@article{DAN_1969_186_4_a5,
     author = {A. Dzhuraev},
     title = {Systems of second order partial differential equations of composite type with one double family of real characteristics},
     journal = {Doklady Akademii Nauk},
     pages = {761--764},
     publisher = {mathdoc},
     volume = {186},
     number = {4},
     year = {1969},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1969_186_4_a5/}
}