@article{JRAM_1981__322_152328,
     author = {Robert Bieri and Ralph Strebel},
     title = {A geometric invariant for modules over an {Abelian} group.},
     journal = {Journal f\"ur die reine und angewandte Mathematik},
     pages = {170--189},
     publisher = {mathdoc},
     volume = {322},
     year = {1981},
     zbl = {0448.20010},
     url = {http://geodesic.mathdoc.fr/item/JRAM_1981__322_152328/}
}