Algebraic and Geometric Topology 5 (2005), paper no. 60, pages 1505-1533.

Ideal triangulations of 3-manifolds II; taut and angle structures

Ensil Kang, J Hyam Rubinstein

Abstract. This is the second in a series of papers in which we investigate ideal triangulations of the interiors of compact 3-manifolds with tori or Klein bottle boundaries. Such triangulations have been used with great effect, following the pioneering work of Thurston. Ideal triangulations are the basis of the computer program SNAPPEA of Weeks and the program SNAP of Coulson, Goodman, Hodgson and Neumann. Casson has also written a program to find hyperbolic structures on such 3-manifolds, by solving Thurston's hyperbolic gluing equations for ideal triangulations. In this second paper, we study the question of when a taut ideal triangulation of an irreducible atoroidal 3-manifold admits a family of angle structures. We find a combinatorial obstruction, which gives a necessary and sufficient condition for the existence of angle structures for taut triangulations. The hope is that this result can be further developed to give a proof of the existence of ideal triangulations admitting (complete) hyperbolic metrics. Our main result answers a question of Lackenby. We give simple examples of taut ideal triangulations which do not admit an angle structure. Also we show that for `layered' ideal triangulations of once-punctured torus bundles over the circle, that if the manodromy is pseudo Anosov, then the triangulation admits angle structures if and only if there are no edges of degree 2. Layered triangulations are generalizations of Thurston's famous triangulation of the Figure-8 knot space. Note that existence of an angle structure easily implies that the 3-manifold has a CAT(0) or relatively word hyperbolic fundamental group.

Keywords. Normal surfaces, 3-manifolds, ideal triangulations, taut and angle structures

AMS subject classification. Primary: 57M25. Secondary: 57N10.

DOI: 10.2140/agt.2005.5.1505

Submitted: 19 May 2005. Accepted: 13 June 2005. Published: 3 November 2005.

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Ensil Kang, J Hyam Rubinstein
Department of Mathematics, College of Natural Sciences, Chosun University
Gwangju 501--759, Korea
Department of Mathematics and Statistics, The University of Melbourne
Parkville, Victoria 3010, Australia

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