private static boolean isPrime( int n )
 {
     if( n == 2 || n == 3 )
         return true;

     if( n == 1 || n % 2 == 0 )
         return false;

     for( int i = 3; i * i <= n; i += 2 )
         if( n % i == 0 )
             return false;

     return true;
 }