Good "Stressfull" Algorithm

I am thinking to make a code contest with my friends, but I need a good algorithm for testing. It should be simple, but should stress on the size and speed of the program. Any suggestions ??
Stress testing is pretty application specific, but generally you want to identify your bottleneck and focus your tests on that. If you don't know where your bottleneck, then either you have other concerns, or your application is small enough that this shouldn't even be a question.
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