@article{DE_1977_13_5_a1,
     author = {N. I. Vulpe and K. S. Sibirskii},
     title = {Geometric classification of a quadratic differential system},
     journal = {Differencialʹnye uravneni\^a},
     pages = {803--814},
     publisher = {mathdoc},
     volume = {13},
     number = {5},
     year = {1977},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1977_13_5_a1/}
}