%0 Journal Article
%A L. Z. Fishman
%T Preservation of the properties of second-order differential
%J Doklady Akademii Nauk
%D 1997
%P 739-741
%V 352
%N 6
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DAN_1997_352_6_a4/
%G ru
%F DAN_1997_352_6_a4