@article{DE_1999_35_7_a0,
     author = {V. V. Amel'kin and K. M. Al-Haider},
     title = {Strong isochronism of polynomial differential systems with a center},
     journal = {Differencialʹnye uravneni\^a},
     pages = {867--873},
     publisher = {mathdoc},
     volume = {35},
     number = {7},
     year = {1999},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1999_35_7_a0/}
}