@article{DE_1991_27_4_a2,
     author = {N. I. Vulpe},
     title = {Coefficient conditions for the number and multiplicity of singular points of a quadratic differential system},
     journal = {Differencialʹnye uravneni\^a},
     pages = {572--577},
     publisher = {mathdoc},
     volume = {27},
     number = {4},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1991_27_4_a2/}
}