# Kerodon

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### 7.2.3 Quillen's Theorem A for $\infty$-Categories

The following result provides a concrete criterion for establishing the cofinality of a functor between $\infty$-categories.

Theorem 7.2.3.1 (Joyal). Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a morphism of simplicial sets, where $\operatorname{\mathcal{D}}$ is an $\infty$-category. Then:

$(1)$

The morphism $F$ is left cofinal if and only if, for every object $X \in \operatorname{\mathcal{D}}$, the simplicial set $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{/X}$ is weakly contractible.

$(2)$

The morphism $F$ is right cofinal if and only if, for every object $X \in \operatorname{\mathcal{D}}$, the simplicial set $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{X/}$ is weakly contractible.

Remark 7.2.3.2. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a morphism of simplicial sets, where $\operatorname{\mathcal{D}}$ is an $\infty$-category. For every object $X \in \operatorname{\mathcal{D}}$, the slice and coslice diagonal morphisms of Construction 4.6.4.12 induce categorical equivalences

$\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{/X} \hookrightarrow \operatorname{\mathcal{C}}\operatorname{\vec{\times }}_{\operatorname{\mathcal{D}}} \{ X\} \quad \quad \operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{X/} \hookrightarrow \{ X\} \operatorname{\vec{\times }}_{ \operatorname{\mathcal{D}}} \operatorname{\mathcal{C}}$

(Example 5.1.6.7). We can therefore reformulate Theorem 7.2.3.1 as follows:

$(1')$

The morphism $F$ is left cofinal if and only if, for every object $X \in \operatorname{\mathcal{D}}$, the simplicial set $\operatorname{\mathcal{C}}\operatorname{\vec{\times }}_{\operatorname{\mathcal{D}}} \{ X\}$ is weakly contractible.

$(2')$

The morphism $F$ is right cofinal if and only if, for every object $X \in \operatorname{\mathcal{D}}$, the simplicial set $\{ X\} \operatorname{\vec{\times }}_{\operatorname{\mathcal{C}}} \operatorname{\mathcal{D}}$ is weakly contractible.

Example 7.2.3.3 (Quillen's Theorem A). Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a functor between categories. Suppose that, for every object $X \in \operatorname{\mathcal{D}}$, the category $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{X/}$ has weakly contractible nerve. Applying Theorem 7.2.3.1, we deduce that the induced morphism of simplicial sets $\operatorname{N}_{\bullet }(F): \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}}) \rightarrow \operatorname{N}_{\bullet }(\operatorname{\mathcal{D}})$ is right cofinal. In particular, it is a weak homotopy equivalence (Proposition 7.2.1.4). This recovers a classical result of Quillen (see ).

Corollary 7.2.3.4. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category. Then:

$(1)$

An object $Y \in \operatorname{\mathcal{C}}$ is initial if and only if the inclusion map $\{ Y\} \hookrightarrow \operatorname{\mathcal{C}}$ is left cofinal.

$(2)$

An object $Y \in \operatorname{\mathcal{C}}$ is final if and only if the inclusion map $\{ Y\} \hookrightarrow \operatorname{\mathcal{C}}$ is right cofinal.

Corollary 7.2.3.5. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category which has either an initial object or a final object. Then $\operatorname{\mathcal{C}}$ is weakly contractible.

Proof. Let $Y$ be an initial object of $\operatorname{\mathcal{C}}$. Then the inclusion $\{ Y\} \hookrightarrow \operatorname{\mathcal{C}}$ is left cofinal (Corollary 7.2.3.4), and is therefore a weak homotopy equivalence (Proposition 7.2.1.4). $\square$

Corollary 7.2.3.6. Let $U: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$ be an inner fibration of $\infty$-categories and let $\operatorname{\mathcal{C}}$ be an $\infty$-category containing an object $Y$. Then:

• If $Y$ is an initial object of $\operatorname{\mathcal{C}}$, then a diagram $\operatorname{\mathcal{C}}^{\triangleleft } \rightarrow \operatorname{\mathcal{D}}$ is a $U$-limit diagram if and only if it carries $\{ X\} ^{\triangleleft } \simeq \Delta ^1$ to a $U$-cartesian morphism of $\operatorname{\mathcal{D}}$.

