%0 Journal Article
%A A. P. Sadovskii
%T Conditions for the problem of the center and focus to arise for an $A_3$-system
%J Differencialʹnye uravneniâ
%D 1990
%P 1743-1753
%V 26
%N 10
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DE_1990_26_10_a11/
%G ru
%F DE_1990_26_10_a11