@article{DE_1993_29_10_a1,
     author = {N. I. Vulpe and I. V. Nikolaev},
     title = {Topological classification of a quadratic system with a four-fold singular point},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1669--1674},
     publisher = {mathdoc},
     volume = {29},
     number = {10},
     year = {1993},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1993_29_10_a1/}
}