• If $Y$ is a final object of $\operatorname{\mathcal{K}}$, then a diagram $\operatorname{\mathcal{C}}^{\triangleleft } \rightarrow \operatorname{\mathcal{D}}$ is a $U$-colimit diagram if and only if it carries $\{ X\} ^{\triangleright } \simeq \Delta ^1$ to a $U$-cocartesian morphism of $\operatorname{\mathcal{D}}$.

Proof. If $Y$ is an initial object of $\operatorname{\mathcal{K}}$, then the inclusion map $\{ Y\} \hookrightarrow \operatorname{\mathcal{K}}$ is left cofinal (Corollary 7.2.3.4). The first assertion now follows by combining Corollary 7.2.2.2 with Example 7.1.7.9. The second assertion follows by a similar argument. $\square$

Corollary 7.2.3.7. Let $\operatorname{\mathcal{C}}$ and $\operatorname{\mathcal{D}}$ be $\infty$-categories. Then:

• If $\operatorname{\mathcal{C}}$ has an initial object $Y$, then a functor $\operatorname{\mathcal{C}}^{\triangleleft } \rightarrow \operatorname{\mathcal{D}}$ is a limit diagram if and only if it carries $\{ Y\} ^{\triangleleft } \simeq \Delta ^1$ to an isomorphism in the $\infty$-category $\operatorname{\mathcal{D}}$.

• If $\operatorname{\mathcal{C}}$ has a final object $Y$, then a functor $\operatorname{\mathcal{C}}^{\triangleright } \rightarrow \operatorname{\mathcal{D}}$ is a colimit diagram if and only if it carries $\{ Y\} ^{\triangleright } \simeq \Delta ^1$ to an isomorphism in the $\infty$-category $\operatorname{\mathcal{D}}$.

Proof. Apply Corollary 7.2.3.6 in the special case $\operatorname{\mathcal{E}}= \Delta ^0$ (and use Example 5.1.1.4). $\square$

Corollary 7.2.3.8. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category and let $\overline{f}: A^{\triangleleft } \rightarrow \operatorname{\mathcal{C}}$ be a diagram, where $A$ is a weakly contractible simplicial set. The following conditions are equivalent:

$(1)$

The diagram $\overline{f}$ carries each edge of $A^{\triangleleft }$ to an isomorphism in $\operatorname{\mathcal{C}}$.

$(2)$

The restriction $f = \overline{f}|_{A}$ carries each edge of $A$ to an isomorphism in $\operatorname{\mathcal{C}}$, and $\overline{f}$ is a limit diagram.

Proof. Without loss of generality, we may assume that $f$ carries each edge of $A$ to an isomorphism in $\operatorname{\mathcal{C}}$. Under this assumption, we can restate $(1)$ and $(2)$ as follows:

$(1')$

For every vertex $a \in A$, the edge

$\Delta ^1 \simeq \{ a\} ^{\triangleleft } \hookrightarrow A^{\triangleleft } \xrightarrow { \overline{f} } \operatorname{\mathcal{C}}$

is an isomorphism in the $\infty$-category $\operatorname{\mathcal{C}}$.

$(2')$

The morphism $\overline{f}$ is a colimit diaram.

Using Corollary 3.1.7.2, we can choose an anodyne morphism $i: A \hookrightarrow B$, where $B$ is a Kan complex. Note that $f$ can be regarded as a morphism from $A$ to the core $\operatorname{\mathcal{C}}^{\simeq }$, which is also a Kan complex (Corollary 4.4.3.11). We can therefore extend $f$ to a morphism of Kan complexes $g: B \rightarrow \operatorname{\mathcal{C}}^{\simeq }$. Moreover, the morphism $i$ is right cofinal (Proposition 7.2.1.4) and therefore right anodyne (Proposition 7.2.1.3). It follows that the induced map $B \coprod _{A} A^{\triangleright } \hookrightarrow B^{\triangleright }$ is inner anodyne (Example 4.3.6.5), so that we can choose a functor $\overline{g}: B^{\triangleright } \rightarrow \operatorname{\mathcal{C}}$ satisfying $\overline{g}|_{B} = g$ and $\overline{g}|_{A^{\triangleright }} = \overline{f}$.

