Homotopy


In topology, two continuous functions from one topological space to another are called homotopic if one can be "continuously deformed" into the other, such a deformation being called a homotopy between the two functions. A notable use of homotopy is the definition of homotopy groups and cohomotopy groups, important invariants in algebraic topology.
In practice, there are technical difficulties in using homotopies with certain spaces. Algebraic topologists work with compactly generated spaces, CW complexes, or spectra.

Formal definition

Formally, a homotopy between two continuous functions f and g from a
topological space X to a topological space Y is defined to be a continuous function from the product of the space X with the unit interval to Y such that and for all.
If we think of the second parameter of H as time then H describes a continuous deformation of f into g: at time 0 we have the function f and at time 1 we have the function g. We can also think of the second parameter as a "slider control" that allows us to smoothly transition from f to g as the slider moves from 0 to 1, and vice versa.
An alternative notation is to say that a homotopy between two continuous functions is a family of continuous functions for such that and, and the map is continuous from to. The two versions coincide by setting. It is not sufficient to require each map to be continuous.
The animation that is looped above right provides an example of a homotopy between two embeddings, f and g, of the torus into. X is the torus, Y is, f is some continuous function from the torus to R3 that takes the torus to the embedded surface-of-a-doughnut shape with which the animation starts; g is some continuous function that takes the torus to the embedded surface-of-a-coffee-mug shape. The animation shows the image of ht as a function of the parameter t, where t varies with time from 0 to 1 over each cycle of the animation loop. It pauses, then shows the image as t varies back from 1 to 0, pauses, and repeats this cycle.

Properties

Continuous functions f and g are said to be homotopic if and only if there is a homotopy H taking f to g as described above. Being homotopic is an equivalence relation on the set of all continuous functions from X to Y.
This homotopy relation is compatible with function composition in the following sense: if are homotopic, and are homotopic, then their compositions and are also homotopic.

Examples

  • If are given by and, then the map given by is a homotopy between them.
  • More generally, if is a convex subset of Euclidean space and are paths with the same endpoints, then there is a linear homotopy given by
  • :
  • Let be the identity function on the unit n-disk; i.e. the set. Let be the constant function which sends every point to the origin. Then the following is a homotopy between them:
  • :

    Homotopy equivalence

Given two topological spaces X and Y, a homotopy equivalence between X and Y is a pair of continuous maps and, such that is homotopic to the identity map idX and is homotopic to idY. If such a pair exists, then X and Y are said to be homotopy equivalent, or of the same homotopy type. This relation of homotopy equivalence is often denoted. Intuitively, two spaces X and Y are homotopy equivalent if they can be transformed into one another by bending, shrinking and expanding operations. Spaces that are homotopy-equivalent to a point are called contractible.

Homotopy equivalence vs. homeomorphism

A homeomorphism is a special case of a homotopy equivalence, in which is equal to the identity map idX, and is equal to idY. Therefore, if X and Y are homeomorphic then they are homotopy-equivalent, but the opposite is not true. Some examples:
  • A solid disk is homotopy-equivalent to a single point, since you can deform the disk along radial lines continuously to a single point. However, they are not homeomorphic, since there is no bijection between them.
  • The Möbius strip and an untwisted strip are homotopy equivalent, since you can deform both strips continuously to a circle. But they are not homeomorphic.

    Examples

  • The first example of a homotopy equivalence is with a point, denoted. The part that needs to be checked is the existence of a homotopy between and, the projection of onto the origin. This can be described as.
  • There is a homotopy equivalence between and.
  • * More generally,.
  • Any fiber bundle with fibers homotopy equivalent to a point has homotopy equivalent total and base spaces. This generalizes the previous two examples since is a fiber bundle with fiber.
  • Every vector bundle is a fiber bundle with a fiber homotopy equivalent to a point.
  • for any, by writing as the total space of the fiber bundle, then applying the homotopy equivalences above.
  • If a subcomplex of a CW complex is contractible, then the quotient space is homotopy equivalent to.
  • A deformation retraction is a homotopy equivalence.

    Null-homotopy

A function is said to be null-homotopic if it is homotopic to a constant function. For example, a map from the unit circle to any space is null-homotopic precisely when it can be continuously extended to a map from the unit disk to that agrees with on the boundary.
It follows from these definitions that a space is contractible if and only if the identity map from to itself—which is always a homotopy equivalence—is null-homotopic.

Invariance

Homotopy equivalence is important because in algebraic topology many concepts are homotopy invariant, that is, they respect the relation of homotopy equivalence. For example, if X and Y are homotopy equivalent spaces, then:
An example of an algebraic invariant of topological spaces which is not homotopy-invariant is compactly supported homology.

Variants

Relative homotopy

In order to define the fundamental group, one needs the notion of homotopy relative to a subspace. These are homotopies which keep the elements of the subspace fixed. Formally: if f and g are continuous maps from X to Y and K is a subset of X, then we say that f and g are homotopic relative to K if there exists a homotopy between f and g such that for all and Also, if g is a retraction from X to K and f is the identity map, this is known as a strong deformation retract of X to K.
When K is a point, the term pointed homotopy is used.

Isotopy

When two given continuous functions f and g from the topological space X to the topological space Y are embeddings, one can ask whether they can be connected 'through embeddings'. This gives rise to the concept of isotopy, which is a homotopy, H, in the notation used before, such that for each fixed t, H gives an embedding.
A related, but different, concept is that of ambient isotopy.
Requiring that two embeddings be isotopic is a stronger requirement than that they be homotopic. For example, the map from the interval into the real numbers defined by f = −x is not isotopic to the identity g = x. Any homotopy from f to the identity would have to exchange the endpoints, which would mean that they would have to 'pass through' each other. Moreover, f has changed the orientation of the interval and g has not, which is impossible under an isotopy. However, the maps are homotopic; one homotopy from f to the identity is H: × → given by H = 2yx − x.
Two homeomorphisms of the unit ball which agree on the boundary can be shown to be isotopic using Alexander's trick. For this reason, the map of the unit disc in defined by f = is isotopic to a 180-degree rotation around the origin, and so the identity map and f are isotopic because they can be connected by rotations.
In geometric topology—for example in knot theory—the idea of isotopy is used to construct equivalence relations. For example, when should two knots be considered the same? We take two knots, K1 and K2, in three-dimensional space. A knot is an embedding of a one-dimensional space, the "loop of string", into this space, and this embedding gives a homeomorphism between the circle and its image in the embedding space. One may try to define knot equivalence based on isotopy instead of the more restricted property of ambient isotopy. That is, two knots are isotopic when there exists a continuous function starting at t = 0 giving the K1 embedding, ending at t = 1 giving the K2 embedding, with all intermediate values corresponding to embeddings. However, this definition would make every knot equivalent to the unknot, as the knotted portions can be "contracted" down to a straight line. The problem is that, while continuous, this is not an injective function of the euclidean space that the knot is embedded in. An ambient isotopy, studied in this context, is an isotopy of the larger space, considered in light of its action on the embedded submanifold. Knots K1 and K2 are considered equivalent when there is a continuous -indexed family of maps which moves K1 to K2 via homeomorphisms of the euclidean space.
Similar language is used for the equivalent concept in contexts where one has a stronger notion of equivalence. For example, a path between two smooth embeddings is a smooth isotopy.