Sierpiński's constant is a mathematical constant usually denoted as K. One way of defining it is as the following limit: K = lim n → ∞ [ ∑ k = 1 n r 2 k − π ln ⁡ n ] {\displaystyle K=\lim _{n\to \infty }\left[\sum _{k=1}^{n}{r_{2} \over k}-\pi \ln n\right]} where r2 is a number of representations of k as a sum of the form a2 + b2 for integer a and b. Wikipedia