@article{DAN_1985_285_4_a10,
     author = {K. S. Sibirskii},
     title = {Centroaffine invariant center and simple saddle-center conditions for a quadratic differential system},
     journal = {Doklady Akademii Nauk},
     pages = {819--823},
     publisher = {mathdoc},
     volume = {285},
     number = {4},
     year = {1985},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1985_285_4_a10/}
}