TY  - JOUR
AU  - N. N. Petrov
TI  - On the sum of squares of vector fields
JO  - Differencialʹnye uravneniâ
PY  - 1997
SP  - 1516
EP  - 1522
VL  - 33
IS  - 11
PB  - mathdoc
UR  - http://geodesic.mathdoc.fr/item/DE_1997_33_11_a10/
LA  - ru
ID  - DE_1997_33_11_a10
ER  -