Contents

model category

for ∞-groupoids

# Contents

## Definition

###### Proposition

(slice model structure)
For $\mathcal{C}$ a model category and $X \in \mathcal{C}$ any object, the slice category $\mathcal{C}_{/X}$ as well as the coslice category $\mathcal{C}^{X/}$ inherit themselves structures of model categories, whose fibrations, cofibrations and weak equivalences are precisely the morphism whose images under the forgetful functors $\mathcal{C}_{/X} \to \mathcal{C}$ or $\mathcal{C}^{X/} \to \mathcal{C}$ are fibrations, cofibrations or weak equivalences, respectively, in $\mathcal{C}$.

## Properties

### Cofibrant generation, properness, combinatoriality

###### Proposition

If $\mathcal{C}$ is

then so are $\mathcal{C}_{/X}$ and $\mathcal{C}^{X/}$.

More in detail, if $I,J \subset Mor(\mathcal{C})$ are the classes of generating cofibrations and of generating acylic cofibrations of $\mathcal{C}$, respectively, then

• the generating (acyclic) cofibrations of $\mathcal{C}^{X/}$ are the image under $X \sqcup(-)$ of those of $\mathcal{C}$.
###### Proposition

If $\mathcal{C}$ is a combinatorial model category, then so is $\mathcal{C}_{/X}$.

###### Proof

By basic properties of locally presentable categories they are stable under slicing. Hence with $\mathcal{C}$ locally presentable also $\mathcal{C}_{/X}$ is, and by prop. with $\mathcal{C}$ cofibrantly generated also $\mathcal{C}_{/X}$ is.

###### Proposition

If $\mathcal{C}$ is an cartesian enriched model category?, then so is $\mathcal{C}_{/X}$.

###### Proof

By basic properties of cartesian enriched categories? they are stable under slicing, where tensoring is computed in $\mathcal{C}$. Hence with $\mathcal{C}$ enriched also $\mathcal{C}_{/X}$ is. The pushout product axiom now follows from the fact that in overcategories pushouts can be computed in the underlying category $\mathcal{C}$. The unit axiom follows from the unit axiom of $\mathcal{C}$ using the fact that tensorings are computed in $\mathcal{C}$.

### As presentations for over (∞,1)-categories

When restricted to fibrant objects, the operation of forming the model structure on an overcategory presents the operation of forming the over (∞,1)-category of an (∞,1)-category.

More explicitly, for any model category $\mathcal{C}$, let

(1)$\gamma \colon \mathcal{C} \longrightarrow L_W \mathcal{C}$

denote the localization of an (infinity,1)-category|localization]] (as an (∞,1)-category) inverting the weak equivalences (as e.g. given by simplicial localization; see also the model structure on relative categories). Then:

###### Proposition

If $\mathcal{C}$ is a model category and $X \in \mathcal{C}$ is fibrant, then $\gamma$ (1) induces an (∞,1)-functor $\mathcal{C}/X \to L_W(\mathcal{C})/\gamma(X)$, which in turn induces an equivalence of (∞,1)-categories

$L_W(\mathcal{C}/X) \overset{\simeq}{\longrightarrow} L_W(\mathcal{C})/\gamma(X) \,.$

This main result is corollary 7.6.13 of Cisinski 20. Model categories are (∞,1)-categories with weak equivalences and fibrations as defined in Cisinski Def. 7.4.12.

We spell out a proof for the special case that $\mathcal{C}$ carries the extra structure of a simplicial model category (this proof was written in 2011 when no comparable statement seemed to be available in the literature):

###### Proposition

If $\mathcal{C}$ is a simplicial model category and $X \in \mathcal{C}$ is fibrant, then the overcategory $\mathcal{C}/X$ with the above slice model structure is a presentation of the over-(∞,1)-category $L_W \mathcal{C} / \gamma(X)$: we have an equivalence of (∞,1)-categories

$L_W(\mathcal{C}/X) \simeq (L_W\mathcal{C}) / \gamma(X) \,.$
###### Proof

We write equivalently $(-)^\circ \coloneqq L_W(-)$.

