On some examples of duality groups

In 1, 3] it was shown: Theorem A. If G is the fundamental group of a finite graph of λ-dimensional duality groups with |G _o(e) : G _e | < ∞ and |G _τ(e) : G _e | < ∞ for every edge e of the corresponding G-tree, then G is an (λ + 1)-dimensional duality group. Here we use the methods of Brown and Geoghegan in 3] to obtain examples of duality groups under weaker conditions than those of Theorem A. Received: 5 June 2007