# Regular homotopy

In the mathematical field of topology, a **regular homotopy** refers to a special kind of homotopy between immersions of one manifold in another. The homotopy must be a 1-parameter family of immersions.

Similar to homotopy classes, one defines two immersions to be in the same regular homotopy class if there exists a regular homotopy between them. Regular homotopy for immersions is similar to isotopy of embeddings: they are both restricted types of homotopies. Stated another way, two continuous functions are homotopic if they represent points in the same path-components of the mapping space , given the compact-open topology. The **space of immersions** is the subspace of consisting of immersions, denote it by . Two immersions are **regularly homotopic** if they represent points in the same path-component of .

## Examples

The **Whitney–Graustein theorem** classifies the regular homotopy classes of a circle into the plane; two immersions are regularly homotopic if and only if they have the same turning number – equivalently, total curvature; equivalently, if and only if their Gauss maps have the same degree/winding number.

Stephen Smale classified the regular homotopy classes of a *k*-sphere immersed in – they are classified by homotopy groups of Stiefel manifolds, which is a generalization of the Gauss map, with here *k* partial derivatives not vanishing. A corollary of his work is that there is only one regular homotopy class of a *2*-sphere immersed in . In particular, this means that sphere eversions exist, i.e. one can turn the 2-sphere "inside-out".

Both of these examples consist of reducing regular homotopy to homotopy; this has subsequently been substantially generalized in the homotopy principle (or *h*-principle) approach.

## References

- Whitney, Hassler (1937). "On regular closed curves in the plane".
*Compositio Mathematica*.**4**: 276–284. - Smale, Stephen (February 1959). "A classification of immersions of the two-sphere" (PDF).
*Transactions of the American Mathematical Society*.**90**(2): 281–290. doi:10.2307/1993205. JSTOR 1993205. - Smale, Stephen (March 1959). "The classification of immersions of spheres in Euclidean spaces" (PDF).
*Annals of Mathematics*.**69**(2): 327–344. doi:10.2307/1970186. JSTOR 1970186.