@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/}
}