Every cusped, finite-volume hyperbolic three-manifold has a canonical decomposition into ideal polyhedra. We study the canonical decomposition of the hyperbolic manifold obtained by filling some (but not all) of the cusps with solid tori: in a broad range of cases, generic in an appropriate sense, this decomposition can be predicted from that of the unfilled manifold (a similar result has been independently announced by Akiyoshi [4]). We also find the canonical decompositions of all hyperbolic Dehn fillings on one cusp of the Whitehead link complement.
51H20; 57M50
IntroductionLet M be a complete cusped hyperbolic 3-manifold of finite volume, and endow the cusps c 1 ; : : : ; c k of M with disjoint simple horoball neighborhoods H 1 ; : : : ; H k . The Ford-Voronoi domain F M consists of all points of M having a unique shortest path to the union of the H i . The complement of F is a compact complex C of totally geodesic polygons. By definition, the canonical decomposition D of M with respect to the H i has one 3-dimensional cell (an ideal polyhedron) per vertex of C , one face per edge of C , and one edge per (polygonal) face of C ; we say that D is dual to C . Other names for D are the geometrically canonical decomposition, or Delaunay (or Delone) decomposition. In [11], Epstein and Penner give a precise description of D in terms of convex hulls in Minkowski space R 3C1 . Weeks' program SnapPea [27] will compute D for most manifolds of moderate size.Akiyoshi [3] proves that, as the volumes of fH i g 1ÄiÄk vary, only finitely many decompositions D arise. By Mostow-Prasad rigidity, the resulting collection of Delaunay decompositions is a complete topological invariant of M . When M has a single cusp there is a unique Delaunay decomposition. When M has multiple cusps one may take all of their volumes to be equal; SnapPea uses the resulting decomposition for rigorous computation of isometry groups and detection of isometric manifolds (see Weeks [28] The present paper offers a relative result: we are interested in how the canonical decomposition D changes when the last cusp c k (where k 2) undergoes a Dehn filling along the slope s . Recall the operation of filling along s removes the interior of H k from M and glues a solid torus X s to the resulting boundary component, yielding the filled manifold M s . Thurston showed that the metric on M s Gromovconverges, with appropriate choices of basepoints, to the metric on M as the length of the filling slope s goes to infinity. Consequently, a Margulis tube (region where the injectivity radius is less than the Margulis constant) appears about the core curve ofExperimentation with SnapPea suggests that, for many manifolds, cusps, and slopes, after filling c k the polyhedra of D outside the Margulis tube undergo only a small geometric perturbation while the combinatorics of D s inside the tube has a predictable structure. To ensure such good behavior we choose the reference horoball neighborhoods fH i g 1Äi