Convexity and monotonicity for the elliptic integrals of the first kind and applications

Abstract

The elliptic integral and its various generalizations are playing very important and basic role in different branches of modern mathematics. It is well known that they cannot be represented by the elementary transcendental functions. Therefore, there is a need for sharp computable bounds for the family of integrals. In this paper, by virtue of two new tools, we study monotonicity and convexity of certain combinations of the complete elliptic integrals of the first kind, and obtain new sharp bounds and inequalities for them. In particular, we prove that the function $\mathcal{K}\left( \sqrt{% x}\right) /\ln \left( c/\sqrt{1-x}\right) $ is concave on $\left( 0,1\right) $ if and only if $c=e^{4/3}$, where $\mathcal{K}$ denotes the complete elliptic integrals of the first kind.