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Re:2^74207281-1 is Prime by theandrewbailey on Tuesday January 19, 2016 @12:25PM (#10932185)
But 5 is not 2^n-1
Re:2^74207281-1 is Prime by eggy on Tuesday January 19, 2016 @03:25PM (#10933697)
5 is a prime number true, but not a Mersenne prime number. The Mersenne prime number 31 has 5 as its exponent n [1].

[1] http://mathworld.wolfram.com/MersennePrime.html