Today is the $\pi$ day. In american date form today is 3/14, a poor approximation to $\pi$.

Some ways to calculate $\pi$ are:

• Leibniz’s series: $\sum_{n=0}^{\infty }{{{\left(-1\right)^{n}}\over{2\,n+1}}}=\frac{1}{1} - \frac{1}{3} + \frac{1}{5} - \frac{1}{7} + \frac{1}{9} - \cdots = \frac{\pi}{4}$
• Euler’s series: $\sum_{n=0}^{\infty }\cfrac{2^n n!^2}{(2n + 1)!}=1 + \frac{1}{3} + \frac{1 \cdot 2}{3 \cdot 5} + \frac{1 \cdot 2 \cdot 3}{3 \cdot 5 \cdot 7} + \cdots = \frac{\pi}{2}$
• Wallis’ product: $\prod_{n=1}^{\infty }{{{4\,n^2}\over{4\,n^2-1}}}=\frac{2}{1} \cdot \frac{2}{3} \cdot \frac{4}{3} \cdot \frac{4}{5} \cdot \frac{6}{5} \cdot \frac{6}{7} \cdot \frac{8}{7} \cdot \frac{8}{9} \cdots = \frac{\pi}{2}$
• Easiest way :P: $atan(1)=\frac{\pi}{4}$

Have fun trying to generate a lot of $\pi$ digits and burn your CPU!