Torus bundle

A torus bundle, in the sub-field of geometric topology in mathematics, is a kind of surface bundle over the circle, which in turn is a class of three-manifolds.

Construction

To obtain a torus bundle: let ${\displaystyle f}$  be an orientation-preserving homeomorphism of the two-dimensional torus ${\displaystyle T}$  to itself. Then the three-manifold ${\displaystyle M(f)}$  is obtained by

• taking the Cartesian product of ${\displaystyle T}$  and the unit interval and
• gluing one component of the boundary of the resulting manifold to the other boundary component via the map ${\displaystyle f}$ .

Then ${\displaystyle M(f)}$  is the torus bundle with monodromy ${\displaystyle f}$ .

Examples

For example, if ${\displaystyle f}$  is the identity map (i.e., the map which fixes every point of the torus) then the resulting torus bundle ${\displaystyle M(f)}$  is the three-torus: the Cartesian product of three circles.

Seeing the possible kinds of torus bundles in more detail requires an understanding of William Thurston's geometrization program. Briefly, if ${\displaystyle f}$  is finite order, then the manifold ${\displaystyle M(f)}$  has Euclidean geometry. If ${\displaystyle f}$  is a power of a Dehn twist then ${\displaystyle M(f)}$  has Nil geometry. Finally, if ${\displaystyle f}$  is an Anosov map then the resulting three-manifold has Sol geometry.

These three cases exactly correspond to the three possibilities for the absolute value of the trace of the action of ${\displaystyle f}$  on the homology of the torus: either less than two, equal to two, or greater than two.

References

• Jeffrey R. Weeks (2002). The Shape of Space (Second ed.). Marcel Dekker, Inc. ISBN 978-0824707095.