@article{DAN_1977_237_3_a2,
     author = {V. A. Kondratiev and S. D. \`Eidel'man},
     title = {On nonnegative solutions of overdetermined systems of partial differential equations},
     journal = {Doklady Akademii Nauk},
     pages = {513--516},
     publisher = {mathdoc},
     volume = {237},
     number = {3},
     year = {1977},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1977_237_3_a2/}
}