We currently don't know the relationship between NP and co-NP, but would it be possible to show whether the intersection is equal to P? I can't think of any languages in both NP and co-NP, but not in P.
closed as off-topic by Emil Jeřábek supports Monica, Gamow, D.W., Jan Johannsen, Aryeh Feb 14 at 14:42
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Proving that $P= NP \cap$ co-$NP$ is an open problem and believed to be unlikely since Integer factoring decision problem is in both $NP$ and co-$NP$ but conjectured to be outside $P$.