Kepler–Bouwkamp constant

Inscribe a regular triangle in this circle.

The radius of the limiting circle is called the Kepler–Bouwkamp constant.

[1] It is named after Johannes Kepler and Christoffel Bouwkamp [de], and is the inverse of the polygon circumscribing constant.

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

If the product is taken over the odd primes, the constant is obtained (sequence A131671 in the OEIS).