@article{DAN_1976_230_2_a0,
     author = {A. D. Bruno},
     title = {Normal form and averaging methods},
     journal = {Doklady Akademii Nauk},
     pages = {257--260},
     publisher = {mathdoc},
     volume = {230},
     number = {2},
     year = {1976},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1976_230_2_a0/}
}