@article{SM_1970_11_2_a4,
     author = {B. E. Kantor},
     title = {On~the problem of the normal image of a~complete surface of negative curvature},
     journal = {Sbornik. Mathematics},
     pages = {197--200},
     year = {1970},
     volume = {11},
     number = {2},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1970_11_2_a4/}
}