@article{SM_1977_33_4_a2,
     author = {A. A. Borisenko},
     title = {Complete $l$-dimensional surfaces of nonpositive extrinsic curvature in {a~Riemannian} space},
     journal = {Sbornik. Mathematics},
     pages = {485--499},
     publisher = {mathdoc},
     volume = {33},
     number = {4},
     year = {1977},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_1977_33_4_a2/}
}