@article{DE_2000_36_8_a3,
     author = {A. F. Filippov},
     title = {The infinitesimal order of perturbations preserving the instability of a singular point},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1037--1044},
     publisher = {mathdoc},
     volume = {36},
     number = {8},
     year = {2000},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_2000_36_8_a3/}
}