Frank E. Harris
Department of Physics, University of Utah
Quantum Theory Project, University of Florida
Applied Numerical Math. 34, 95-98 (2000)
The exponential integral occurs in many physical problems and is relatively cumbersome to evaluate for an intermediate range of its argument. It would be helpful to expand that integral as a quadratic series in spherical Bessel functions, but the mathematics literature contains no expansions of this type parallel to the well-known such expansion of the sine integral. The missing expansion, and an analogous expansion of cos 2x, are presented and verified.