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The antipodal map preserves orientation (is homotopic to the identity map) [2] when is odd, and reverses it when is even. Its degree is ( − 1 ) n + 1 . {\displaystyle (-1)^{n+1}.} If antipodal points are identified (considered equivalent), the sphere becomes a model of real projective space .
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 X {\displaystyle X} is contractible if and only if the identity map from X {\displaystyle X} to itself—which is ...
the inclusion, a retraction is a continuous map r such that =, that is, the composition of r with the inclusion is the identity of A. Note that, by definition, a retraction maps X onto A. A subspace A is called a retract of X if such a retraction exists. For instance, any non-empty space retracts to a point in the obvious way (any constant map ...
The degree of a map is a homotopy invariant; moreover for continuous maps from the sphere to itself it is a complete homotopy invariant, i.e. two maps ,: are homotopic if and only if = (). In other words, degree is an isomorphism between [ S n , S n ] = π n S n {\displaystyle \left[S^{n},S^{n}\right]=\pi _{n}S^{n}} and Z {\displaystyle ...
Two maps , are called homotopic relative to A if they are homotopic by a basepoint-preserving homotopy : [,] such that, for each p in and t in [,], the element (,) is in A. Note that ordinary homotopy groups are recovered for the special case in which A = { x 0 } {\displaystyle A=\{x_{0}\}} is the singleton containing the base point.
The null homotopic class acts as the identity of the group addition, and for X equal to S n (for positive n) — the homotopy groups of spheres — the groups are abelian and finitely generated. If for some i all maps are null homotopic, then the group π i consists of one element, and is called the trivial group.
Indeed, both above composites are homotopic, for example, to the loop that traverses all three loops ,, with triple speed. The set of based loops up to homotopy, equipped with the above operation therefore does turn π 1 ( X , x 0 ) {\displaystyle \pi _{1}(X,x_{0})} into a group.
For example, the category of (reasonable) topological spaces has a structure of a model category where a weak equivalence is a weak homotopy equivalence, a cofibration a certain retract and a fibration a Serre fibration. [20] Another example is the category of non-negatively graded chain complexes over a fixed base ring. [21