@article{SM_1968_4_1_a6,
     author = {A. L. Verner},
     title = {On the extrinsic geometry of elementary complete surfaces with nonpositive {curvature.~II}},
     journal = {Sbornik. Mathematics},
     pages = {99--123},
     publisher = {mathdoc},
     volume = {4},
     number = {1},
     year = {1968},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1968_4_1_a6/}
}