int is_prime(unsigned long n)
{
    unsigned long i, n_sqrt;

    if ( n<=3 )
      return 1;
    if ( (n%2)==0 )
      return 0;

    n_sqrt = (unsigned long)sqrt((double)n);
    n_sqrt++;
    for (i = 3; i <= n_sqrt; i += 2)
       if (n % i == 0)
         return 0;

    return 1;
}