@article{MZ_1996__222_3_174878,
     author = {W. Ballmann and S. Buyalo},
     title = {Nonpositively curved metrics on 2-polyhedra.},
     journal = {Mathematische Zeitschrift},
     pages = {97--134},
     publisher = {mathdoc},
     volume = {222},
     number = {3},
     year = {1996},
     zbl = {0843.53036},
     url = {http://geodesic.mathdoc.fr/item/MZ_1996__222_3_174878/}
}