%0 Journal Article
%A V. V. Amel'kin
%T Letter to the editors: ``On the question of the isochrony of the center of two-dimensional analytic differential systems'' (Differencial'nye Uravnenija \textbf{13} (1977), no.~6, 971--980)
%J Differencialʹnye uravneniâ
%D 1977
%P 1537
%V 13
%N 8
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DE_1977_13_8_a28/
%G ru
%F DE_1977_13_8_a28