It is clear that we have an essentially surjective (∞,1)-functor $\mathcal{C}^\circ/X \to (\mathcal{C}/X)^\circ$. What has to be shown is that this is a full and faithful (∞,1)-functor in that it is an equivalence on all hom-∞-groupoids $\mathcal{C}^\circ/X(a,b) \simeq (\mathcal{C}/X)^\circ(a,b)$.

To see this, notice that the hom-space in an over-(∞,1)-category $\mathcal{C}^\circ/X$ between objects $a \colon A \to X$ and $b \colon B \to X$ is given (as discussed there) by the (∞,1)-pullback

$\array{ \mathcal{C}^\circ/X(A \stackrel{a}{\to} X, B \stackrel{b}{\to} X) &\to& \mathcal{C}^\circ(A,B) \\ \big\downarrow && \big\downarrow^{\mathrlap{b_*}} \\ {*} &\stackrel{a}{\to}& \mathcal{C}^\circ(A,X) }$

in ∞Grpd.

Let $A \in C$ be a cofibrant representative and $b \colon B \to X$ be a fibration representative in $C$ (which always exists) of the objects of these names in $C^\circ$, respectively. In terms of these we have a cofibration

$\array{ \emptyset &&\hookrightarrow&& A \\ & \searrow && \swarrow_{\mathrlap{a}} \\ && X }$

in $\mathcal{C}/X$, exhibiting $a$ as a cofibrant object of $\mathcal{C}/X$; and a fibration

$\array{ B &&\stackrel{b}{\to}&& X \\ & {}_{\mathllap{b}}\searrow && \swarrow_{\mathrlap{Id}} \\ && X }$

in $\mathcal{C}/X$, exhibiting $b$ as a fibrant object in $\mathcal{C}/X$.

Moreover, the diagram in sSet given by

$\array{ \mathcal{C}/X(a, b) &\longrightarrow& \mathcal{C}(A,B) \\ \big\downarrow && \big\downarrow^{\mathrlap{b_*}} \\ {*} &\stackrel{a}{\to}& \mathcal{C}(A,X) }$

is

1. a pullback diagram in sSet (by the definition of morphism in an ordinary overcategory);

2. a homotopy pullback in the model structure on simplicial sets, because by the pullback power axiom on the sSet${}_{Quillen}$ enriched model category $C$ and the above (co)fibrancy assumptions, all objects are Kan complexes and the right vertical morphism is a Kan fibration;

3. has in the top left the correct derived hom-space in $C/X$ (since $a$ is cofibrant and $b$ fibrant).

This means that this correct hom-space $\mathcal{C}/X(a,b) \simeq (\mathcal{C}/X)^\circ(a,b) \in sSet$ is indeed a model for $\mathcal{C}^\circ/X(a,b) \in \infty Grpd$.

#### Quillen base change

###### Proposition

For $\mathcal{C}$ a model category and $c_1 \xrightarrow{f} c_2$ any morphism in $\mathcal{C}$, the left base change adjunction $(f_1 \dashv f^\ast)$ along $f$ (where $f_!$ is postcomposition with and $f^\ast$ is pullback along $f$) is a Quillen adjunction:

$\mathcal{C}_{/c_1} \underoverset {\underset{f^\ast}{\longleftarrow}} {\overset{f_!}{\longrightarrow}} {\;\;\;\;\bot_{\mathrlap{Qu}}\;\;\;\;} \mathcal{C}_{/c_2} \,.$

###### Proof

Since the left adjoint $f_1$ is the postcomposition operation, it manifestly preserves the classes of underlying morphisms, hence in particular preserves the classes of (acyclic) cofibrations in the slice model structure (by Prop. ), hence is a left Quillen functor.

If this adjunction is a Quillen equivalence, then $f$ must be a weak equivalence. In general, the converse can be proven only if $c_1$ and $c_2$ are fibrant.

###### Proposition

The following are equivalent:

1. $\mathcal{C}$ is a right proper model category.

2. If $f \colon c_1 \to c_2$ is any weak equivalence in $\mathcal{C}$, then the left base change Quillen adjunction $(f_! \dashv f^\ast)$ (from Prop. ) is a Quillen equivalence.

