%0 Journal Article
%A K. S. Sibirskii
%T On some questions of the existence of the center
%J Differencialʹnye uravneniâ
%D 1968
%P 1931-1938
%V 4
%N 11
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DE_1968_4_11_a0/
%G ru
%F DE_1968_4_11_a0