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@article{MM2_1989__64_2_155397, author = {Viktor Schroeder}, title = {Structure of flat subspaces in low-dimensional manilfolds of nonpositiv curvature.}, journal = {Manuscripta mathematica}, pages = {77--106}, publisher = {mathdoc}, volume = {64}, number = {2}, year = {1989}, zbl = {0678.53035}, url = {http://geodesic.mathdoc.fr/item/MM2_1989__64_2_155397/} }
Viktor Schroeder. Structure of flat subspaces in low-dimensional manilfolds of nonpositiv curvature.. Manuscripta mathematica, Tome 64 (1989) no. 2, pp. 77-106. http://geodesic.mathdoc.fr/item/MM2_1989__64_2_155397/