No-retraction theorem

This article describes a theorem about spheres

Statement

The sphere $S^{n}$ is not a retract of the disc $D^{n+1}$. In other words, the sphere is not a retract of the disc that it bounds.

Equivalently, the identity map from $S^{n}$ to itself is not nullhomotopic, and hence $S^{n}$ is not contractible.

Relation with other results

Results proved using the no-retraction theorem

• Complex numbers are algebraically closed uses the two-dimensional case of this theorem