@article{DE_1997_33_11_a10,
     author = {N. N. Petrov},
     title = {On the sum of squares of vector fields},
     journal = {Differencialʹnye uravneni\^a},
     pages = {1516--1522},
     publisher = {mathdoc},
     volume = {33},
     number = {11},
     year = {1997},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1997_33_11_a10/}
}