Hyperbolic Dehn surgery

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In mathematics, hyperbolic Dehn surgery refers to an operation by which one can obtain further hyperbolic 3-manifolds from a given cusped hyperbolic 3-manifold. Hyperbolic Dehn surgery exists only in dimension three and is the main reason which distinguishes hyperbolic geometry in three dimensions from other dimensions.

Such an operation is often also called hyperbolic Dehn filling, as Dehn surgery proper refers to a "drill and fill" operation on a link which consists of drilling out a neighborhood of the link and then filling back in with solid tori. Hyperbolic Dehn surgery actually only involves "filling".

Note that we will generally assume that a hyperbolic 3-manifold is complete, except when we are explicitly discussing deformations of hyperbolic structures, in which case incomplete hyperbolic metrics arise necessarily as a consequence of Mostow rigidity.

Suppose M is a cusped hyperbolic 3-manifold with n cusps. M can be thought of, topologically, as the interior of a compact manifold with toral boundary. Suppose we have chosen a meridian and longitude for each boundary torus, i.e. simple closed curves that are generators for the fundamental group of the torus. Let denote the manifold obtained from M by filling in the i-th boundary torus with a solid torus using the slope u_i = p_i / q_i where each pair p_i and q_i are coprime integers. We allow a u_i to be which means we do not fill in that cusp, i.e. do the "empty" Dehn filling. So M = .

We equip the space H of finite volume hyperbolic 3-manifolds with the geometric topology.

Thurston's hyperbolic Dehn surgery theorem states: is hyperbolic as long as a finite set of slopes E_i is avoided for the i-th cusp for each i. In addition, converges to M in H as all for all p_i / q_i corresponding to non-empty Dehn fillings u_i.

This theorem is due to William Thurston and fundamental to the theory of hyperbolic 3-manifolds. It shows that nontrivial limits exist in H. Troels Jorgensen's study of the geometric topology further shows that all nontrivial limits arise by Dehn filling as in the theorem.

Another important result by Thurston is that volume decreases under hyperbolic Dehn filling. In fact, the theorem states that volume decreases under topological Dehn filling, assuming of course that the Dehn-filled manifold is hyperbolic. The proof relies on basic properties of the Gromov norm.

Jorgensen also showed that the volume function on this space is a continuous, proper function. Thus by the previous results, nontrivial limits in H are taken to nontrivial limits in the set of volumes. In fact, one can further conclude, as did Thurston, that the set of volumes of finite volume hyperbolic 3-manifolds has ordinal type ω^ω.

[edit] References

• William Thurston, The geometry and topology of 3-manifolds, Princeton lecture notes (1978-1981).