@article{DE_1965_1_3_a1,
     author = {N. A. Lukashevich},
     title = {The isochronism of a center of certain systems of differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {295--302},
     publisher = {mathdoc},
     volume = {1},
     number = {3},
     year = {1965},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1965_1_3_a1/}
}