# Sierpiński's constant

Sierpiński's constant is a mathematical constant usually denoted as K. One way of defining it is by limiting the expression:

${\displaystyle K=\lim _{n\to \infty }\left[\sum _{k=1}^{n}{r_{2}(k) \over k}-\pi \ln n\right]}$

where r2(k) is a number of representations of k as a sum of the form a2 + b2 for integer a and b.

It can be given in closed form as:

{\displaystyle {\begin{aligned}K&=\pi \left(2\ln 2+3\ln \pi +2\gamma -4\ln \Gamma \left({\tfrac {1}{4}}\right)\right)\\&=\pi \ln \left({\frac {4\pi ^{3}e^{2\gamma }}{\Gamma \left({\tfrac {1}{4}}\right)^{4}}}\right)\\&=\pi \ln \left({\frac {e^{2\gamma }}{2G^{2}}}\right)\\&=2.584981759579253217065893587383\dots \end{aligned}}}

where ${\displaystyle G}$ is Gauss's constant and ${\displaystyle \gamma }$ is the Euler-Mascheroni constant.