@article{DAN_1961_137_3_a4,
     author = {N. V. Efimov and \`E. G. Poznyak},
     title = {Generalization of {Hilbert's} theorem on surfaces of constant negative curvature},
     journal = {Doklady Akademii Nauk},
     pages = {509--512},
     publisher = {mathdoc},
     volume = {137},
     number = {3},
     year = {1961},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1961_137_3_a4/}
}