Leibniz's Formula for Pi

Theorem

 * $\dfrac \pi 4 = 1 - \dfrac 1 3 + \dfrac 1 5 - \dfrac 1 7 + \dfrac 1 9 - \cdots \approx 0 \cdotp 78539 \, 81633 \, 9744 \ldots$

That is:
 * $\ds \pi = 4 \sum_{k \mathop \ge 0} \paren {-1}^k \frac 1 {2 k + 1}$

Also known as
Some sources refer to this as Gregory's series for $\pi$.

Also see

 * Gregory Series