It follows from Corollary 7.2.2.3 $\overline{f}$ is a colimit diagram if and only if $\overline{g}$ is a colimit diagram. Since $A$ is weakly contractible, the Kan complex $B$ is contractible. In particular, every vertex $a \in A$ can be regarded as a final object of $B$. The equivalence of $(1')$ and $(2')$ now follows from Corollary 7.2.3.7. $\square$

Corollary 7.2.3.9. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category and let $U: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$ be a functor of $\infty$-categories. Then:

• If $\operatorname{\mathcal{C}}$ has an initial object $Y$ and $\overline{F}: \operatorname{\mathcal{C}}^{\triangleleft } \rightarrow \operatorname{\mathcal{D}}$ is a functor which carries $\{ Y\} ^{\triangleleft } \simeq \Delta ^1$ to an isomorphism in $\operatorname{\mathcal{D}}$, then $\overline{F}$ is a $U$-limit diagram.

• If $\operatorname{\mathcal{C}}$ has an initial object $Y$ and $\overline{F}: \operatorname{\mathcal{C}}^{\triangleright } \rightarrow \operatorname{\mathcal{D}}$ is a functor which carries $\{ Y\} ^{\triangleright } \simeq \Delta ^1$ to an isomorphism in $\operatorname{\mathcal{D}}$, then $\overline{F}$ is a $U$-limit diagram.

Corollary 7.2.3.10. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a functor of $\infty$-categories. Then:

$(1)$

If $F$ is a left adjoint, then it is left cofinal.

$(2)$

If $F$ is a right adjoint, then it is right cofinal.

Proof. We will prove $(1)$; the proof of $(2)$ is similar. Suppose that $F$ admits a right $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$. For every object $X \in \operatorname{\mathcal{D}}$, Corollary 7.1.3.13 guarantees that the $\infty$-category $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{/X}$ has a final object. In particular, the $\infty$-category $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{/X}$ is weakly contractible (Corollary 7.2.3.5). Allowing $X$ to vary and applying Theorem 7.2.3.1, we conclude that $F$ is left cofinal. $\square$

Corollary 7.2.3.11. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category and let $K$ be a simplicial set. The following conditions are equivalent:

$(1)$

The diagonal map $\delta : \operatorname{\mathcal{C}}\rightarrow \operatorname{Fun}(K,\operatorname{\mathcal{C}})$ is right cofinal.

$(2)$

For every diagram $f: K \rightarrow \operatorname{\mathcal{C}}$, the coslice $\infty$-category $\operatorname{\mathcal{C}}_{f/}$ is weakly contractible.

Proof. By virtue of Remark 7.2.3.2, condition $(1)$ is equivalent to the requirement that for every diagram $f: K \rightarrow \operatorname{\mathcal{C}}$, the oriented fiber product $\{ f\} \operatorname{\vec{\times }}_{ \operatorname{Fun}(K, \operatorname{\mathcal{C}}) } \operatorname{\mathcal{C}}$ is weakly contractible. The equivalence of $(1)$ and $(2)$ now follows from Theorem 4.6.4.16. $\square$

Corollary 7.2.3.12. Let $\operatorname{{\bf \Delta }}$ denote the simplex category (Definition 1.1.1.2), and let $\operatorname{{\bf \Delta }}_{\operatorname{inj}} \subset \operatorname{{\bf \Delta }}$ denote the non-full subcategory whose morphisms are strictly increasing functions $[m] \hookrightarrow [n]$ (Variant 1.1.1.6). Then the inclusion of simplicial sets $\operatorname{N}_{\bullet }( \operatorname{{\bf \Delta }}_{\operatorname{inj}} ) \hookrightarrow \operatorname{N}_{\bullet }( \operatorname{{\bf \Delta }})$ is left cofinal.

