An object $A$ in a category is called a retract of an object $B$ if there are morphisms $i\colon A\to B$ and $r \colon B\to A$ such that $r \circ i = id_A$. In this case $r$ is called a retraction of $B$ onto $A$.
Here $i$ may also be called a section of $r$. (In particular if $r$ is thought of as exhibiting a bundle; the terminology originates from topology.)
Hence a retraction of a morphism $i \;\colon\; A \to B$ is a left-inverse.
In this situation, $r$ is a split epimorphism and $i$ is a split monomorphism. The entire situation is said to be a splitting of the idempotent
Accordingly, a split monomorphism is a morphism that has a retraction; a split epimorphism is a morphism that is a retraction.
Retracts are clearly preserved by any functor.
A split epimorphism $r; B \to A$ is the strongest of various notions of epimorphism (e.g., it is a regular epimorphism, in fact an absolute? coequalizer, being the coequalizer of a pair $(e, 1_B)$ where $e = i \circ r: B \to B$ is idempotent). Dually, a split monomorphism is the strongest of various notions of monomorphism.
If an object $B$ has the left lifting property against a morphism $X \to Y$, then so does every of its retracts $A \to B$:
Let $C$ be a category with split idempotents and write $PSh(C) = [C^{op}, Set]$ for its presheaf category. Then a retract of a representable functor $F = PSh(C)$ is itself representable.
This appears as (Borceux, lemma 6.5.6)
The inclusion of standard topological horns into the topological simplex $\Lambda^n_k \hookrightarrow \Delta^n$ is a retract in Top.
Let $\Delta[1] = \{0 \to 1\}$ be the interval category. For every category $C$ the functor category $[\Delta[1], C]$ is the arrow category of $C$.
In the theory of weak factorization systems and model categories, an important role is played by retracts in $C^{\Delta[1]}$, the arrow category of $C$. Explicitly spelled out in terms of the original category $C$, a morphism $f:X\to Y$ is a retract of a morphism $g:Z\to W$ if we have commutative squares
such that the top and bottom rows compose to identities.
Classes of morphisms in a category $C$ that are given by a left or right lifting property are preserved under retracts in the arrow category $[\Delta[1],C]$. In particular the defining classes of a model category are closed under retracts.
This is fairly immediate, a proof is made explicit here.
This implies:
In every category $C$ the class of isomorphisms is preserved under retracts in the arrow category $[\Delta[1], C]$
This is also checked directly: for
a retract diagram and $a_2 \to b_2$ an isomorphism, the inverse to $a_1 \to b_1$ is given by the composite
where $b_2 \to a_2$ is the inverse of the middle morphism.
For the following, let $C$ and $J$ be categories and write $J^{\triangleleft}$ for the join of $J$ with a single initial object, so that functors $J^{\triangleleft} \to C$ are precisely cones over functors $J \to C$. Write
for the canonical inclusion and hence $i^* F$ for the underlying diagram of a cone $F : J^{\triangleleft} \to C$. Finally, write $[J^{\triangleleft}, C]$ for the functor category.
If $Id: F_1 \hookrightarrow F_2 \to F_1$ is a retract in the category $[J^{\triangleleft}, C]$ and $F_2 : J^{\triangleleft} \to C$ is a limit cone over the diagram $i^* F_2 : J \to C$, then also $F_1$ is a limit cone over $i^* F_1$.
We give a direct and a more abstract argument.
Direct argument. We can directly check the universal property of the limit: for $G$ any other cone over $i^* F_1$, the composite $i^* G = i^* F_1 \to i^* F_2$ exhibits $G$ also as a cone over $i^* F_2$. By the pullback property of $F_2$ this extends to a morphism of cones $G \to F_2$. Postcomposition with $F_2 \to F_1$ makes this a morphism of cones $G \to F_1$. By the injectivity of $F_1 \to F_2$ and the universality of $F_2$, any two such cone morphisms are equals.
More abstract argument. The limiting cone over a diagram $D : J \to C$ may be regarded as the right Kan extension $i_* D := Ran_i D$ along $i$
Therefore a cone $F : J^{\triangleleft} \to C$ is limiting precisely if the $(i^* \dashv i_*)$-unit
is an isomorphism. Since this unit is a natural transformation it follows that applied to the retract diagram
it yields the retract diagram
in $[\Delta[1], [J^{\triangleleft}, C]]$. Here by assumption the middle morphism is an isomorphism. Since isomorphisms are stable under retract, by prop. 4, also the left and right vertical morphism is an isomorphism, hence also $F_1$ is a limiting cone.
This argument generalizes form limits to homotopy limits.
For that, let now $C$ be a category with weak equivalences and write $Ho(C) : Diagram^{op} \to Cat$ for the corresponding derivator: $Ho(C)(J) := [J,C](W^J)^{-1}$ is the homotopy category of $J$-diamgrams in $C$, with respect to the degreewise weak equivalences in $C$.
Let
be a retract in $Ho(C)(J^{\triangleleft})$. If $F_2$ is a homotopy limit cone over $i^* F_2$, then also $F_1$ is a homotopy limit cone over $i^* F_1$.
By the discussion at derivator we have that
$i_* : Ho(C)(J) \to Ho(C)(J^{\triangleleft})$ forms homotopy limit cones;
$F \to i_* i^* F$ is an isomorphism precisely if $F$ is a homotopy limit cone.
With this the claim follows as in prop. 5.
In
the definition appears as def. 1.7.3. Properties are discussed in section 6.5
Last revised on January 23, 2018 at 12:49:11. See the history of this page for a list of all contributions to it.