@article{Stegun:1974:ACM,
author = {Stegun, Irene A. and Zucker, Ruth},
title = {Automatic computing methods for special functions. {I}{I}.
{T}he exponential integral $E_{n}(x)$},
journal = {J. Res. Nat. Bur. Standards Sect. B},
volume = {78B},
year = {1974},
pages = {199–216},
note = {Double-precision Fortran, maximum accuracy 16S.},
mrclass = {65D20},
mrnumber = {MR0362844 (50 \#15282)},
mrreviewer = {Bernard H. Rosman},
zblno = {0307.33001}}