@article{verbeeck:1970:rae,
    author = {Verbeeck, Paul},
     title = {Rational approximations for exponential integrals
              $E_{n}(x)$},
   journal = {Acad. Roy. Belg. Bull. Cl. Sci. (5)},
    volume = {56},
      year = {1970},
     pages = {1064–1072},
   mrclass = {41.17},
  mrnumber = {MR0283460 (44 \#691)},
mrreviewer = {A. Magnus},
     zblno = {0216.23005}}