%0 Journal Article
%A A. P. Sadovskii
%T On the problem of discrimination of center and focus
%J Differencialʹnye uravneniâ
%D 1969
%P 326-330
%V 5
%N 2
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DE_1969_5_2_a10/
%G ru
%F DE_1969_5_2_a10