@article{DAN_1965_161_2_a12,
     author = {K. S. Sibirskii},
     title = {On the number of limit cycles arising from a singular point of focus or center type},
     journal = {Doklady Akademii Nauk},
     pages = {304--307},
     publisher = {mathdoc},
     volume = {161},
     number = {2},
     year = {1965},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1965_161_2_a12/}
}