Conjecture: suppose n is an integer larger than 1 and n is not prime. Then 2^n-1 is not prime. Proof attached. Could someone please explain to me how they got to xy= 2^(ab)-1. I see the -1 part. Also I think I do not understand the concept of 2^((a-1)b) I mean is it some index or some way of showing it is finite? I am confused.