nLab
deformation retract

Contents

Idea

A deformation retract is a retract which is also a section up to homotopy. Equivalently, it is a homotopy equivalence one of whose two homotopies is in fact an identity.

Definition

Let 𝒞\mathcal{C} be a category equipped with a notion of homotopy between its morphisms. Then a deformation retraction of a morphism

i:AX i : A \to X

(the deformation retract itself) is another morphism

r:XA r : X \to A

such that

X i = r A = A \array{ && X \\ & {}^{\mathllap{i}}\nearrow &\Downarrow^=& \searrow^{\mathrlap{r}} \\ A &&\stackrel{=}{\to}&& A }

and

A r i X = X. \array{ && A \\ & {}^{\mathllap{r}}\nearrow &\Downarrow^{\simeq}& \searrow^{\mathrlap{i}} \\ X &&\stackrel{=}{\to}&& X } \,.

In particular, if “homotopy” in 𝒞\mathcal{C} means left homotopy with respect to an cylinder object IXI \otimes X

X d 0 Id X IX σ X X d 1 Id X X, \array{ X \\ \downarrow^{\mathrlap{d_0}} & \searrow^{\mathrlap{Id_X}} \\ I \otimes X &\stackrel{\sigma_X}{\to}& X \\ \uparrow^{\mathrlap{d_1}} & \nearrow_{\mathrlap{Id_X}} \\ X } \,,

then a deformation retract of i:AXi : A \to X is a morphism r:XAr : X \to A such that ri=id Ar \circ i = id_A and such that there exists a morphism η:IXX\eta : I \otimes X \to X fitting into a diagram

X r A d 0 i IX η X d 1 Id X X. \array{ X &\stackrel{r}{\to}& A \\ \downarrow^{\mathrlap{d_0}} && \downarrow^{\mathrlap{i}} \\ I \otimes X &\stackrel{\eta}{\to}& X \\ \uparrow^{\mathrlap{d_1}} & \nearrow_{\mathrlap{Id_X}} \\ X } \,.

If the cylinder object assignment here is functorial, we say that η\eta is a strong deformation retract if moreover

η(Ii)=σ X(Ii) \eta \circ (I \otimes i) = \sigma_X \circ (I \otimes i)

(hence if the homotopy restricted to the inclusion is “constant” as seen by the chosen cylinder object).

In parts of the literature, deformation retracts are required to be strong by default.

Examples

In topological spaces

In the category Top of topological spaces the standard cylinder object is given by cartesian product with the interval I:=[0,1]I := [0,1].

With respect to the corresponding notion of left homotopy, if XX is a topological space and AXA\subset X a subspace, then AA is a strong deformation retract of XX if there exists a continuous map H:X×IXH:X\times I\to X such that H(a,t)=aH(a,t)=a for all aAa\in A, tI=[0,1]t\in I=[0,1], H(x,0)=xH(x,0) = x for all xXx\in X and H(x,1)AH(x,1)\in A for all xXx\in X.

Equivalently, there are continuous maps i:AXi:A\to X and r:XAr:X\to A such that ri=id Ar\circ i = id_A and irid X(relA)i\circ r\sim id_X (rel A), where (relA)\sim (rel A) denotes homotopy with fixed AA. More generally, for any continuous map j:ZYj:Z\to Y we say that it is deformation retractable if there is r:YZr:Y\to Z such that jrid Yj\circ r\sim id_Y and rj=id Zr\circ j = id_Z.

A pair (X,A)(X,A) is an NDR-pair if there are two continuous maps, u:XI,H:X×IXu:X\to I,\; H:X\times I\to X such that H(a,t)=aH(a,t)=a for all aAa\in A and all tt, H(x,0)=xH(x,0)=x for all xXx\in X, u 1(0)=Au^{-1}(0)=A and H(x,1)AH(x,1)\in A for all xx such that u(x)<1u(x)\lt 1. If (X,A)(X,A) is an NDR-pair, then the inclusion has a left homotopy inverse iff AA is also a retract of XX (in Top, in the standard categorical sense).

The pair (X,A)(X,A) is a DR-pair if it is a deformation retract and there is a function u:XIu:X\to I such that A=u 1(0)A=u^{-1}(0) (i.e. it gives simultaneously a deformation retract and a NDR-pair). If (X,A)(X,A) is an NDR-pair then the inclusion AXA\hookrightarrow X is a homotopy equivalence iff AA is a deformation retract of XX. Any map f:XYf:X\to Y is a homotopy equivalence iff XX is the deformation retract of the mapping cylinder of ff. If (X,A)(X,A) is an NDR-pair and AA is contractible, then the quotient map XX/AX\to X/A is a homotopy equivalence.

References

For instance

Revised on April 4, 2012 11:22:45 by Urs Schreiber (82.172.178.200)