%0 Journal Article
%A A. P. Sadovskii
%T The problem of the center and focus for analytic systems with a zero linear part. I
%J Differencialʹnye uravneniâ
%D 1989
%P 790-799
%V 25
%N 5
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DE_1989_25_5_a7/
%G ru
%F DE_1989_25_5_a7