@article{DE_1983_19_3_a0,
     author = {N. I. Vulpe},
     title = {Affine-invariant conditions for topological distinction of quadratic systems in the presence of a center},
     journal = {Differencialʹnye uravneni\^a},
     pages = {371--379},
     publisher = {mathdoc},
     volume = {19},
     number = {3},
     year = {1983},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1983_19_3_a0/}
}