@article{DE_1974_10_12_a2,
     author = {N. I. Vulpe and K. S. Sibirskii},
     title = {Affine classification of a quadratic system},
     journal = {Differencialʹnye uravneni\^a},
     pages = {2111--2124},
     publisher = {mathdoc},
     volume = {10},
     number = {12},
     year = {1974},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1974_10_12_a2/}
}