@article{PMIHES_1995__82__169_0,
     author = {Werner Ballmann and Michael Brin},
     title = {Orbihedra of nonpositive curvature},
     journal = {Publications Math\'ematiques de l'IH\'ES},
     pages = {169--209},
     year = {1995},
     publisher = {Institut des Hautes \'Etudes Scientifiques},
     volume = {82},
     doi = {10.1007/BF02698640},
     mrnumber = {97i:53049},
     zbl = {0866.53029},
     language = {en},
     url = {https://pmihes.centre-mersenne.org/articles/10.1007/BF02698640/}
}