<complexity> The set or property of problems for which no polynomial-time algorithm is known.
This includes problems for which the only known algorithms require a number of steps which increases exponentially with the size of the problem, and those for which no algorithm at all is known. Within these two there are problems which are "provably difficult" and "provably unsolvable".
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Nearby terms: non-optimal solution « Nonpareil « non parity « non-polynomial » Non Return to Zero Inverted » nontrivial » Non-Uniform Memory Access