@article{DE_1977_13_8_a28,
     author = {V. V. Amel'kin},
     title = {Letter to the editors: {{\textquotedblleft}On} the question of the isochrony of the center of two-dimensional analytic differential systems{\textquotedblright} {(Differencial'nye} {Uravnenija} 13 (1977), no.~6, 971{\textendash}980)},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1537},
     year = {1977},
     volume = {13},
     number = {8},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1977_13_8_a28/}
}