# Pullback bundle

In mathematics, a pullback bundle or induced bundle [1][2][3] is a useful construction in the theory of fiber bundles. Given a fiber bundle π : E → B and a continuous map f : B′ → B one can define a "pullback" of E by f as a bundle f*E over B′. The fiber of f*E over a point b′ in B′ is just the fiber of E over f(b′). Thus f*E is the disjoint union of all these fibers equipped with a suitable topology.

## Formal definition

Let π : EB be a fiber bundle with abstract fiber F and let f : B′ → B be a continuous map. Define the pullback bundle by

$f^{*}E = \{(b',e) \in B' \times E \mid f(b') = \pi(e)\}\subset B'\times E$

and equip it with the subspace topology and the projection map π′ : f*EB′ given by the projection onto the first factor, i.e.,

$\pi'(b',e) = b'.\,$

The projection onto the second factor gives a map $\tilde f \colon f^{*}E \to E$ such that the following diagram commutes:

$\begin{array} {ccc} f^{\ast}E & \stackrel {\tilde f} {\longrightarrow} & E\\ {\pi}' \downarrow & & \downarrow \pi\\ B' & \stackrel f {\longrightarrow} & B \end{array}$

If (U, φ) is a local trivialization of E then (f−1U, ψ) is a local trivialization of f*E where

$\psi(b',e) = (b', \mbox{proj}_2(\varphi(e))).\,$

It then follows that f*E is a fiber bundle over B′ with fiber F. The bundle f*E is called the pullback of E by f or the bundle induced by f. The map $\tilde f$ is then a bundle morphism covering f.

## Properties

Any section s of E over B induces a section of f*E, called the pullback section f*s, simply by defining $f^*s=s\circ f$.

If the bundle EB has structure group G with transition functions tij (with respect to a family of local trivializations {(Ui, φi)} ) then the pullback bundle f*E also has structure group G. The transition functions in f*E are given by

$f^{*}t_{ij} = t_{ij} \circ f.$

If EB is a vector bundle or principal bundle then so is the pullback f*E. In the case of a principal bundle the right action of G on f*E is given by

$(x,e)\cdot g = (x,e\cdot g)$

It then follows that the map $\tilde f$ is equivariant and so defines a morphism of principal bundles.

In the language of category theory, the pullback bundle construction is an example of the more general categorical pullback. As such it satisfies the corresponding universal property.

The construction of the pullback bundle can be carried out in subcategories of the category of topological spaces, such as the category of smooth manifolds. The latter construction is useful in differential geometry and topology

Examples: It is illuminating to consider the pullback of the degree 2 map from the circle to itself over the degree 3 or 4 map from the circle to itself. In such examples one sometimes gets a connected and sometimes disconnected space, but always several copies of the circle.

## Bundles and sheaves

Bundles may also be described by their sheaves of sections. The pullback of bundles then corresponds to the inverse image of sheaves, which is a contravariant functor. A sheaf, however, is more naturally a covariant object, since it has a pushforward, called the direct image of a sheaf. The tension and interplay between bundles and sheaves, or inverse and direct image, can be advantageous in many areas of geometry. However, the direct image of a sheaf of sections of a bundle is not in general the sheaf of sections of some direct image bundle, so that although the notion of a 'pushforward of a bundle' is defined in some contexts (for example, the pushforward by a diffeomorphism), in general it is better understood in the category of sheaves, because the objects it creates cannot in general be bundles.

## References

1. ^ Steenrod, Norman (1951). The Topology of Fibre Bundles. Princeton: Princeton University Press. ISBN 0-691-00548-6. page 47
2. ^ Husemoller, Dale (1994). Fibre Bundles (Third ed.). New York: Springer. ISBN 978-0-387-94087-8. page 18
3. ^ Lawson, H. Blaine; Michelsohn, Marie-Louise (1989). Spin Geometry. Princeton University Press. ISBN 978-0-691-08542-5. page 374

## Books

• Steenrod, Norman (1951). The Topology of Fibre Bundles. Princeton: Princeton University Press. ISBN 0-691-00548-6.
• Husemoller, Dale (1994). Fibre Bundles (Third ed.). New York: Springer. ISBN 978-0-387-94087-8.
• Sharpe, R. W. (1997). Differential Geometry: Cartan's Generalization of Klein's Erlangen Program. New York: Springer. ISBN 0-387-94732-9.