NP-hard problems are a class of problems in computational complexity theory that are at least as hard as the hardest problems in NP (nondeterministic polynomial time). The key properties of NP-hard problems include: 1. **Definition**: A problem is considered NP-hard if every problem in NP can be reduced to it in polynomial time. This means that if you could solve an NP-hard problem quickly (in polynomial time), you could also solve all NP problems quickly.
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