%0 Journal Article
%A A. P. Sadovskii
%T A solution for the problem of center and focus for certain systems of non-linear oscillations
%J Differencialʹnye uravneniâ
%D 1978
%P 380-382
%V 14
%N 2
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DE_1978_14_2_a25/
%G ru
%F DE_1978_14_2_a25