# nLab simplicial localization

### Context

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

(∞,1)-category theory

# Simplicial localisation

## Idea

A category with weak equivalences or homotopical category is a category $C$ equipped with the information that some of its morphisms, specifically, a subcategory $W \supset Core(C)$, are to be regarded as “weakly invertible”. One way to make this notion precise is through the concept of simplicial localization:

The simplicial localization $L C$ of a category $C$ is an (∞,1)-category realized concretely as a simplicially enriched category which is such that the original category injects into it, $C \hookrightarrow L C$, such that every morphism in $C$ that is labeled as a weak equivalence becomes an actual equivalence in the sense of morphisms in (∞,1)-categories in $L C$. And $L C$ is in some sense universal with this property.

Passing to the homotopy category of an (∞,1)-category of $L C$ then reproduces the homotopy category that can also directly be obtained from $C$:

$Ho_C(a,b) \simeq \Pi_0 (L C(a,b))$

(where $\Pi_0$ gives the 0th simplicial homotopy groupoid).

If the homotopical structure on $C$ extends to that of a (combinatorial) model category, then there is another procedure to obtain a simplicially enriched category from $C$, the (∞,1)-category presented by a combinatorial model category. This $(\infty,1)$-category is equivalent to the one obtained by simplicial localization but typically more explicit and more tractable.

## Definition

### “Standard” simplicial localization

Let $U : \mathbf{Cat} \to \mathbf{Grph}$ be the forgetful functor that sends a (small) category to its underlying reflexive graph, and let $F : \mathbf{Grph} \to \mathbf{Cat}$ be its left adjoint. We then get a comonad $\mathbb{G} = (G, \epsilon, \delta)$ on $\mathbf{Cat}$, and as usual this defines a functor $G_\bullet : \mathbf{Cat} \to [\mathbf{\Delta}^{op}, \mathbf{Cat}]$ from Cat to simplicial objects in Cat equipped with a canonical augmentation, where $G_n C = G^{n+1} C$.

###### Definition

The standard resolution of a small category $C$ is defined to be the simplicial category $G_\bullet C$.

Note that this is also a simplicial category in the strong sense, i.e. $ob G_\bullet C$ is discrete! Thus we may also regard $G_\bullet C$ as an sSet-category. This is a resolution in the sense that the augmentation $\epsilon : G_\bullet C \to C$ is a Dwyer-Kan equivalence. (In fact, for objects $X$ and $Y$ in $C$, the morphism $G_\bullet C (X, Y) \to C (X, Y)$ admits an extra degeneracy and hence a contracting homotopy.)

###### Definition

The standard simplicial localization of a relative category $(C, W)$ is the simplicial category $L_\bullet (C, W)$ where $L_n (C, W) = G_n C [{G_n W}^{-1}]$.

This appears as (DwyerKanLocalizations, def. 4.1). Again, $L_\bullet C$ is a simplicial category in the strong sense, because $G_\bullet C$ is.

### Hammock localization

###### Definition

Let $(C,W)$ be a category with weak equivalences. For $X,Y \in C$ any two objects, write

$L^H C(X,Y) \in sSet$

for the simplicial set defined as follows. For each natural number $n$ there is a category defined as follows:

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

$X \stackrel{\simeq}{\leftarrow} K_1 \to K_2 \stackrel{\simeq}{\leftarrow} K_3 \to \cdots \to Y \,,$

where the left-pointing morphisms are to be in $W$;

• its morphisms are “natural transformations” between such objects, fixing the endpoints:

$\array{ && K_1 &\to& K_2 &\stackrel{\simeq}{\leftarrow}& \cdots \\ & {}^{\mathllap{\simeq}}\swarrow &&&&& && \searrow^{} \\ X && \downarrow^{\simeq}&& \downarrow^{\simeq}& \cdots&& && Y \\ & {}_{\mathllap{\simeq}}\nwarrow &&&&& && \nearrow^{} \\ && L_1 &\to& L_2 &\stackrel{\simeq}{\leftarrow}& \cdots } \;$

$L^H C(X,Y)$ is obtained by

• taking the coproduct of the nerves of these categories over all $n$, and

• quotienting by the equivalence relation generated by inserting or removing identity morphisms and composing composable morphisms.

For $X,Y,Z$ three objects, there is an evident compositing morphism

$L^H C(X,Y) \times L^H C(Y,Z) \to L^H C(X,Z)$

given by horizontally concatenating hammock diagrams as above.

The simplicially enriched category $L^H C$ obtained this way is the hammock localization of $(C,W)$.

This appears as (DwyerKanCalculating, def. 2.1).