This is due to Rezk 02, Prop. 2.5.

###### Proposition

$\mathcal{D} \underoverset {\underset{R}{\longrightarrow}} {\overset{L}{\longleftarrow}} {\;\;\;\;\bot_{\mathrlap{Qu}}\;\;\;\;} \mathcal{C} \,,$

then:

1. for any object $b \in \mathcal{C}$ the sliced adjunction over $b$ is a Quillen adjunction between the corresponding slice model categories (Prop. ):

(2)$\mathcal{D}_{/L(c)} \underoverset {\underset{\;\;\;\;R_{/b}\;\;\;\;}{\longrightarrow}} {\overset{\;\;\;\;L_{/b}\;\;\;\;}{\longleftarrow}} {\bot_{\mathrlap{Qu}}} \mathcal{C}_{/b} \mathrlap{\,,}$
2. for any object $b \in \mathcal{D}$ the sliced adjunction over $b$ is a Quillen adjunction between the corresponding slice model categories (Prop. ):

(3)$\mathcal{D}_{/c} \underoverset {\underset{\;\;\;\;R_{/b}\;\;\;\;}{\longrightarrow}} {\overset{\;\;\;\;L_{/b}\;\;\;\;}{\longleftarrow}} {\bot_{\mathrlap{Qu}}} \mathcal{C}_{/R(b)} \mathrlap{\,,}$

(e.g. Li 2016, Prop. 2.5 (2))

###### Proof

Consider the first case: By the nature of the sliced adjunction, its left adjoint $L_{/b}$ acts as $L$ on underlying morphisms. But since $L$ is assumed to be a left Quillen functor and since the (acyclic) cofibrations of the slice model structure are those of underlying morphisms (Prop. ), $L_{/b}$ preserves them and is hence itself a left Quillen functor.

The second case is directly analogous: Here it is evident that $R_{/b}$ is a right Quillen functor, since it acts via $R$ on underlying morphisms, and $R$ is right Quillen by assumption.

###### Proposition

(sliced Quillen equivalences)
Consider a Quillen equivalence

$\mathcal{D} \underoverset {\underset{R}{\longrightarrow}} {\overset{L}{\longleftarrow}} {\;\;\;\;\simeq_{\mathrlap{Qu}}\;\;\;\;} \mathcal{C} \,.$

Then:

1. for a cofibrant object $b \in \mathcal{C}$ such that $L(b)$ is a fibrant object, the sliced Quillen adjunction (2) from Prop. is itself a Quillen equivalence:
$\mathcal{D}_{/L(b)} \underoverset {\underset{\;\;\;\;R_{/b}\;\;\;\;}{\longrightarrow}} {\overset{\;\;\;\;L_{/b}\;\;\;\;}{\longleftarrow}} {\simeq_{\mathrlap{Qu}}} \mathcal{C}_{/b}$
1. for a fibrant object $b \in \mathcal{D}$, the sliced Quillen adjunction (3) from Prop. is itself a Quillen equivalence:
$\mathcal{D}_{/b} \underoverset {\underset{\;\;\;\;R_{/b}\;\;\;\;}{\longrightarrow}} {\overset{\;\;\;\;L_{/b}\;\;\;\;}{\longleftarrow}} {\simeq_{\mathrlap{Qu}}} \mathcal{C}_{/R(b)}$

(e.g. Li 16, Prop. 3.1)
###### Proof

It is sufficient to check that the derived adjunction unit and derived adjunction counit are weak equivalences (…)

## Examples

###### Example

The classical model structure on pointed topological spaces is the model structure on the undercategory under the point (the category of pointed objects) of the classical model structure on topological spaces.

