@article{1214440722,
author = {Ballmann, Werner and Eberlein, Patrick},
title = {Fundamental groups of manifolds of nonpositive curvature},
journal = {J. Differential Geom.},
volume = {25},
number = {1},
year = {1987},
pages = { 1-22},
language = {en},
url = {http://dml.mathdoc.fr/item/1214440722}
}
Ballmann, Werner; Eberlein, Patrick. Fundamental groups of manifolds of nonpositive curvature. J. Differential Geom., Tome 25 (1987) no. 1, pp. 1-22. http://gdmltest.u-ga.fr/item/1214440722/