@article{DE_1969_5_5_a20,
     author = {V. I. Chemodanov},
     title = {Isochronism in the case of a focus},
     journal = {Differencialʹnye uravneni\^a},
     pages = {964--966},
     publisher = {mathdoc},
     volume = {5},
     number = {5},
     year = {1969},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1969_5_5_a20/}
}