Math 2.0

Computer science Computational complexity: resources and classes Concept

NP-hardness

Summary

A problem is NP-hard when every problem in NP reduces to it by a polynomial-time reduction. An NP-hard problem is at least as hard as every problem in NP. A polynomial-time algorithm for one NP-hard problem would give one for all of NP, and so would prove that P equals NP. NP-hard problems need not lie in NP; some, such as the halting problem, are not even decidable.

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Further reading

  1. Web https://en.wikipedia.org/wiki/NP-hardness
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Changed3 November 2026

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