## Properties

### Basic properties

###### Proposition

For $(C,W)$ a category with weak equivalences, write $L^H C \in sSet Cat$ for its hammock localization and $C[W^{-1}] \in Cat$ for its ordinary localization. Write $Ho(L^H C) \in Cat$ for the category with the same objects as $C$ and morphisms between $X$ and $Y$ being $\pi_0 L^H C(X,Y)$.

There is an equivalence of categories

$Ho L^H C \simeq C[W^{-1}] \,.$

This appears as (DwyerKanCalculating, prop. 3.1).

###### Proposition

Let $(C,W)$ be a category with weak equivalences, and let

$(f : X \to Y) \in W \subset Mor(C)$

be a weak equivalence. Then for all objects $U \in C$ we have that the to concatenation operations on hammocks induce weak homotopy equivalences

$f_* : L^H C(U,X) \stackrel{\simeq}{\to} L^H C(U,Y)$

and

$f^* : L^H C(Y,U) \stackrel{\simeq}{\to} L^H C(X, U) \,.$

This appears as (DwyerKanCalculating, prop. 3.3).

### Simplical localization gives all $(\infty,1)$-categories

###### Proposition

Up to Dwyer-Kan equivalence –the weak equivalences in the model structure on sSet-categories – every simplicial category is the simplicial localization of a category with weak equivalences.

This is (DwyerKan 87, 2.5).

If the category with weak equivalences in question happens to carry even the structure of a model category there exist more refined tools for computing the SSet-hom object of the simplicial localization. These are described at (∞,1)-categorical hom-space.

### Equivalences between simplicial localizations

###### Proposition

Let $(C,W)$ and $(C', W')$ be categories with weak equivalences. Write $L^H C, L^H C' \in sSet Cat$ for the corresponding hammock localizations.

Then for $F_1, F_2 : C \to C'$ two homotopical functors (functors respecting the weak equivalences, i.e. $F_i(W) \subset W'$) with

$\eta : F_1 \Rightarrow F_2$

a natural transformation with components in the $W'$, we have that for all objects $X,Y \in C$, there is induced a natural homotopy between morphisms of simplicial sets

$\array{ && L^H C'(F_1(X), F_1(Y) ) \\ & {}^{\mathllap{L^H F_1}}\nearrow && \searrow^{\mathrlap{\eta(Y)_*}} \\ L^H C(X,Y) && \Downarrow && L^H C'(F_1(X), F_2(Y)) \\ & {}_{\mathllap{L^H F_2}}\searrow && \nearrow_{\mathrlap{\eta(X)^*}} \\ && L^H C' (F_2(X), F_2(Y)) } \,.$

This is (DwyerKanComputations, prop. 3.5).

###### Corollary

Let $i : (C_1, W_1) \hookrightarrow (C_2, W_2)$ be a full subcategory such that

1. $i$ is homotopy-essentially surjective: for every object $c_2 \in C_2$ there is an object $c_1 \in C_1$ and a weak equivalence $c_2 \stackrel{\simeq}{\to} i(c_1)$;

2. there is a functor $Q : (C_2,W_2) \to (C_1, W_1)$ and a natural transformation

$i \circ Q \Rightarrow Id_{C_2} \,.$

Then we have an equivalence of (∞,1)-categories

$L^H C_1 \simeq L^H C_2 \,.$
###### Proof

We have to check that $i$ is an essentially surjective (∞,1)-functor and a full and faithful (∞,1)-functor.

The first condition is immediate from the first assumption. The second follows with prop. 4 (using prop. 2) from the second assumption.

### Simplicial localization of model categories

###### Proposition

Let $C$ be a simplicial model category. Write $C^\circ$ for the full $Set$-subcategory on the fibrant and cofibrant objects.

Then $C^\circ$ and $L^H C$ are connected by an equivalence of (∞,1)-categories.

This is one of the central statements in (DwyerKanFunctionComplexes). The weak homotopy equivalence between $C^\circ(X,Y)$ and $L^H C(X,Y)$ is in corollary 4.7. The equivalence of $\infty$-categories stated above follows with this and the diagram of morphisms of simplicial categories in prop. 4.8.

## References

The original articles are

A survey of the general topic involved here can be found in the following paper:

• Tim Porter, $S$-Categories, $S$-groupoids, Segal categories and quasicategories (arXiv)

Hammock localization is described in Section 4.1 there.

A useful quick collection of facts can be found at the beginning of Section 2 in the following paper:

Revised on March 29, 2016 06:04:58 by Urs Schreiber (195.37.209.180)