@article{SM_1967_3_2_a4,
     author = {A. L. Verner},
     title = {On the extrinsic geometry of elementary complete surfaces with nonpositive curvature. {I}},
     journal = {Sbornik. Mathematics},
     pages = {205--224},
     year = {1967},
     volume = {3},
     number = {2},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1967_3_2_a4/}
}