@article{DE_1982_18_6_a18,
     author = {V. V. Amel'kin},
     title = {Isochrony in cases of a center and a structurally unstable focus},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1073--1075},
     publisher = {mathdoc},
     volume = {18},
     number = {6},
     year = {1982},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1982_18_6_a18/}
}