M000002008

$$\pi=8\arctan\left(\frac{1}{2}\right)-16\arctan\left(\frac{1}{4}\right)+16\arctan\left(\frac{1}{5}\right)-4\arctan\left(\frac{1}{7}\right)+16\arctan\left(\frac{1}{21}\right)$$

Compact formula	`8[2] - 16[4] + 16[5] - 4[7] + 16[21]`
Lehmer's measure	8.35316