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Okay, it is trivially true that if n = 1, then a can not just be 2 but, indeed, any prime number. After looking at the question, it turns out that I've overlooked a small detail to exclude what is called the trivial case.
By the way, these trivial cases are more common than one might think but they're generally discarded 'cause... well... they're trivial!
Anyway, here's the amendment. Enjoy!
Prove that for n > 1, if a^n - 1 is prime, then a = 2.
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