@article{IM_1987__90_143518,
     author = {W.D. Neumann and R. Bieri and R. Strebel},
     title = {A geometric invariant of discrete groups.},
     journal = {Inventiones mathematicae},
     pages = {451--478},
     publisher = {mathdoc},
     volume = {90},
     year = {1987},
     zbl = {0642.57002},
     url = {http://geodesic.mathdoc.fr/item/IM_1987__90_143518/}
}