%0 Journal Article
%A W.D. Neumann
%A R. Bieri
%A R. Strebel
%T A geometric invariant of discrete groups.
%J Inventiones mathematicae
%D 1987
%P 451-478
%V 90
%I mathdoc
%U http://geodesic.mathdoc.fr/item/IM_1987__90_143518/
%F IM_1987__90_143518