@article{MZ_1993__214_1_174579,
     author = {Wolfgang Woess},
     title = {Fixed sets and free subgroups of groups acting on metric spaces.},
     journal = {Mathematische Zeitschrift},
     pages = {425--440},
     year = {1993},
     volume = {214},
     number = {1},
     zbl = {0892.54022},
     url = {http://geodesic.mathdoc.fr/item/MZ_1993__214_1_174579/}
}