Sharp bounds for generalized elliptic integrals of the first kind

In this paper, we prove that the double inequality \begin{equation*} 1+\alpha r’^2<\frac{$\mathcal{K}$_{a}®}{\sin(\pi a)\log(e^{R(a)/2}/r’)}<1+\beta r’^2 \end{equation*} holds for all $a\in (0,1\mathrm{/}2]$ and $r\in (0,1)$ if and only if $\alpha \le \pi \mathrm{/}[R(a)\sin (\pi a)]-1$ and $\beta \ge a(1-a)$, where $r^{\mathrm{\prime }}=\sqrt{1-r^2}$, ${\mathcal{K}}_a(r)$ is the generalized elliptic integral of the first kind and $R(x)$ is the Ramanujan constant function. Besides, as the key tool, the series expression for the Ramanujan constant function $R(x)$ is given.