@article{IM_1993__112_3_144098,
     author = {Jens Heber},
     title = {On the geometric rank of homogeneous spaces of nonpositive curvature.},
     journal = {Inventiones mathematicae},
     pages = {151--170},
     publisher = {mathdoc},
     volume = {112},
     number = {3},
     year = {1993},
     zbl = {0792.53048},
     url = {http://geodesic.mathdoc.fr/item/IM_1993__112_3_144098/}
}