Asymptotic approximations for the first incomplete elliptic integral near logarithmic singularity

Abstract

We find two convergent series expansions for Legendre's first incomplete elliptic integral $F(\lambda,k)$ in terms of recursively computed elementary functions. Both expansions are valid at every point of the unit square $0<\lambda,k<1$. Truncated expansions yield asymptotic approximations for $F(\lambda,k)$ as $\lambda$ and/or $k$ tend to unity, including the case when logarithmic singularity $\lambda=k=1$ is approached from any direction. Explicit error bounds are given at every order of approximation. For the reader's convenience we present explicit expressions for low-order approximations and numerical examples to illustrate their accuracy. Our derivation is based on rearrangements of some known double series expansions, hypergeometric summation algorithms and inequalities for hypergeometric functions.