2/PI = 2/2 * ( 2 + 2 )/2 * (2 + ( ( 2 + 2) ) ) * ...c

Leibnitz's Formula

PI/4 = 1/1 - 1/3 + 1/5 - 1/7 + ...

Wallis Product

PI/2 = 2/1 * 2/3 * 4/3 * 4/5 * 6/5 * 6/7 * ...

2/PI = (1 - 1/2^2)(1 - 1/4^2)(1 - 1/6^2)...

Lord Brouncker's Formula

4/PI = 1 +        1
           ----------------
           2 +     3^2
               ------------
               2 +   5^2
                  ---------
                  2 + 7^2 ...

(PI^2)/8 = 1/1^2 + 1/3^2 + 1/5^2 + ...

(PI^2)/24 = 1/2^2 + 1/4^2 + 1/6^2 + ...