%0 Journal Article
%A A. P. Sadovskii
%T The problem of distinguishing the center and the focus
%J Differencialʹnye uravneniâ
%D 1973
%P 650-659
%V 9
%N 4
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DE_1973_9_4_a8/
%G ru
%F DE_1973_9_4_a8