@article{MAT_1971_15_4_a2,
     author = {L. Nirenberg and F. Tr\`eves},
     title = {On local solvability of linear partial differential equations. {II.~Sufficient} conditions},
     journal = {Matematika},
     pages = {68--110},
     publisher = {mathdoc},
     volume = {15},
     number = {4},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MAT_1971_15_4_a2/}
}