# Deformation retract

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{{#invoke:Hatnote|hatnote}}Template:Main other In topology, a branch of mathematics, a retraction,[1] as the name suggests, "retracts" an entire space into a subspace. A deformation retraction is a map which captures the idea of continuously shrinking a space into a subspace.

## Definitions

### Retract

Let X be a topological space and A a subspace of X. Then a continuous map

${\displaystyle r:X\to A}$

is a retraction if the restriction of r to A is the identity map on A; that is, r(a) = a for all a in A. Equivalently, denoting by

${\displaystyle \iota :A\hookrightarrow X}$

the inclusion, a retraction is a continuous map r such that

${\displaystyle r\circ \iota =id_{A},}$

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 space retracts to a point in the obvious way (the constant map yields a retraction). If X is hausdorff, then A must be closed.

A space X is known as an absolute retract (or AR) if for every normal space Y that embeds X as a closed subset, X is a retract of Y. The unit cube In as well as the Hilbert cube Iω are absolute retracts.

### Neighborhood retract

If there exists an open set U such that

${\displaystyle A\subset U\subset X}$

and A is a retract of U, then A is called a neighborhood retract of X.

A space X is an absolute neighborhood retract (or ANR) if for every normal space Y that embeds X as a closed subset, X is a neighborhood retract of Y. The n-sphere Sn is an absolute neighborhood retract.

### Deformation retract and strong deformation retract

A continuous map

${\displaystyle F:X\times [0,1]\to X\,}$

is a deformation retraction of a space X onto a subspace A if, for every x in X and a in A,

${\displaystyle F(x,0)=x,\;F(x,1)\in A,\quad {\mbox{and}}\quad F(a,1)=a.}$

In other words, a deformation retraction is a homotopy between a retraction and the identity map on X. The subspace A is called a deformation retract of X. A deformation retract is a special case of homotopy equivalence.

A retract need not be a deformation retract. For instance, having a single point as a deformation retract would imply a space is path connected (in fact, it would imply contractibility of the space).

Note: An equivalent definition of deformation retraction is the following. A continuous map r: XA is a deformation retraction if it is a retraction and its composition with the inclusion is homotopic to the identity map on X. In this formulation, a deformation retraction carries with it a homotopy between the identity map on X and itself.

If, in the definition of a deformation retraction, we add the requirement that

${\displaystyle F(a,t)=a\,}$

for all t in [0, 1], F is called a strong deformation retraction. In other words, a strong deformation retraction leaves points in A fixed throughout the homotopy. (Some authors, such as Allen Hatcher, take this as the definition of deformation retraction.)

## Properties

Deformation retraction is a particular case of homotopy equivalence. In fact, two spaces are homotopy equivalent if and only if they are both deformation retracts of a single larger space.

Any topological space which deformation retracts to a point is contractible. Contractibility, however, is a weaker condition, as contractible spaces exist which do not deformation retract to a point.[2]

## Notes

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