Kerodon

$\Newextarrow{\xRightarrow}{5,5}{0x21D2}$ $\newcommand\empty{}$
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$

5.5.4 Elements of $\operatorname{\mathcal{QC}}$-Valued Functors

Let $\operatorname{QCat}$ denote the ordinary category whose objects are $\infty $-categories and whose morphisms are functors (Construction 5.4.4.1). To every functor $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{QCat}$, the weighted nerve construction of Definition 5.5.3.1 supplies a cocartesian fibration of $\infty $-categories $U: \int ^{\mathrm{s}}_{\operatorname{\mathcal{C}}}\mathscr {F} \rightarrow \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})$ (Proposition 5.5.3.13), whose fiber over an object $C \in \operatorname{\mathcal{C}}$ is isomorphic to the $\infty $-category $\mathscr {F}(C)$ (Example 5.5.3.7). The utility of this construction is limited by the fact that it applies only to strictly commutative diagrams in $\operatorname{QCat}$: that is, Definition 5.5.3.1 requires $\operatorname{\mathcal{C}}$ to be an ordinary category and $\mathscr {F}$ to be a functor of ordinary categories. Our goal in this section is to introduce a homotopy coherent variant of the weighted nerve which is associated to any functor of $\infty $-categories $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{QC}}$; here $\operatorname{\mathcal{QC}}$ denotes the $\infty $-category of $\infty $-categories introduced in Construction 5.4.4.1.

Definition 5.5.4.1. Let $\operatorname{\mathcal{C}}$ be a simplicial set and let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }})$ be a morphism of simplicial sets. We let $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ denote the fiber product $\operatorname{\mathcal{C}}\times _{ \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }}) } \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }})_{ \Delta ^{0} / }$, so that we have a pullback diagram of simplicial sets

\[ \xymatrix@R =50pt@C=50pt{ \int _{\operatorname{\mathcal{C}}} \mathscr {F} \ar [r] \ar [d]^-{U} & \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }})_{\Delta ^{0}/} \ar [d] \\ \operatorname{\mathcal{C}}\ar [r]^-{\mathscr {F}} & \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }}). } \]

We will refer to $U: \int _{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \operatorname{\mathcal{C}}$ as the projection map.

The simplicial set $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ of Definition 5.5.4.1 is defined for an arbitrary morphism $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{N}_{\bullet }^{\operatorname{hc}}(\operatorname{\mathcal{C}})$. However, we will be primarily interested in the case where $\mathscr {F}$ takes values in the simplicial subset $\operatorname{\mathcal{QC}}\subseteq \operatorname{N}_{\bullet }^{\operatorname{hc}}(\operatorname{\mathcal{C}})$ introduced in Construction 5.4.4.1.

Proposition 5.5.4.2. Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{QC}}$ be a morphism of simplicial sets. Then the projection map $U: \int _{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \operatorname{\mathcal{C}}$ is a cocartesian fibration of simplicial sets.

Proof. By construction, the morphism $U$ fits into a pullback diagram

\[ \xymatrix@R =50pt@C=50pt{ \int _{\operatorname{\mathcal{C}}} \mathscr {F} \ar [r] \ar [d]^-{U} & \operatorname{\mathcal{QC}}_{\operatorname{Obj}} \ar [d] \\ \operatorname{\mathcal{C}}\ar [r]^-{\mathscr {F}} & \operatorname{\mathcal{QC}}, } \]

where

\[ \operatorname{\mathcal{QC}}_{\operatorname{Obj}} = \operatorname{\mathcal{QC}}\times _{ \operatorname{N}_{\bullet }^{\operatorname{hc}}(\operatorname{Set_{\Delta }})} \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }})_{ \Delta ^{0} / } \]

is the $\infty $-category introduced in Construction 5.4.6.10. It will therefore suffice to show that the projection map $\operatorname{\mathcal{QC}}_{\operatorname{Obj}} \rightarrow \operatorname{\mathcal{QC}}_{\ast }$ is a cocartesian fibration of simplicial sets, which follows from Proposition 5.4.6.11. $\square$

Corollary 5.5.4.3. Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{QC}}$ be a functor of $\infty $-categories. Then the simplicial set $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ of Definition 5.5.4.1 is an $\infty $-category.

Definition 5.5.4.4. Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{QC}}$ be a functor of $\infty $-categories. We will refer to $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ as the $\infty $-category of elements of $\mathscr {F}$.

