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# 3-manifold

In mathematics, a 3-manifold is a 3-dimensional manifold. The topological, piecewise-linear, and smooth categories are all equivalent in three dimensions, so little distinction is usually made in whether we are dealing with say, topological 3-manifolds, or smooth 3-manifolds.

Phenomena in three dimensions can be strikingly different from that for other dimensions, and so there is a prevalence of very specialized techniques that do not generalize to dimensions greater than three. Perhaps surprisingly, this special role has led to the discovery of close connections to a diversity of other fields, such as knot theory, geometric group theory, hyperbolic geometry, number theory, Teichmüller theory, topological quantum field theory, gauge theory, Floer homology, and partial differential equations. 3-manifold theory is considered a part of low-dimensional topology or geometric topology.

A key idea in the theory is to study a 3-manifold by considering special surfaces embedded in it. One can choose the surface to be nicely placed in the 3-manifold, which leads to the idea of an incompressible surface and the theory of Haken manifolds, or one can choose the complementary pieces to be as nice as possible, leading to structures such as Heegaard splittings, which are useful even in the non-Haken case.

Thurston's contributions to the theory allow one to also consider, in many cases, the additional structure given by a particular Thurston model geometry (of which there are eight). The most prevalent geometry is hyperbolic geometry. Using a geometry in addition to special surfaces is often fruitful.

The fundamental groups of 3-manifolds strongly reflect the geometric and topological information belonging to a 3-manifold. Thus, there is an interplay between group theory and topological methods.

## Important examples of 3-manifolds

### Hyperbolic link complements

The following examples are particularly well-known and studied.

## Some important classes of 3-manifolds

The classes are not necessarily mutually exclusive!

## Foundational results

Some results are named as conjectures as a result of historical artifacts.

We begin with the purely topological:

Theorems where geometry plays an important role in the proof:

Results explicitly linking geometry and topology:

## Important conjectures

Some of these are thought to be solved, as of March 2007. Please see specific articles for more information.

## References

• Hempel, 3-manifolds, American Mathematical Society, ISBN 0-8218-3695-1
• Jaco, Lectures on three-manifold topology, American Mathematical Society, ISBN 0-8218-1693-4
• Rolfsen, Knots and Links, American Mathematical Society, ISBN 0-914098-16-0
• Thurston, Three-dimensional geometry and topology, Princeton University Press, ISBN 0-691-08304-5
• Adams, The Knot Book, American Mathematical Society, ISBN 0-8050-7380-9
• Hatcher, Notes on basic 3-manifold topology, available online
• R. H. Bing, The Geometric Topology of 3-Manifolds, (1983) American Mathematical Society Colloquium Publications Volume 40, Providence RI, ISBN 0-8218-1040-5.

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