Sharp double inequality for complete elliptic integral of the first kind

Abstract

For $r\in (0,1)$, the function $\K(r)=\int_0^{π/2}(1-r^2\sin^2t)^{-1/2}dt$ is known as the complete elliptic integral of the first kind. In this paper, we prove the absolute monotonicity of two functions involving $\K(r)$. As a consequence, we improve Alzer and Richards' result.