# Kepler–Bouwkamp constant

In plane geometry, the Kepler–Bouwkamp constant (or polygon inscribing constant) is obtained as a limit of the following sequence. Take a circle of radius 1. Inscribe a regular triangle in this circle. Inscribe a circle in this triangle. Inscribe a square in it. Inscribe a circle, regular pentagon, circle, regular hexagon and so forth. The radius of the limiting circle is called the Kepler–Bouwkamp constant. It is named after Johannes Kepler and Christoffel Bouwkamp [de], and is the inverse of the polygon circumscribing constant.

## Numerical value

The decimal expansion of the Kepler–Bouwkamp constant is (sequence A085365 in the OEIS)

$\prod _{k=3}^{\infty }\cos \left({\frac {\pi }{k}}\right)=0.1149420448\dots .$ The natural logarithm of the Kepler-Bouwkamp constant is given by
$-2\sum _{k=1}^{\infty }{\frac {2^{2k}-1}{2k}}\zeta (2k)\left(\zeta (2k)-1-{\frac {1}{2^{2k}}}\right)$ where $\zeta (s)=\sum _{n=1}^{\infty }{\frac {1}{n^{s}}}$ is the Riemann zeta function.

If the product is taken over the odd primes, the constant

$\prod _{k=3,5,7,11,13,17,\ldots }\cos \left({\frac {\pi }{k}}\right)=0.312832\ldots$ is obtained (sequence A131671 in the OEIS).