@article{DE_1987_23_12_a25,
     author = {K. S. Sibirskii},
     title = {Coefficient conditions for the center in the sense of {Dulac} of a differential system with a zero characteristic root and homogeneous nonlinearities},
     journal = {Differencialʹnye uravneni\^a},
     pages = {2171--2173},
     publisher = {mathdoc},
     volume = {23},
     number = {12},
     year = {1987},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1987_23_12_a25/}
}