@article{SM_1983_44_4_a9,
     author = {Yu. V. Egorov},
     title = {Equations of principal type. {Sufficient} conditions for local solvability},
     journal = {Sbornik. Mathematics},
     pages = {517--525},
     publisher = {mathdoc},
     volume = {44},
     number = {4},
     year = {1983},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1983_44_4_a9/}
}