@article{702180,
     title = {Asymptotic formulas for the solutions of linear differential equations of the second order},
     booktitle = {Differential Equations and Their Applications},
     series = {GDML\_Books},
     publisher = {Publishing House of the Czechoslovak Academy of Sciences},
     address = {Prague},
     year = {1963},
     pages = {131-135},
     mrnumber = {MR0170064},
     zbl = {0138.32503},
     url = {http://dml.mathdoc.fr/item/702180}
}

Ráb, M. Asymptotic formulas for the solutions of linear differential equations of the second order, dans Differential Equations and Their Applications, GDML_Books,  (1963), pp. 131-135. http://gdmltest.u-ga.fr/item/702180/