2^n-1, 2^n, 2^n+1 are three consecutive numbers.
2^n-1 is not divisible by 3 because it it prime.
2^n is not divisible by 3 because it is divisible by 2 and its powers.
Then 2^n+1 must be divisible by 3.
Therefore, 2^n+1 is composite.
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回复:A Better Proof
-passenger101-
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01/29/2008 postreply
10:34:51
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Great! -eom
-yma16-
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01/29/2008 postreply
11:19:01