@article{DE_1968_4_11_a8,
     author = {A. P. Sadovskii},
     title = {The problem of center and focus},
     journal = {Differencialʹnye uravneni\^a},
     pages = {2002--2009},
     publisher = {mathdoc},
     volume = {4},
     number = {11},
     year = {1968},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1968_4_11_a8/}
}