@article{DAN_1957_112_5_a6,
     author = {M. A. Lavrent'ev and B. V. Shabat},
     title = {The geometric properties of solutions of non-linear systems of partial differential equations},
     journal = {Doklady Akademii Nauk},
     pages = {810--811},
     publisher = {mathdoc},
     volume = {112},
     number = {5},
     year = {1957},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1957_112_5_a6/}
}