Question

Prove that (2^n) + 1 cannot be a prime number if n has an odd prime divisor.

Answer 1

So n=(2k+1)*m. Thus 2^n +1 = (2+1)*(...)

Answer 2

2^n+1 or 2^n-1?

Answer 3

kinda looks like this one
http://en.wikipedia.org/wiki/Mersenne_prime