NP Hard, NP Complete

• Reductions : use one problem to solve another

• For example, if we have an algorithm for HALT, we could use it to solve ACC.

• A problem is NP hard if solving it means we can solve every problem in NP

  • Specifically, there is a polytime reduction to the NP hard problem

• A problem is NP complete if it is NP hard and it is also in NP

  • These problems epitomize the class NP.

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