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I'm taking an algorithms course and we are covering polynomial time reductions, and I've read online that many decision problems are polynomial-time reducible to their complements.

Can anyone give me an example of one such decision problem?

I tried to reduce the Composite decision problem to Prime, but don't really know how to go about it.

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    $\begingroup$ I did a Google search for "decision problems are polynomial-time reducible to their complements" and got a number of hits. Some of them look promising. $\endgroup$ – marty cohen Mar 26 '15 at 20:49
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Here's how to polynomial-time reduce an instance of the COMPOSITE decision problem to the PRIME decision problem.

  1. Consider an instance of COMPOSITE in the form of a number $n$.
  2. In polynomial time, determine if $n$ is prime (using the AKS algorithm).
  3. If $n$ is prime, let $m = 4$, otherwise let $m = 2$.
  4. $m$ is the corresponding instance of PRIME.
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