@article{AA_1991_3_1_a3,
     author = {S. V. Buyalo},
     title = {Euclidean planes in open three-dimensional manifolds of nonpositive curvature},
     journal = {Algebra i analiz},
     pages = {102--117},
     publisher = {mathdoc},
     volume = {3},
     number = {1},
     year = {1991},
     language = {ru},
     url = {http://geodesic.mathdoc.fr/item/AA_1991_3_1_a3/}
}