \overline {3SAT} $

So, $X \leq 3SAT \leq \overline {3SAT}$

But don't we have to also prove that

$Y \leq 3SAT $?


2 Answers 2


For $A \in \mathsf{NP}$ you have $ A \le_p 3SAT \le_p \overline{3SAT} \in \mathsf{co{\text -}NP}$, which implies $A \in \mathsf{co{\text -}NP}$ and hence $\mathsf{NP} \subseteq \mathsf{co{\text -}NP}$.

Simmetrically, for $A \in \mathsf{co{\text -}NP}$, you have $A \le_p \overline{3SAT} \le_p 3SAT \in \mathsf{NP}$, which implies $A \in \mathsf{NP}$ and hence $\mathsf{co{\text -}NP} \subseteq \mathsf{NP}$.

From $\mathsf{NP} \subseteq \mathsf{co{\text -}NP} \subseteq \mathsf{NP}$, it follows that $\mathsf{NP} = \mathsf{co{\text -}NP}$.

  • $\begingroup$ don't understand why $\overline {3SAT} \le_p 3SAT$ as only the opposite is given @Steven $\endgroup$
    – Hjm
    Jan 10, 2023 at 12:40
  • 1
    $\begingroup$ @Hjm for that, use the hint 2 in my answer. $\endgroup$
    – Nathaniel
    Jan 10, 2023 at 13:15
  • $\begingroup$ Is it true that $\overline {\overline {3SAT} } =3SAT$? $\endgroup$
    – Hjm
    Jan 10, 2023 at 13:48
  • $\begingroup$ Yes, by definition of complement. $\endgroup$
    – Steven
    Jan 10, 2023 at 14:56

Hint 1: $Y\in \text{co}\mathsf{NP}$ if and only if $\overline{Y}\in\mathsf{NP}$.

Hint 2: $A\leqslant B$ if and only if $\overline{A}\leqslant \overline{B}$.

  • $\begingroup$ If I use hint 1: $3SAT \in coNP$ (because $3SAT \leq \overline {3SAT}$) so $\overline {3SAT} \in NP $and so every Y in coNp is also in NP? $\endgroup$
    – Hjm
    Jan 10, 2023 at 11:26
  • $\begingroup$ While the result would be what we want to obtain, the process is not quite it. I used the notation $Y$ to represent any problem $Y$ in $\text{co}\mathsf{NP}$, like in your post. $\endgroup$
    – Nathaniel
    Jan 10, 2023 at 11:41
  • $\begingroup$ I don't get what I have done wrong, can you help me? I suposse that I have to start with an $L \in coNP $ and so that it also belongs to NP. $\endgroup$
    – Hjm
    Jan 10, 2023 at 11:58

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