Proof. By virtue of Theorem 7.2.3.1, it will suffice to show that for every integer $n \geq 0$, the category $\operatorname{\mathcal{C}}= \operatorname{{\bf \Delta }}_{\operatorname{inj}} \times _{\operatorname{{\bf \Delta }}} \operatorname{{\bf \Delta }}_{ / [n] }$ has weakly contractible nerve. Let $C_0 \in \operatorname{\mathcal{C}}$ denote the object corresponding to the inclusion map $[0] \simeq \{ n\} \hookrightarrow [n]$. For every object $C \in \operatorname{\mathcal{C}}$, given by a nondecreasing function $\alpha : [m] \rightarrow [n]$, we let $F(C) \in \operatorname{\mathcal{C}}$ denote the object given by the nondecreasing function $\alpha ^{+}: [m+1] \rightarrow [n]$ given by the formula

$\alpha ^{+}(i) = \begin{cases} \alpha (i) & \text{ if } 0 \leq i \leq m \\ n & \text{ if } i = m+1. \end{cases}$

Note that we have canonical maps $C \xrightarrow {\beta _{-}} F(C) \xleftarrow {\beta _+} C_0$, given by the inclusions

$\{ 0 < 1 < \cdots < m \} \hookrightarrow \{ 0 < 1 < \cdots < m+1 \} \hookleftarrow \{ m+1 \} .$

These morphisms depend functorially on $C$, and therefore furnish natural transformations of functors $\operatorname{id}_{\operatorname{\mathcal{C}}} \rightarrow F \leftarrow \underline{C}_0$, where $\underline{C}_0: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{C}}$ denotes the constant functor taking the value $C_0$. It follows that the identity morphism of $\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ is homotopic to the constant morphism $\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}}) \twoheadrightarrow \{ C_0 \} \hookrightarrow \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$, so that the simplicial set $\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ is contractible (and, in particular, it is weakly contractible). $\square$

Our proof of Theorem 7.2.3.1 will use a variant of Proposition 5.2.6.24, which is of independent interest:

Proposition 7.2.3.13. Suppose we are given a pullback diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}' \ar [d] \ar [r]^-{F} & \operatorname{\mathcal{C}}\ar [d]^-{\pi } \\ \operatorname{\mathcal{D}}' \ar [r]^-{\overline{F}} & \operatorname{\mathcal{D}}. }$

If $\pi$ is a cocartesian fibration and $\overline{F}$ is right cofinal, then $F$ is right cofinal.

Proof. By virtue of Corollary 7.2.1.13, it will suffice to prove Proposition 7.2.3.13 in the special case where $\overline{F}$ is right anodyne. Let $S$ be the collection of all morphisms of simplicial sets $\overline{F}: \operatorname{\mathcal{D}}' \rightarrow \operatorname{\mathcal{D}}$ having the property that, for every cocartesian fibration $\pi : \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$, the induced map $F: \operatorname{\mathcal{D}}' \times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{C}}$ is right anodyne. We wish to show show that every right anodyne morphism belongs to $S$. It follows immediately from the definitions that $S$ is weakly saturated, in the sense of Definition 1.4.4.15. It will therefore suffice to show that $S$ contains every horn inclusion $\Lambda ^{n}_{i} \hookrightarrow \Delta ^ n$ for $0 < i \leq n$. In other words, we are reduced to proving Proposition 7.2.3.13 in the special case where $\operatorname{\mathcal{D}}= \Delta ^ n$ is a standard simplex and $\overline{F}$ is the inclusion of the horn $\Lambda ^{n}_{i} \subseteq \Delta ^ n$.

For $0 \leq j \leq n$, let $\operatorname{\mathcal{C}}(j)$ denote the fiber $\{ j\} \times _{\Delta ^ n} \operatorname{\mathcal{C}}$. Applying Proposition 5.2.6.18, we deduce that there exists a scaffold

$U: M( \operatorname{\mathcal{C}}(0) \rightarrow \operatorname{\mathcal{C}}(1) \rightarrow \cdots \rightarrow \operatorname{\mathcal{C}}(n) ) \rightarrow \operatorname{\mathcal{C}}$

for the cocartesian fibration $\pi$ (see Definition 5.2.6.12). Set $M = M(\operatorname{\mathcal{C}}(0) \rightarrow \cdots \rightarrow \operatorname{\mathcal{C}}(n) )$, so that we have a commutative diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ \Lambda ^{n}_{i} \times \operatorname{\mathcal{C}}(0) \ar [r] \ar [d]^{F''} & \Lambda ^{n}_{i} \times _{\Delta ^ n} M \ar [r] \ar [d]^{F'} & \Lambda ^{n}_{i} \times _{\Delta ^ n} \operatorname{\mathcal{C}}\ar [d]^{F} \\ \Delta ^ n \times \operatorname{\mathcal{C}}(0) \ar [r] & M \ar [r]^-{U} & \operatorname{\mathcal{C}}. }$