###### Proposition

(induced Quillen adjunction on model categories of pointed objects)
Given a Quillen adjunction between model categories

$\mathcal{D} \underoverset {\underset{R}{\longrightarrow}} {\overset{L}{\longleftarrow}} {\;\;\;\;\;\bot_{\mathrlap{{}_{Qu}}}\;\;\;\;\;} \mathcal{C} \,,$

there is induced a Quillen adjunction between the corresponding model categories of pointed objects

$\mathcal{D}^{\ast\!/} \underoverset {\underset{R^{\ast\!/}}{\longrightarrow}} {\overset{L^{\ast\!/}}{\longleftarrow}} {\;\;\;\;\;\bot_{\mathrlap{{}_{Qu}}}\;\;\;\;\;} \mathcal{C}^{\ast\!/} \,,$

where

• the right adjoint acts directly as $R$ on the triangular commuting diagrams in $\mathcal{C}$ that define the morphisms in $\mathcal{C}^{\ast\!/}$;

• the left adjoint is the composite of the corresponding direct application of $L$ followed by pushout along the adjunction counit $L(\ast) \simeq L \circ R(\ast) \xrightarrow{ \;\epsilon_\ast \; } \ast$ (using that $R(\ast) \simeq \ast$ since right adjoints preserve limits and hence terminal objects):

$L^{\ast\!/} \;\colon\; \mathcal{C}^{\ast\!/} \xrightarrow{ \;\; L \;\; } \mathcal{D}^{L(\ast)\!/} \;\simeq\; \mathcal{D}^{L\circ R(\ast)\!/} \xrightarrow{ \;\; (-) \sqcup \epsilon_\ast \;\; } \mathcal{D}^{\ast\!/} \,.$

###### Proof

It is fairly straightforward to check this directly (e.g. Hovey 1999, Prop. 1.3.5), but it is also a special case of Prop. — to make this explicit, notice that passing to opposite categories with their opposite model structures turns the original Quillen adjunction into the opposite Quillen adjunction:

$\mathcal{C}^{op} \underoverset {\underset{L^{op}}{\longrightarrow}} {\overset{R^{op}}{\longleftarrow}} {\;\;\;\;\;\bot_{\mathrlap{{}_{Qu}}}\;\;\;\;\;} \mathcal{D}^{op} \,.$

Now the passage to pointed objects corresponds to slicing (instead of co-slicing), since

(4)$\mathcal{C}^{\ast\!/} \;\; \simeq \big( \mathcal{C}^{op}_{/\ast} \big)^{op} \,, \;\;\;\;\;\;\; \text{similarly} \;\;\; \mathcal{D}^{\ast\!/} \;\; \simeq \big( \mathcal{D}^{op}_{/R(\ast)} \big)^{op} \,,$

whence item (1) in Prop. says that there is a Quillen adjunction of the form

$\mathcal{C}^{op}_{/R(\ast)} \underoverset {\underset{L^{op}_{/\ast}}{\longrightarrow}} {\overset{R^{op}_{/\ast}}{\longleftarrow}} {\;\;\;\;\;\bot_{\mathrlap{{}_{Qu}}}\;\;\;\;\;} \mathcal{D}^{op}_{/\ast} \,,$

hence with opposite Quillen adjunction of the required form

$\mathcal{D}^{\ast\!/} \simeq \big(\mathcal{D}^{op}_{/R(\ast)}\big)^{op} \underoverset {\underset{ R^{\ast\!/} \,\coloneqq\, \big( R^{op}_{/\ast} \big)^{op} }{\longrightarrow}} {\overset{ L^{\ast\!/} \,\coloneqq\, \big( L^{op}_{/\ast} \big)^{op} }{\longleftarrow}} {\;\;\;\;\;\bot_{\mathrlap{{}_{Qu}}}\;\;\;\;\;} \big(\mathcal{C}^{op}_{/\ast}\big)^{op} \simeq \mathcal{C}^{\ast\!/} \,,$

with $R^{op}$ acting directly as $R$ on underlying diagrams, and with $L^{op}$ acting as the composite of $L$ following by pullback – in $\mathcal{C}^{op}$ – along the adjunction unit of $(R^{op} \dashv L^{op})$. Since the component morphism of the unit of the opposite adjunction $(R^{op} \dashv L^{op})$ is that of the adjunction unit of $(L \dashv R)$, and since pullback in an opposite category is pushout in the original category, this implies the claim.