@article{10_1090_S0894_0347_97_00238_5,
     author = {Eskin, Alex and Farb, Benson},
     title = {Quasi-flats and rigidity in higher rank symmetric spaces},
     journal = {Journal of the American Mathematical Society},
     pages = {653--692},
     year = {1997},
     volume = {10},
     number = {3},
     doi = {10.1090/S0894-0347-97-00238-5},
     url = {http://geodesic.mathdoc.fr/articles/10.1090/S0894-0347-97-00238-5/}
}