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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.
Let $\mathcal{C}$ be a category equipped with a notion of homotopy between its morphisms. Then a deformation retraction of a morphism
(the deformation retract itself) is another morphism
such that
and
In particular, if “homotopy” in $\mathcal{C}$ means left homotopy with respect to an cylinder object $I \otimes X$
then a deformation retract of $i : A \to X$ is a morphism $r : X \to A$ such that $r \circ i = id_A$ and such that there exists a morphism $\eta : I \otimes X \to X$ fitting into a diagram
Hence a deformation retract is a (left) homotopy equivalence where one of the two homotopies occuring is in fact an identity.
If the cylinder object assignment here is functorial, we say that $\eta$ is a strong deformation retract if moreover
(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.
In the category Top of topological spaces the standard cylinder object is given by cartesian product with the interval $I \coloneqq [0,1]$.
With respect to the corresponding notion of left homotopy, if $X$ is a topological space and $A\subset X$ a subspace, then $A$ is a strong deformation retract of $X$ if there exists a continuous map $H \colon X\times I\to X$ such that $H(a,t) = a$ for all $a\in A$, $t\in I=[0,1]$, $H(x,0) = x$ for all $x\in X$ and $H(x,1)\in A$ for all $x\in X$.
Equivalently, there are continuous maps $i \colon A\to X$ and $r \colon X\to A$ such that $r \circ i = id_A$ and $i\circ r\sim id_X (rel A)$, where $\sim (rel A)$ denotes homotopy with fixed $A$. More generally, for any continuous map $j \colon Z\to Y$ we say that it is deformation retractable if there is $r \colon Y\to Z$ such that $j\circ r\sim id_Y$ and $r\circ j = id_Z$.
A pair $(X,A)$ is an NDR-pair if there is a pair of continuous maps, $u \colon X\to I,\; H \colon X\times I\to X$ such that $H(a,t)=a$ for all $a\in A$ and all $t$, $H(x,0)=x$ for all $x\in X$, $u^{-1}(0)=A$ and $H(x,1)\in A$ for all $x$ such that $u(x)\lt 1$. If $(X,A)$ is an NDR-pair, then the inclusion has a left homotopy inverse iff $A$ is also a retract of $X$ (in Top, in the standard category-theoretic sense).
The pair $(X,A)$ is a DR-pair if it is a deformation retract and there is a function $u \colon X\to I$ such that $A=u^{-1}(0)$ (i.e. it gives simultaneously a deformation retract and a NDR-pair). If $(X,A)$ is an NDR-pair then the inclusion $A\hookrightarrow X$ is a homotopy equivalence iff $A$ is a deformation retract of $X$. Any map $f:X\to Y$ is a homotopy equivalence iff $X$ is the deformation retract of the mapping cylinder of $f$. If $(X,A)$ is an NDR-pair and $A$ is contractible, then the quotient map $X\to X/A$ is a homotopy equivalence.
(Eilenberg-Zilber/Alexander-Whitney deformation retraction)
Let
and denote
by $N(A), N(B) \,\in\, Ch^+_\bullet =$ ConnectiveChainComplexes their normalized chain complexes,
by $A \otimes B \,\in\, sAb$ the degreewise tensor product of abelian groups,
by $N(A) \otimes N(B)$ the tensor product of chain complexes.
Then there is a deformation retraction
where
$\nabla_{A,B}$ is the Eilenberg-Zilber map;
$\Delta_{A,B}$ is the Alexander-Whitney map.
For unnormalized chain complexes, where we have a homotopy equivalence, this is the original Eilenberg-Zilber theorem (Eilenberg & Zilber 1953, Eilenberg & MacLane 1954, Thm. 2.1). The above deformation retraction for normalized chain complexes is Eilenberg & MacLane 1954, Thm. 2.1a. Both are reviewed in May 1967, Cor. 29.10. Explicit description of the homotopy operator is given in Gonzalez-Diaz & Real 1999.
There is also the notion of a deformation retract of a homotopical category, which has a similar feel in some ways but is not closely related. (It should not be confused with the idea of a deformation retract in a model category, which is a direct generalization of the notion described above for Top.)
Textbook accounts
George Whitehead, chapter 1 of: Elements of Homotopy Theory, Springer 1978 (doi:10.1007/978-1-4612-6318-0)
Peter May, Section 6.4 of: A concise course in algebraic topology, University of Chicago Press 1999 (ISBN: 9780226511832, pdf)