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Given that $n \ge 2$ and $a^n - 1$ is prime, prove that $a = 2$, and $n$ must be prime.
Problem ID: 285 (01 Aug 2006) Difficulty: 3 Star
Given that $n \ge 2$ and $a^n - 1$ is prime, prove that $a = 2$, and $n$ must be prime.
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