Sphere bundle
# Sphere bundle

In the mathematical field of topology, a sphere bundle is a fiber bundle in which the fibers are spheres $S^{n}$ of some dimension n. Similarly, in a disk bundle, the fibers are disks $D^{n}$. From a topological perspective, there is no difference between sphere bundles and disk bundles: this is a consequence of the Alexander trick, which implies $\operatorname {BTop} (D^{n+1})\simeq \operatorname {BTop} (S^{n}).$

An example of a sphere bundle is the torus, which is orientable and has $S^{1}$ fibers over an $S^{1}$ base space. The non-orientable Klein bottle also has $S^{1}$ fibers over an $S^{1}$ base space, but has a twist that produces a reversal of orientation as one follows the loop around the base space.

A circle bundle is a special case of a sphere bundle.

## Orientation of a sphere bundle

A sphere bundle that is a product space is orientable, as is any sphere bundle over a simply connected space.

If E be a real vector bundle on a space X and if E is given an orientation, then a sphere bundle formed from E, Sph(E), inherits the orientation of E.

## Spherical fibration

A spherical fibration, a generalization of the concept of a sphere bundle, is a fibration whose fibers are homotopy equivalent to spheres. For example, the fibration

    $\operatorname {BTop} (\mathbb {R} ^{n})\to \operatorname {BTop} (S^{n})$

has fibers homotopy equivalent to $S^n.$

