clipped from: mathworld.wolfram.com   
Tupper%27s%20formula

J. Tupper concocted the amazing formula

1/2%3C%7C_mod%28%7C_y/%2817%29_%7C2%5E%28-17%7C_x_%7C-mod%28%7C_y_%7C%2C17%29%29%2C2%29_%7C%2C