@article{MMO_1972_26_a6,
     author = {A. D. Bruno},
     title = {Analytic form of differential equations},
     journal = {Trudy Moskovskogo matemati\v{c}eskogo ob\^{s}estva},
     pages = {199--239},
     publisher = {mathdoc},
     volume = {26},
     year = {1972},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MMO_1972_26_a6/}
}