@article{MAT_1971_15_3_a5,
     author = {L. Nirenberg and F. Tr\`eves},
     title = {On local solvability of linear partial differential equations. {I.~Necessary} conditions},
     journal = {Matematika},
     pages = {142--172},
     publisher = {mathdoc},
     volume = {15},
     number = {3},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MAT_1971_15_3_a5/}
}