@article{DE_1986_22_9_a7,
     author = {M. N. Popa and K. S. Sibirskii},
     title = {Focal cyclicity of critical points of a differential system},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1539--1545},
     publisher = {mathdoc},
     volume = {22},
     number = {9},
     year = {1986},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1986_22_9_a7/}
}