@article{IM_1987__90_143519,
     author = {K.S. Brown},
     title = {Trees, valuations, and the {Bieri-Neumann-Strebel} invariant.},
     journal = {Inventiones mathematicae},
     pages = {479--504},
     publisher = {mathdoc},
     volume = {90},
     year = {1987},
     zbl = {0663.20033},
     url = {http://geodesic.mathdoc.fr/item/IM_1987__90_143519/}
}