@article{olver:1965:eba,
    author = {Olver, F. W. J. and Stenger, F.},
     title = {Error bounds for asymptotic solutions of second-order
              differential equations having an irregular singularity of
              arbitrary rank},
   journal = {J. Soc. Indust. Appl. Math. Ser. B Numer. Anal.},
    volume = {2},
      year = {1965},
    number = {2},
     pages = {244–249},
   mrclass = {41.50 (34.00)},
  mrnumber = {MR0185351 (32 \#2819)},
       doi = {10.1137/0702018},
     zblno = {0173.34001}}