@article{CMH_1982__57_139893,
     author = {Robert Bieri},
     title = {A 1 1/2-dimensional version of {Hopf's} {Theorem} on the number of ends of a group.},
     journal = {Commentarii mathematici Helvetici},
     pages = {25--40},
     publisher = {mathdoc},
     volume = {57},
     year = {1982},
     zbl = {0485.20046},
     url = {http://geodesic.mathdoc.fr/item/CMH_1982__57_139893/}
}