@article{GDZPPN002229757,
title = {Structure of flat subspaces in low-dimensional manilfolds of nonpositiv curvature.},
journal = {Manuscripta mathematica},
volume = {64},
year = {1989},
pages = {77-106},
zbl = {0678.53035},
url = {http://dml.mathdoc.fr/item/GDZPPN002229757}
}
Schroeder, Viktor. Structure of flat subspaces in low-dimensional manilfolds of nonpositiv curvature.. Manuscripta mathematica, Tome 64 (1989) pp. 77-106. http://gdmltest.u-ga.fr/item/GDZPPN002229757/