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@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/} }
S. V. Buyalo. Euclidean planes in three-dimensional manifolds of nonpositive curvature. Matematičeskie zametki, Tome 43 (1988) no. 1, pp. 103-114. http://geodesic.mathdoc.fr/item/MZM_1988_43_1_a11/