@article{DE_1976_12_7_a10,
     author = {A. P. Sadovskii},
     title = {The problem of distinguishing the center and a focus for systems with nonzero linear part},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1238--1246},
     publisher = {mathdoc},
     volume = {12},
     number = {7},
     year = {1976},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1976_12_7_a10/}
}