# nLab (infinity,1)-categorical hom-space

Contents

### Context

#### $(\infty,1)$-Category theory

(∞,1)-category theory

## Model structures

for ∞-groupoids

### for $(\infty,1)$-sheaves / $\infty$-stacks

#### Mapping space

internal hom/mapping space

# Contents

## Idea

Where an ordinary category has a hom-set, an (∞,1)-category has an ∞-groupoid of morphisms between any two objects, a hom-space.

There are several ways to present an (∞,1)-category $\mathbf{C}$ by an ordinary category $C$ equipped with some extra structure: for instance $C$ may be a category with weak equivalences or a model category or even a simplicial model category. In all of these presentations, given two objects $X, Y \in C$, there is a way to construct a simplicial sets $\mathbb{R}\mathbf{C}(X,Y)$ that presents the hom-∞-groupoid $\mathbf{C}(X,Y)$. This simplicial set – or rather its homotopy type – is called the derived hom space or homotopy function complex and denoted $\mathbf{R}Hom(X,Y)$ or similarly.

## Presentations

There are many ways to present an (∞,1)-category by category theoretic data, and for each of these there are corresponding tools for explicitly computing the derived hom spaces.

The most basic data is that of a category with weak equivalences. Here the derived hom spaces can be constructed in terms of zig-zags of morphisms by a process called simplicial localization. This we discuss below in For a category with weak equivalences.

Particularly useful extra structure on a category with weak equivalences that helps with computing the derived hom spaces is the structure of a model category. Using this one can construct simplicial resolutions of objects – called framings – that generalize cylinder objects and path objects, and then construct the derived hom spaces in terms of direct morphisms between these resolutions. This we discuss below in For a model category.

Still a bit more helpful structure on top of a bare model category is that of a simplicial model category. Here, after a choice of cofibrant and fibrant resolutions of opjects, the derived hom spaces are given already by the sSet-hom objects. This we discuss below in For a simplicial model category.

### For a category with weak equivalences

Let $(C,W \subset Mor(C))$ be a category with weak equivalences.

###### Definition

Fix $n \in \mathbb{N}$. For $X,Y \in Obj(C)$, define a category $wMor_C^n(X,Y)$

• whose objects are zig-zags of morphisms in $C$ of length $n$

$X = X_0 \leftarrow X_1 \to X_2 \leftarrow \cdots \to X_{n-1} \leftarrow X_n = Y$

such that each morphism going to the left, $X_{2k}\leftarrow X_{2k +1}$, is a weak equivalence, an element in $W$;

• morphisms between such objects $(X,X_i,Y) \to (X',X'_i,Y')$ are collections of weak equivalences $(X_i \to X'_i)$ for all $0 \lt i \lt n$ such that all triangles and squares commute.

###### Definition

Write $N(wMor_C^n(X,Y))$ for the nerve of this category, a simplicial set.

The hammock localization $L_W^H C$ of $C$ is the simplicially enriched category with objects those of $C$ and hom-objects given by the colimit over the length of these hammock hom-categories

$L^H C(X,Y) := \lim_{\to_n} N(wMor_C^n(X,Y)) \,.$

The Kan fibrant replacement of this simplicial set is the derived hom-space between $X$ and $Y$ of the $(\infty,1)$-category modeled by $(C,W)$.

### For a model category

The derived hom spaces of a model category $C$ may always be computed in terms of simplicial resolutions with respect to the Reedy model structure $[\Delta^{op}, C]_{Reedy}$. These resolutions are often called framings (Hovey). These constructions are originally due to (Dwyer-Hirschhorn-Kan).

Let $C$ be any model category.

###### Observation

$(const \dashv ev_0 \dashv (-)^{\times^\bullet}) : C \stackrel{\overset{const}{\longrightarrow}}{\stackrel{\overset{ev_0}{\longleftarrow}}{\underset{(-)^{\times^\bullet}}{\longrightarrow}}} \,, [\Delta^{op}, C] \,,$

where

1. $const X : [n] \mapsto X$;

2. $ev_0 X_\bullet = X_0$;

3. $X^{\times^\bullet} : [n] \mapsto X^{\times^{n+1}}$.

