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Interesting proof...
I was reading an old math book that had this interesting problem in it. It's not a text book, just a book on mathematical reasoning and problem solving, much like George Polya's classic, How to solve it!
My book is called Doing Mathematics: an Introduction to Proofs and Problem Solving, by Steven Galovich.
Prove that if a^n - 1 is prime, then a = 2.
Although the proof is far from difficult, I wouldn't be surprised if this thread ends up as popular as blizzak's thread, Very Difficult Proof...
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