%0 Journal Article
%A A. P. Sadovskii
%T The problem of center and focus
%J Differencialʹnye uravneniâ
%D 1968
%P 2002-2009
%V 4
%N 11
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DE_1968_4_11_a8/
%G ru
%F DE_1968_4_11_a8