\[ 1 - {1 \over 3} + {1 \over 5} - {1 \over 7} + {1 \over 9} - \cdots = {\pi \over 4} \]
\[\sum_{n=0}^\infty {(-1)^n \over 2n + 1} = {\pi \over 4}\]
\[{1 \over \pi} = {2 \sqrt{2} \over 99^2}\sum_{n=0}^\infty {(4n)!(1103 + 26390n) \over (4^n99^nn!)^4}\]