Geodesic
On the sum of squares of vector fields
Differencialʹnye uravneniâ, Tome 33 (1997) no. 11, pp. 1516-1522 
N. N. Petrov. On the sum of squares of vector fields. Differencialʹnye uravneniâ, Tome 33 (1997) no. 11, pp. 1516-1522. http://geodesic.mathdoc.fr/item/DE_1997_33_11_a10/
@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},
     year = {1997},
     volume = {33},
     number = {11},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1997_33_11_a10/}
}
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