@article{DE_1980_16_10_a21,
     author = {N. V. Pyzhkova and A. P. Sadovskii},
     title = {A method for solving the problem of isochronism of the center},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1893--1895},
     publisher = {mathdoc},
     volume = {16},
     number = {10},
     year = {1980},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1980_16_10_a21/}
}