@article{DE_1973_9_4_a8,
     author = {A. P. Sadovskii},
     title = {The problem of distinguishing the center and the focus},
     journal = {Differencialʹnye uravneni\^a},
     pages = {650--659},
     publisher = {mathdoc},
     volume = {9},
     number = {4},
     year = {1973},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1973_9_4_a8/}
}