complexity
(Co-NP) The set (or property) of problems with a yes/no answer where the complementary no/yes problem takes nondeterministic polynomial time (NP).
For example, "Is n prime" is Co-NP and "Is n not prime" is NP, since it is only necessary to find one factor to prove that n is not prime whereas to prove that it is prime all possible factors must be eliminated.
(2009-05-21)