@article{DE_1984_20_11_a7,
     author = {G. S. Rychkov},
     title = {The absence in the equation $P_1(x,y)dx=(y-P_3(x))dy$ of limit cycles surrounding three singular points},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1906--1910},
     publisher = {mathdoc},
     volume = {20},
     number = {11},
     year = {1984},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1984_20_11_a7/}
}