@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}}