###### Remark

For $X \in C$ fibrant, $X^{\times^\bullet}$ is fibrant in the Reedy model structure $[\Delta^{op}, C]_{Reedy}$.

###### Proof

The matching morphisms are in fact isomorphisms.

###### Definition

Let $C$ be a model category.

1. For $X \in C$ any object, a simplicial frame on $X$ is a factorization of $const X \to X^{\times^\bullet}$ into a weak equivalence followed by a fibration in the Reedy model structure $[\Delta^{op}, C]_{Reedy}$.

2. A right framing in $C$ is a functor $(-)_\bullet : C \to [\Delta^{op}, C]$ with a natural isomorphism $(X)_0 \simeq X$ such that $X_\bullet$ is a simplicial frame on $X$.

Dually for cosimplicial frames.

This appears as (Hovey, def. 5.2.7).

###### Remark

By remark a simplicial frame $X_\bullet$ in the above is in particular fibrant in $[\Delta^{op}, C]_{Reedy}$.

###### Proposition

For $X \in C$ cofibrant and $A \in C$ fibrant, there are weak equivalences in $sSet_{Quillen}$

$Hom_C(X^\bullet, A) \stackrel{\simeq}{\to} diag Hom_C(X^\bullet, A_\bullet) \stackrel{\simeq}{\leftarrow} Hom_C(X, A_\bullet) \,,$

(where in the middle we have the diagonal of the bisimplicial set $Hom(X^\bullet, A_\bullet)$).

This appears as (Hovey, prop. 5.4.7).

Either of these simplicial sets is a model for the derived hom-space $\mathbb{R}Hom(X,A)$.

###### Remark

By developing these constructions further, one obtains a canonical simplicial model category-resolution of (left proper and combinatorial) model categories $C$, such that the simplicial resolutions given by framings are just the cofibrant$\to$fibrant $sSet$-hom objects as discussed below.

This is discussed at Simplicial Quillen equivalent models.

###### Proposition

Let $C$ be a model category, let $\mathrm{c}_\mathrm{w} C$ be the full subcategory of $[\Delta, C]$ spanned by the cosimplicial objects whose coface and codegeneracy operators are weak equivalences, and let $\mathrm{s}_\mathrm{w} C$ be the full subcategory of $[\Delta^{op}, C]$ spanned by the simplicial objects whose face and degeneracy operators are weak equivalences.

1. $const : C \to \mathrm{c}_\mathrm{w} C$ is the right half of an adjoint homotopical equivalence of homotopical categories, and $const : C \to \mathrm{s}_\mathrm{w} C$ is the left half of an adjoint homotopical equivalence of homotopical categories.
2. The functor $\operatorname{diag} Hom_C : (\mathrm{c}_\mathrm{w} C)^{op} \times \mathrm{s}_\mathrm{w} C \to sSet$ admits a right derived functor.
3. The induced functor $(\operatorname{Ho} C)^{op} \times \operatorname{Ho} C \to \operatorname{Ho} sSet$ is the derived hom-space functor.

### For a simplicial model category

We describe here in more detail properties of derived hom-functors (see there for more) in a simplicial model category.

The crucial axiom used for this is the axiom of an enriched model category $C$ which says that

• $\cdot : C \times SSet \to C$

is a Quillen bifunctor;

• or equivalently that for $X \to Y$ a cofibration and $A \to B$ a fibration the induced morphism

$C(Y, A) \to C(X,A) \times_{C(X,B)} C(Y,B)$

is a fibration, which is acyclic if either $X \to Y$ or $A \to B$ is.

