prime number algorithm
bool IsPrime(int n) { if (n == 2 || n == 3) return true; if (n <= 1 || n % 2 == 0 || n % 3 == 0) return false; for (int i = 5; i * i <= n; i += 6) { if (n % i == 0 || n % (i + 2) == 0) return false; } return true; }
Source: en.wikipedia.org