# 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 $f,g:M\to N$ are homotopic if they represent points in the same path-components of the mapping space $C(M,N)$, given the compact-open topology. The space of immersions is the subspace of $C(M,N)$ consisting of immersions, denoted by $\operatorname {Imm} (M,N)$. Two immersions $f,g:M\to N$ are regularly homotopic if they represent points in the same path-component of $\operatorname {Imm} (M,N)$.

## Examples

Any two knots in 3-space are equivalent by regular homotopy, though not by isotopy.
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 $\mathbb {R} ^{n}$ – they are classified by homotopy groups of Stiefel manifolds, which is a generalization of the Gauss map, with here k partial derivatives not vanishing. More precisely, the set $I(n,k)$ of regular homotopy classes of embeddings of sphere $S^{k}$ in $\mathbb {R} ^{n}$ is in one-to-one correspondence with elements of group $\pi _{k}\left(V_{k}\left(\mathbb {R} ^{n}\right)\right)$. In case $k=n-1$ we have $V_{n-1}\left(\mathbb {R} ^{n}\right)\cong SO(n)$. Since $SO(1)$ is path connected, $\pi _{2}(SO(3))\cong \pi _{2}\left(\mathbb {R} P^{3}\right)\cong \pi _{2}\left(S^{3}\right)\cong 0$ and $\pi _{6}(SO(6))\to \pi _{6}(SO(7))\to \pi _{6}\left(S^{6}\right)\to \pi _{5}(SO(6))\to \pi _{5}(SO(7))$ and due to Bott periodicity theorem we have $\pi _{6}(SO(6))\cong \pi _{6}(\operatorname {Spin} (6))\cong \pi _{6}(SU(4))\cong \pi _{6}(U(4))\cong 0$ and since $\pi _{5}(SO(6))\cong \mathbb {Z} ,\ \pi _{5}(SO(7))\cong 0$ then we have $\pi _{6}(SO(7))\cong 0$. Therefore all immersions of spheres $S^{0},\ S^{2}$ and $S^{6}$ in euclidean spaces of one more dimension are regular homotopic. In particular, spheres $S^{n}$ embedded in $\mathbb {R} ^{n+1}$ admit eversion if $n=0,2,6$. A corollary of his work is that there is only one regular homotopy class of a 2-sphere immersed in $\mathbb {R} ^{3}$. 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.

## Non-degenerate homotopy

For locally convex, closed space curves, one can also define non-degenerate homotopy. Here, the 1-parameter family of immersions must be non-degenerate (i.e. the curvature may never vanish). There are 2 distinct non-degenerate homotopy classes. Further restrictions of non-vanishing torsion lead to 4 distinct equivalence classes.