%0 Journal Article
%A A. P. Sadovskii
%T Formal transformations and the problem of distinguishing the center and focus
%J Differencialʹnye uravneniâ
%D 1978
%P 2185-2189
%V 14
%N 12
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DE_1978_14_12_a10/
%G ru
%F DE_1978_14_12_a10