Note that the morphism $F''$ is right anodyne (Proposition 4.2.5.3). Since the diagram on the left is a pushout square, it follows that $F'$ is also right anodyne, and therefore right cofinal (Proposition 7.2.1.3). Corollary 5.2.6.21 guarantees that the right horizontal maps are categorical equivalences, so that $F$ is also right cofinal (Corollary 7.2.1.20). $\square$

Corollary 7.2.3.14. Suppose we are given a pullback diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}' \ar [d] \ar [r]^-{F} & \operatorname{\mathcal{C}}\ar [d]^-{\pi } \\ \operatorname{\mathcal{D}}' \ar [r]^-{\overline{F}} & \operatorname{\mathcal{D}}. }$

If $\pi$ is a cocartesian fibration and $\overline{F}$ is right anodyne, then $F$ is right anodyne.

Proof of Theorem 7.2.3.1. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a morphism of simplicial sets, where $\operatorname{\mathcal{D}}$ is an $\infty$-category. We will show that $F$ is right cofinal if and only if, for every object $X \in \operatorname{\mathcal{D}}$, the simplicial set $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{X/}$ is weakly contractible; the analogous characterization of left cofinal morphisms follows by a similar argument.

Suppose first that $F$ is right cofinal. For every object $X \in \operatorname{\mathcal{D}}$, the projection map $\operatorname{\mathcal{D}}_{X/} \rightarrow \operatorname{\mathcal{D}}$ is a left fibration (Proposition 4.3.6.1), and therefore a cocartesian fibration (Proposition 5.1.4.14). Applying Proposition 7.2.3.13, we conclude that the projection map $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{X/} \rightarrow \operatorname{\mathcal{D}}_{X/}$ is also right cofinal. In particular, it is a weak homotopy equivalence (Proposition 7.2.1.4). Since the $\infty$-category $\operatorname{\mathcal{D}}_{X/}$ has an initial object (Proposition 7.1.3.6), it is weakly contractible, so that the fiber product $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{X/}$ is also weakly contractible.

We now prove the converse. Assume that, for every object $X \in \operatorname{\mathcal{D}}$, the simplicial set $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{X/}$ is weakly contractible. We wish to show that $F$ is right cofinal. Using Proposition 4.1.3.2, we can factor $F$ as a composition

$\operatorname{\mathcal{C}}\xrightarrow {F'} \operatorname{\mathcal{C}}' \xrightarrow {F''} \operatorname{\mathcal{D}},$

where $F'$ is inner anodyne and $F''$ is an inner fibration. Since $F'$ is left cofinal (Proposition 7.2.1.3), it will suffice to show that $F''$ is right cofinal (Proposition 7.2.1.5). For every object $X \in \operatorname{\mathcal{D}}$, Proposition 5.2.6.24 guarantees that the induced map $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{X/} \hookrightarrow \operatorname{\mathcal{C}}' \times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{X/}$ is a categorical equivalence. In particular, it is a weak homotopy equivalence (Remark 4.5.2.4), so that $\operatorname{\mathcal{C}}' \times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{X/}$ is also weakly contractible. We may therefore replace $\operatorname{\mathcal{C}}$ by $\operatorname{\mathcal{C}}'$ and thereby reduce to the case where $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ is an inner fibration, so that $\operatorname{\mathcal{C}}$ is also an $\infty$-category (Remark 4.1.1.9).

Let $\operatorname{ev}_0, \operatorname{ev}_1: \operatorname{Fun}( \Delta ^1, \operatorname{\mathcal{D}}) \rightarrow \operatorname{\mathcal{D}}$ denote the functors given by evaluation at the vertices $0,1 \in \operatorname{\mathcal{D}}$, and let $\delta : \operatorname{\mathcal{D}}\hookrightarrow \operatorname{Fun}( \Delta ^1, \operatorname{\mathcal{D}})$ be the diagonal map. Note that there is a unique natural transformation from $\operatorname{id}_{ \Delta ^1 }$ to the constant map $\Delta ^1 \twoheadrightarrow \{ 1\} \hookrightarrow \Delta ^1$, which induces a natural transformation $h: \operatorname{id}_{ \operatorname{Fun}(\Delta ^1, \operatorname{\mathcal{D}}) } \rightarrow \delta \circ \operatorname{ev}_{1}$. Let $\operatorname{\mathcal{M}}$ denote the oriented fiber product $\operatorname{\mathcal{D}}\operatorname{\vec{\times }}_{\operatorname{\mathcal{D}}} \operatorname{\mathcal{C}}= \operatorname{Fun}( \Delta ^1, \operatorname{\mathcal{D}}) \times _{ \operatorname{Fun}( \{ 1\} , \operatorname{\mathcal{D}}) } \operatorname{Fun}(\{ 1\} , \operatorname{\mathcal{C}})$ of Construction 4.6.4.1, so that $\operatorname{ev}_{0}$ and $\operatorname{ev}_{1}$ lift to functors

