@article{DE_1986_22_2_a18,
     author = {V. M. Filippov},
     title = {The variational principle for hypoelliptic equations with constant coefficients},
     journal = {Differencialʹnye uravneni\^a},
     pages = {338--343},
     publisher = {mathdoc},
     volume = {22},
     number = {2},
     year = {1986},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DE_1986_22_2_a18/}
}