@article{JRAM_1988__390_153057,
     author = {Viktor Schroeder},
     title = {Existence of immersed tori in manifolds of nonpositive curvature.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {32--46},
     publisher = {mathdoc},
     volume = {390},
     year = {1988},
     zbl = {0641.53042},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1988__390_153057/}
}