@article{DE_1991_27_11_a22,
     author = {N. L. Shcheglova},
     title = {A criterion for the coexistence of a center and a focus of the equation $y'=(x+\lambda y+Q_3)/(-y+\lambda x+P_3)$},
     journal = {Differencialʹnye uravneni\^a},
     pages = {2001--2005},
     publisher = {mathdoc},
     volume = {27},
     number = {11},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1991_27_11_a22/}
}