%0 Journal Article
%A N. N. Petrov
%T On the sum of squares of vector fields
%J Differencialʹnye uravneniâ
%D 1997
%P 1516-1522
%V 33
%N 11
%I mathdoc
%U http://geodesic.mathdoc.fr/item/DE_1997_33_11_a10/
%G ru
%F DE_1997_33_11_a10