@article{MZM_1988_43_1_a11,
     author = {S. V. Buyalo},
     title = {Euclidean planes in three-dimensional manifolds of nonpositive curvature},
     journal = {Matemati\v{c}eskie zametki},
     pages = {103--114},
     publisher = {mathdoc},
     volume = {43},
     number = {1},
     year = {1988},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/MZM_1988_43_1_a11/}
}