@article{RM_1977_32_5_a13,
     author = {Yu. V. Egorov},
     title = {The smoothness of the solutions of equations of principal type},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     publisher = {mathdoc},
     volume = {32},
     number = {5},
     year = {1977},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/RM_1977_32_5_a13/}
}