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I know one can show that by $\mathsf{coNP}\subseteq\mathsf{NEXP}=\mathsf{MIP}$. But here I would like to start with a $\mathsf{coNP}$-complete problem and show there is a two-prover one-round interactive proof system for it.

I read Luca Trevisan's lecture note, which says that it was once conjectured that $\mathsf{coNP}\not\subseteq\mathsf{MIP}$, but it turned out Babai, Fortnow and Lund showed $\mathsf{MIP}=\mathsf{NEXP}$. Also $\mathsf{coNP}$ does not have a constant-round interactive proof system unless the polynomial hierarchy collapses. So perhaps the question is not totally trivial, at least to me.

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    $\begingroup$ Do you have any reason to believe this is easier than the general MIP = NEXP result? $\endgroup$ Jun 13, 2023 at 6:51
  • $\begingroup$ Thanks for asking. That's exactly what I'm wondering. I would like to get intuition on why $\mathsf{coNP}\subseteq\mathsf{MIP}$ without knowing $\mathsf{MIP}=\mathsf{NEXP}$. $\endgroup$
    – user50394
    Jun 13, 2023 at 17:44
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    $\begingroup$ You can look at the SUMCHECK protocol---which shows that #P (and in particular coNP) is in IP---without reading the MIP = NEXP paper. It'd be interesting if multiple provers make this easier though (i.e., if coNP in MIP is easier than coNP in IP). $\endgroup$ Jun 14, 2023 at 17:21

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