@article{SM_1982_42_3_a0,
     author = {A. A. Borisenko},
     title = {Surfaces of nonpositive extrinsic curvature in spaces of constant curvature},
     journal = {Sbornik. Mathematics},
     pages = {297--310},
     publisher = {mathdoc},
     volume = {42},
     number = {3},
     year = {1982},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1982_42_3_a0/}
}