@article{SM_1969_8_4_a1,
     author = {M. V. Fedoryuk},
     title = {Asymptotic methods in the theory of ordinary linear differential equations},
     journal = {Sbornik. Mathematics},
     pages = {451--491},
     year = {1969},
     volume = {8},
     number = {4},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1969_8_4_a1/}
}