@article{DE_1974_10_4_a1,
     author = {V. V. Amel'kin and N. A. Lukashevich},
     title = {Tests for the existence of the center, and its isochronism},
     journal = {Differencialʹnye uravneni\^a},
     pages = {585--590},
     publisher = {mathdoc},
     volume = {10},
     number = {4},
     year = {1974},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1974_10_4_a1/}
}