First of all the first statement directly implies that for $\emptyset \in C$ the initial object and $A \in C$ any object, the simplicial set $C(\emptyset,A) = {*}$ is the terminal simplicial set (see also this Prop.): because for any simplicial set $S$

\begin{aligned} SSet(S,C(\emptyset, A)) & = Hom_C(\emptyset \cdot S, A) \\ & = Hom_C(colim_{\emptyset} \cdot S, A) \\ & = Hom_C(\emptyset, A) \\ &= {*} \end{aligned} \,,

where we use that the tensor Quillen bifunctor is required to respect colimits and that the empty colimit is the initial object. (All equality signs here denote isomorphisms, to distinguish them from weak equivalences.)

Similarly one has for all $X$ that $C(X,{*}) = {*}$.

Using this, the second equivalent form of the enrichment axiom has as a special case the following statement.

###### Lemma

In a simplicial model category $C$, for $X \in C$ cofibrant and $A \in C$ fibrant, the simplicial set $C(X,A)$ is a Kan complex.

###### Proof

We apply the enriched model category axiom to the cofibration $\emptyset \to X$ and the fibration $A \to {*}$ to obtain a fibration

$C(X,A) \to C(\emptyset, A) \times_{C(\emptyset,{*})} C(X,{*}) \,.$

The right hand is the pullback of the terminal simplicial set ${*} = \Delta^0$ to itself, hence is itself the point. So we have a fibration $C(X,A) \to {*}$ and $C(X,A)$ is a fibrant object in the standard model structure on simplicial sets, hence a Kan complex. .

###### Lemma

In a simplicial model category $C$, for $X \in C$ cofibrant and $f : A \to B$ a fibration, the morphism of simplicial sets $C(X,f) : C(X,A) \to C(X,B)$ is a Kan fibration that is a weak homotopy equivalence if $f$ is acyclic.

Dually, for $i : X \to Y$ a cofibration and $A$ fibrant, the morphism $C(i,A) : C(X,A) \to C(Y,A)$ is a cofibration of simplicial sets.

###### Proof

This is as before. Explicity, consider the first case, the second one is the formal dual of that:

We enter the enrichment axiom with the morphisms $\emptyset \to X$ and $A \to B$ and find for the required pullback that

$C(\emptyset,A) \times_{C(\emptyset, B)} C(X,B) = {*} \times_{*} C(X,B) = C(X,B)$

and hence that $C(X,A) \to C(X,B)$ is, indeed, a fibration, which is acyclic if $A \to B$ is.

###### Proposition

Let $C$ be a simplicial model category.

Then for $X$ a cofibant object and

$f : A \stackrel{\simeq}{\to} B$

a weak equivalence between fibrant objects, the enriched hom-functor

$C(X,f) : C(X,A) \to C(X,B)$

is a weak homotopy equivalence of Kan complexes.

Similarly, for $A$ a fibrant object and $j : X \stackrel{\simeq}{\to} Y$ a weak equivalence between cofibrant objects, the morphism

$C(j,A) : C(X,A) \to C(Y,A)$

is a weak homotopy equivalence of Kan complexes.

###### Proof

The second case is formally dual to the first, so we restrict attention to the first one.

By the above, the axioms of an enriched model category ensure that the above statement is true when $f$ is in addition a fibration. So we reduce the situation to that case.

This is possible because both $A$ and $B$ are assumed to be fibrant. This allows to apply the factorization lemma that is described in great detail at category of fibrant objects. By this lemma, for every morphism $f : A \to B$ between fibrant objects there is a commutative diagram

$\array{ && \mathbf{E}_f B \\ & {}^{\mathllap{\in fib \cap W}}\swarrow && \searrow^{\mathrlap{\in fib}} \\ A &&\stackrel{\simeq}{\to}&& B }$

Since $f$ is assumed a weak equivalence it follows by 2-out-of-3 that $\mathbf{E}_f B$ is also a weak equivalence.

