@article{ZNSL_1990_186_a16,
     author = {I. N. Yakushina},
     title = {Asymptotic expansions of the solutions of the $n$-order differential equations with two turning points},
     journal = {Zapiski Nauchnykh Seminarov POMI},
     pages = {172--179},
     year = {1990},
     volume = {186},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/ZNSL_1990_186_a16/}
}