@article{DAN_1994_335_4_a2,
     author = {A. D. Bruno},
     title = {First approximations of differential equations},
     journal = {Doklady Akademii Nauk},
     pages = {413--416},
     publisher = {mathdoc},
     volume = {335},
     number = {4},
     year = {1994},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1994_335_4_a2/}
}