%0 Journal Article
%A E. D. Zhitelzeif
%T Tests for the existence of limit cycles for systems of limit cycles for systems of the form $\dot{x}=y$, $\dot{y}=-\sum_{k=0}^n f_k(x)y^k$
%J Differencialʹnye uravneniâ
%D 1975
%P 1702-1704
%V 11
%N 9
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DE_1975_11_9_a19/
%G ru
%F DE_1975_11_9_a19