@article{RM_1974_29_2_a12,
     author = {Yu. V. Egorov and P. R. Popivanov},
     title = {Equations of principal type witgout solution},
     journal = {Trudy Matematicheskogo Instituta imeni V.A. Steklova},
     pages = {176--194},
     publisher = {mathdoc},
     volume = {29},
     number = {2},
     year = {1974},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/RM_1974_29_2_a12/}
}