@article{DE_1983_19_4_a1,
     author = {N. I. Vulpe},
     title = {Minimal complete system of orthogonal invariants of a two-dimensional differential system},
     journal = {Differencialʹnye uravneni\^a},
     pages = {564--569},
     publisher = {mathdoc},
     volume = {19},
     number = {4},
     year = {1983},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1983_19_4_a1/}
}