@article{DE_1965_1_4_a1,
     author = {A. P. Vorob'ev},
     title = {Qualitative investigation in the large of integral curves of isochronous systems of two differential equations},
     journal = {Differencialʹnye uravneni\^a},
     pages = {439--441},
     publisher = {mathdoc},
     volume = {1},
     number = {4},
     year = {1965},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1965_1_4_a1/}
}