The Symplectic Geometry of Polygons in Hyperbolic 3-space∗

We study the symplectic geometry of the moduli spaces Mr = Mr(H) of closed n-gons with fixed side-lengths in hyperbolic three-space. We prove that these moduli spaces have almost canonical symplectic structures. They are the symplectic quotients of Bn by the dressing action of SU(2) (here B is the subgroup of the Borel subgroup of SL2(C) defined below). We show that the hyperbolic Gauss map sets up a real analytic isomorphism between the spaces Mr and the weighted quotients of (S2)n by PSL2(C) studied by Deligne and Mostow. We construct an integrable Hamiltonian system on Mr by bending polygons along nonintersecting diagonals. We describe angle variables and the momentum polyhedron for this system. The results of this paper are the analogues for hyperbolic space of the results of [KM2] for Mr(E), the space of n-gons with fixed side-lengths in E3. We prove Mr(H) and Mr(E) are symplectomorphic.