@article{DE_1975_11_9_a19,
     author = {E. D. Zhitelzeif},
     title = {Tests for the existence of limit cycles for systems of limit cycles for systems of the form $\dot{x}=y$, $\dot{y}=-\sum_{k=0}^n f_k(x)y^k$},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1702--1704},
     publisher = {mathdoc},
     volume = {11},
     number = {9},
     year = {1975},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1975_11_9_a19/}
}