@article{CMH_1988__63_4_140130,
     author = {Robert Bieri and Burkhardt Renz},
     title = {Valuations on free resolutions and higher geometric invariants of groups.},
     journal = {Commentarii mathematici Helvetici},
     pages = {464--497},
     publisher = {mathdoc},
     volume = {63},
     number = {4},
     year = {1988},
     zbl = {0654.20029},
     url = {http://geodesic.mathdoc.fr/item/CMH_1988__63_4_140130/}
}