@article{DE_1972_8_9_a18,
     author = {M. V. Dolov},
     title = {Limit cycles in the case of the center},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1691--1692},
     publisher = {mathdoc},
     volume = {8},
     number = {9},
     year = {1972},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1972_8_9_a18/}
}