@article{MMO_1971_24_a1,
     author = {Yu. V. Egorov},
     title = {Necessary conditions for the solvability of pseudodifferential equations of principal type},
     journal = {Trudy Moskovskogo matemati\v{c}eskogo ob\^{s}estva},
     pages = {29--41},
     publisher = {mathdoc},
     volume = {24},
     year = {1971},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MMO_1971_24_a1/}
}