Let P denote the class of decision problems solvable in polynomial time by a deterministic Turing machine. Let NP denote the class of decision problems for which there exists a polynomial p over N and a polynomial-time computable relation R on pairs of binary strings such that, for every input string x, the problem answers "yes" on x if and only if there exists a witness string w with ∣w∣≤p(∣x∣) for which R(x,w) holds. Let coNP denote the class of decision problems whose complements belong to NP. Does NP=coNP?