@article{IVM_1958_1_a5,
     author = {E. I. Zheleznov},
     title = {Some sufficient conditions for the existence of limit cycles},
     journal = {Izvesti\^a vys\v{s}ih u\v{c}ebnyh zavedenij. Matematika},
     pages = {56--59},
     publisher = {mathdoc},
     number = {1},
     year = {1958},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/IVM_1958_1_a5/}
}