@article{DE_1969_5_2_a10,
     author = {A. P. Sadovskii},
     title = {On the problem of discrimination of center and focus},
     journal = {Differencialʹnye uravneni\^a},
     pages = {326--330},
     publisher = {mathdoc},
     volume = {5},
     number = {2},
     year = {1969},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1969_5_2_a10/}
}