@article{DE_1978_14_12_a10,
     author = {A. P. Sadovskii},
     title = {Formal transformations and the problem of distinguishing the center and focus},
     journal = {Differencialʹnye uravneni\^a},
     pages = {2185--2189},
     publisher = {mathdoc},
     volume = {14},
     number = {12},
     year = {1978},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1978_14_12_a10/}
}