computational complexity theory, co-NP is a complexity class. A problem is a member of co-NP if and only if its complement is in complexity class NP. In simple terms, co-NP is the class of problems for which efficiently verifiable proofs of "no" instances, sometimes called "counterexamples", exist.
An example of an NP-complete problem is the
subset sum problem: given a finite set of integers is there a non-empty subset which sums to zero? The complementary problem is in co-NP and asks: "given a finite set of integers, does every non-empty subset have a nonzero sum?" To give a proof of a "no" instance one must specify a non-empty subset which does sum to zero. This proof is then easy to verify.
P, the class of polynomial time solvable problems, is a subset of both NP and co-NP. P is thought to be a strict subset in both cases (and demonstrably cannot be strict in one case but not the other). NP and co-NP are also thought to be unequal. If so, then no NP-complete problem can be in co-NP and no co-NP-complete problem can be in NP.
This can be shown as follows. Assume that there is an NP-complete problem that is in co-NP. Since all problems in NP can be reduced to this problem it follows that for all problems in NP we can construct a non-deterministic Turing machine that decides the complement of the problem in polynomial time, i.e., NP is a subset of co-NP. From this it follows that the set of complements of the problems in NP is a subset of the set of complements of the problems in co-NP, i.e., co-NP is a subset of NP. Since we already knew that NP is a subset of co-NP it follows that they are the same. The proof for the fact that no co-NP-complete problem can be in NP is symmetrical.
If a problem can be shown to be in both NP and co-NP, that is generally accepted as strong evidence that the problem is probably not NP-complete (since otherwise NP = co-NP).
An example of a problem which is known to be in NP and in co-NP is
integer factorization: given positive integers "m" and "n" determine if "m" has a factor less than "n" and greater than one. Membership in NP is clear; if "m" does have such a factor then the factor itself is a certificate. Membership in co-NP is more subtle; one must list the prime factors of "m" and provide a primality certificatefor each one. Integer factorizationis often confused with the closely related primalityproblem. Both primality testing and factorization have long been known to be NP and co-NP problems. The AKS primality test, published in 2002, proves that primality testing also lies in P, while factorization may or may not have a polynomial-time algorithm. [Manindra Agrawal, Neeraj Kayal, Nitin Saxena, " [http://www.cse.iitk.ac.in/users/manindra/algebra/primality_v6.pdf PRIMES is in P] ", "Annals of Mathematics" 160 (2004), no. 2, pp. 781�793.]
* Complexity Zoo: [http://qwiki.caltech.edu/wiki/Complexity_Zoo#conp coNP]
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