Well, for the mathematically curious, here are a few other interesting prime number theroms: For any number n which is prime, (2^n)-1 is also prime (Mersenne's theorem). For any number n (2^(2^n))+1 is prime. (I might have that wrong, I don't remember exactly) For any number n, if the square root of (n!)+1 is an integer, it is also prime. (This is interesting, but rather useless in practice)