# Thread: Primes

1. ## Primes

Prove that if a is grater than or equal to 2, n is grater than or equal to 2, and (a^n) -1 is prime, then a = 2 and n is a prime.

2. Originally Posted by wrighchr
Prove that if a is grater than or equal to 2, n is grater than or equal to 2, and (a^n) -1 is prime, then a = 2 and n is a prime.
Hint: $x^n - 1 = (x-1)(x^{n-1}+...+x+1)$.