%0 Journal Article
%A G. E. Popov
%T On the absence of periodic solutions taking the value $x=0$ in a class of differential equations of the form $\ddot x+x=F(x,\,\dot x)$
%J Izvestiâ vysših učebnyh zavedenij. Matematika
%D 1964
%P 120-122
%N 3
%I mathdoc
%U http://geodesic.mathdoc.fr/item/IVM_1964_3_a17/
%G ru
%F IVM_1964_3_a17