@article{DE_1977_13_6_a0,
     author = {V. V. Amel'kin},
     title = {On the question of the isochronism of the center of two-dimensional analytic differential systems},
     journal = {Differencialʹnye uravneni\^a},
     pages = {971--980},
     publisher = {mathdoc},
     volume = {13},
     number = {6},
     year = {1977},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1977_13_6_a0/}
}