@article{DAN_1970_194_4_a1,
     author = {I. S. Brandt},
     title = {Surfaces of negative extrinsic curvature in a {Riemannian} space with nonpositive {Riemannian} curvature},
     journal = {Doklady Akademii Nauk},
     pages = {747--749},
     publisher = {mathdoc},
     volume = {194},
     number = {4},
     year = {1970},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/DAN_1970_194_4_a1/}
}