The goal of this short "footnote" is to prove the following theorem used in the discussion of Mersenne primes.
Theorem.
If for some positive integer n, 2n-1 is prime, then so is n.
Proof.
Let r and s be positive integers, then the polynomial xrs-1 is xs-1 times xs(r-1) + xs(r-2) + ... + xs + 1. So if n is composite (say r.s with 1