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Possible Duplicate:
Simple Mersenne prime divisibility proofs

I'm taking elementary number theory and there is this one question that I don't know where to start at... Please help me, thanks!

Prove that if $n$ is composite, then $2^n - 1$ is composite.

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    (And abstractly a duplicate of so many other questions that it's actually hard to find them all, but...)2012-09-24

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