[science_man_88]

Originally Posted by Blackadder

Is that rhetorical? I really should get back into my math books.

Special as in: why are we calculating for 2^p-1 and not 2^p-2 as well. I guess the latter can't be prime?

no number divisible by a number other than 1 or itself is prime 2^p-2 = 2(2^p-1-1) therefore is divisible by 2 except when p-1 = 1 and therefore p=2 -- 2^p-1-1 = 1