Sentence:  Let A be a prime number for which 2 is a <WSD>primitive element</WSD>. Then 2^{A-1}-1 is divisible by A. 
Meaning of the word marked with <WSD> tags:  Given a modulus "n", a number "g" such that every number coprime to "n" is congruent (modulo "n") to some power of "g"; "equivalently", a generator of the multiplicative field of integers modulo "n".