$\operatorname{\mathcal{D}}\xleftarrow { \widetilde{\operatorname{ev}}_0 } \operatorname{\mathcal{M}}\xrightarrow { \widetilde{ \operatorname{ev}}_{1} } \operatorname{\mathcal{C}},$

the diagonal map $\delta$ lifts to a functor $\widetilde{\delta }: \operatorname{\mathcal{C}}\hookrightarrow \operatorname{\mathcal{M}}$, and $h$ lifts to a natural transformation $\widetilde{h}: \operatorname{id}_{\operatorname{\mathcal{M}}} \rightarrow \widetilde{\delta } \circ \widetilde{\operatorname{ev}}_1$. Note that $\widetilde{h}$ can be identified with a morphism of simplicial sets $\Delta ^1 \times \operatorname{\mathcal{M}}\rightarrow \operatorname{\mathcal{M}}$ which fits into a commutative diagram

$\xymatrix@R =50pt@C=50pt{ \{ 0\} \times \operatorname{\mathcal{C}}\ar [r] \ar [d]^{\widetilde{\delta }} & (\Delta ^1 \times \operatorname{\mathcal{C}}) \coprod _{ (\{ 1\} \times \operatorname{\mathcal{C}}) } (\{ 1\} \times \operatorname{\mathcal{M}}) \ar [r] \ar [d]^{\iota } & \operatorname{\mathcal{C}}\ar [d]^{ \widetilde{\delta } } \\ \{ 0\} \times \operatorname{\mathcal{M}}\ar [r] & \Delta ^1 \times \operatorname{\mathcal{M}}\ar [r]^-{ \widetilde{h} } & \operatorname{\mathcal{M}}, }$

where the horizontal compositions are the identity. It follows that $\widetilde{\delta }$ is a retract of $\iota$. Since $\iota$ is right anodyne (Proposition 4.2.5.3), $\widetilde{\delta }$ is also right anodyne, and therefore right cofinal (Proposition 7.2.1.3).

The functor $\widetilde{\operatorname{ev}}_0: \operatorname{\mathcal{M}}\rightarrow \operatorname{\mathcal{D}}$ is a cartesian fibration (Proposition 5.1.7.1). Moreover, for each object $X \in \operatorname{\mathcal{D}}$, the fiber $\widetilde{\operatorname{ev}}_{0}^{-1} \{ X\} \simeq \{ X\} \operatorname{\vec{\times }}_{\operatorname{\mathcal{D}}} \operatorname{\mathcal{C}}$ is equivalent to the $\infty$-category $\operatorname{\mathcal{C}}\times _{\operatorname{\mathcal{D}}} \operatorname{\mathcal{D}}_{X/}$ (Example 5.1.6.7), and is therefore weakly contractible. Applying Corollary 6.3.5.3, we deduce that the functor $\widetilde{\operatorname{ev}}_1$ exhibits $\operatorname{\mathcal{D}}$ as a localization of $\operatorname{\mathcal{D}}\operatorname{\vec{\times }}_{\operatorname{\mathcal{D}}} \operatorname{\mathcal{C}}$, and is therefore right cofinal (Proposition 7.2.1.8). We now observe that the functor $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ factors as a composition

$\operatorname{\mathcal{C}}\xrightarrow { \widetilde{\delta } } \operatorname{\mathcal{M}}\xrightarrow { \widetilde{\operatorname{ev}}_1 } \operatorname{\mathcal{D}},$

and is therefore also right cofinal (Proposition 7.2.1.5). $\square$