@article{DE_1970_6_10_a2,
     author = {N. F. Otrokov},
     title = {Conditions for the existence of a loop of a separatrix},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1761--1771},
     publisher = {mathdoc},
     volume = {6},
     number = {10},
     year = {1970},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1970_6_10_a2/}
}