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@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/} }
W.D. Neumann; R. Bieri; R. Strebel. A geometric invariant of discrete groups.. Inventiones mathematicae, Tome 90 (1987), pp. 451-478. http://geodesic.mathdoc.fr/item/IM_1987__90_143518/