Therefore by the above properties of simpliciall enriched categories we obtain a span of acyclic fibrations of Kan complexes

$C(X,A) \stackrel{\simeq}{\leftarrow} C(X, \mathbf{E}_f B) \stackrel{\simeq}{\to} C(X,B) \,.$

By the Whitehead theorem every weak equivalence of Kan complexes is a homotopy equivalence, hence there is a weak equivalence

$C(X,A) \stackrel{\simeq}{\to} C(X,\mathbf{E}_f B) \stackrel{\simeq}{\to} C(X,B)$

that is homotopic to our $C(X,f)$. Therefore this is also a weak equivalence.

### Comparison

Let $C$ be a model category. We discuss how its simplicial function complexes from prop. are related to the simplicial localization from def. and def. .

Suppose now that $Q : C \to C$ is a cofibrant replacement functor and $R : C \to C$ a fibrant replacement functor, $\Gamma^\bullet : C \to (cC)_c$ a cosimplicial resolution functor and $\Lambda_\bullet : C \to (sC)_f$ a simplicial resolution functor in the model category $C$.

###### Theorem

(Dwyer–Kan)

There are natural weak equivalences between the following equivalent realizations of this SSet hom-object:

$\array{ Mor_C(\Gamma^\bullet X, R Y) &\stackrel{\simeq}{\to}& diag Mor_C(\Gamma^\bullet X, \Lambda_\bullet Y) &\stackrel{\simeq}{\leftarrow}& Mor_C(Q X, \Lambda_\bullet Y) \\ && \uparrow^\simeq \\ && hocolim_{p,q \in \Delta^{op} \times \Delta^{op}} Mor_C(\Gamma^p X, \Lambda_q Y) \\ &&\downarrow^\simeq \\ &&N wMor_C^3(X,Y) \\ &&\downarrow^\simeq \\ &&Mor_{L^H C}(X,Y) } \,.$

The top row weak equivalences are those of prop.

### In a category of fibrant objects

There is also an explicit simplicial construction of the derived hom spaces for a homotopical category that is equipped with the structure of a category of fibrant objects. This is described in (Cisinksi 10) and (Nikolaus-Schreiber-Stevenson 12, section 3.6.2).

## Properties

### Hom-spaces of equivalences

###### Theorem

For $C$ a simplicial model category and $X$ an object, the delooping of the automorphism ∞-group

$Aut_W(X) \subset \mathbb{R}Hom(X,X)$

has the homotopy type of the component on $X$ of the nerve $N(C_W)$ of the subcategory of weak equivalences:

$\mathbf{B} Aut_W(X) \simeq N(C_W)_X \,.$

The equivalence is given by a finite sequence of zig-zags and is natural with respect to sSet-enriched functors of simplicial model categories that preserve weak equivalences and send a fibrant cofibrant model for $X$ again to a fibrant cofibrant object.

This is Dwyer-Kan 84, 2.3, 2.4.

###### Corollary

For $C$ a model category, the simplicial set $N(C_W)$ is a model for the core of the (∞,1)-category determined by $C$.

###### Proof

That core, like every ∞-groupoid is equivalent to the disjoint union over its connected components of the deloopings of the automorphism $\infty$-groups of any representatives in each connected component.

homotopycohomologyhomology
$[S^n,-]$$[-,A]$$(-) \otimes A$
category theorycovariant homcontravariant homtensor product
homological algebraExtExtTor
enriched category theoryendendcoend
homotopy theoryderived hom space $\mathbb{R}Hom(S^n,-)$cocycles $\mathbb{R}Hom(-,A)$derived tensor product $(-) \otimes^{\mathbb{L}} A$

For some original references by William Dwyer and Dan Kan see simplicial localization. For instance

• William Dwyer, Dan Kan, A classification theorem for diagrams of simplicial sets, Topology 23 (1984), 139-155.

Discussion in terms of quasi-categories:

The theory of framings is due to

and in parallel section 5 of

and in sections 16, 17 of

A useful quick review of the interrelation of the various constructions of derived hom spaces is page 14, 15 of

Discussion of derived hom spaces for categories of fibrant objects is in

• Denis-Charles Cisinski, Invariance de la K-théorie par equivalences dérivées, J. K-theory, 6 (2010), 505–546.

and section 3.6.2 of