Remark 5.5.4.5. Let $\operatorname{\mathcal{C}}$ be an ordinary category equipped with a strictly unitary functor of $2$-categories $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \mathbf{Cat}$. Then the construction $C \mapsto \operatorname{N}_{\bullet }(\mathscr {F}(C) )$ determines a functor of $\infty $-categories $\operatorname{N}_{\bullet }(\mathscr {F}): \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}}) \rightarrow \operatorname{\mathcal{QC}}$ (see Remark 5.4.4.9). In §5.5.5, we will construct a canonical isomorphism

\[ \int _{\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})} \operatorname{N}_{\bullet }(\mathscr {F} ) \simeq \operatorname{N}_{\bullet }( \int _{\operatorname{\mathcal{C}}} \mathscr {F} ), \]

where the simplicial set on the left hand side is given by Definition 5.5.4.1 and $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ is the category of elements introduced in Definition 5.5.2.1 (see Proposition 5.5.5.3). Stated more informally, we can regard the $\infty $-category of elements construction (Definition 5.5.4.4) as a generalization of the classical category of elements construction (Definition 5.5.2.1).

Warning 5.5.4.6. In §5.5.2, we introduced a variant of the category of elements construction for contravariant $\mathbf{Cat}$-valued functors $\mathscr {F}: \operatorname{\mathcal{C}}^{\operatorname{op}} \rightarrow \mathbf{Cat}$ (see Definition 5.5.2.3), which is characterized by the formula

\[ \int ^{\operatorname{\mathcal{C}}} \mathscr {F} = ( \int _{\operatorname{\mathcal{C}}^{\operatorname{op}} } \mathscr {F}^{\operatorname{op}} )^{\operatorname{op}}. \]

In the $\infty $-categorical setting, the situation is more subtle: the involution $\operatorname{\mathcal{E}}\mapsto \operatorname{\mathcal{E}}^{\operatorname{op}}$ does not preserve the simplicial structure on the category $\operatorname{QCat}$ and therefore does not induce an involution on the simplicial set $\operatorname{\mathcal{QC}}= \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{QCat})$. We will return to this point in §.

Warning 5.5.4.7. Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{QCat}$ be a functor of ordinary categories. Passing to the homotopy coherent nerve, we obtain a functor of $\infty $-categories $\operatorname{N}_{\bullet }^{\operatorname{hc}}(\mathscr {F}): \operatorname{N}_{\bullet }(\operatorname{\mathcal{C}}) \rightarrow \operatorname{\mathcal{QC}}$. Beware that the simplicial set $\int _{\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})} \operatorname{N}_{\bullet }^{\operatorname{hc}}(\mathscr {F})$ is usually not isomorphic to the weighted nerve $\int ^{\mathrm{s}}_{\operatorname{\mathcal{C}}}\mathscr {F}$ of Definition 5.5.3.1, even in the special case $\operatorname{\mathcal{C}}= \Delta ^{0}$. However, in §5.5.6 we will construct a comparison map

\[ \int ^{\mathrm{s}}_{\operatorname{\mathcal{C}}}\mathscr {F} \rightarrow \int _{\operatorname{N}_{\bullet }(\operatorname{\mathcal{C}})} \operatorname{N}_{\bullet }^{\operatorname{hc}}(\mathscr {F}) \]

which is an equivalence of $\infty $-categories (Proposition 5.5.6.8).

Example 5.5.4.8 ($\operatorname{Set}$-Valued Functors). Let $\operatorname{Set}$ denote the category of sets, and let us regard the nerve $\operatorname{N}_{\bullet }(\operatorname{Set})$ as a simplicial subset of the homotopy coherent nerve $\operatorname{N}_{\bullet }^{\operatorname{hc}}(\operatorname{Set_{\Delta }})$. Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{N}_{\bullet }(\operatorname{Set})$ be a morphism of simplicial sets, which we can identify with a functor of categories $\mathrm{h} \mathit{\mathscr {F}}: \mathrm{h} \mathit{\operatorname{\mathcal{C}}} \rightarrow \operatorname{Set}$. Using Example 5.4.3.9 and Remark 5.5.1.6, we obtain a canonical isomorphism of simplicial sets

\[ \int _{\operatorname{\mathcal{C}}} \mathscr {F} \simeq \operatorname{\mathcal{C}}\times _{ \operatorname{N}_{\bullet }( \mathrm{h} \mathit{\operatorname{\mathcal{C}}} ) } \operatorname{N}_{\bullet }( \int _{\mathrm{h} \mathit{\operatorname{\mathcal{C}}} } \mathrm{h} \mathit{\mathscr {F}}), \]

where $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ is the simplicial set of Definition 5.5.4.1 and $\int _{\mathrm{h} \mathit{\operatorname{\mathcal{C}}}} \mathrm{h} \mathit{\mathscr {F}}$ is the category of elements introduced in Construction 5.5.1.1. In particular, the projection map $\int _{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \operatorname{\mathcal{C}}$ is a left covering map.

Example 5.5.4.9 ($\operatorname{\mathcal{S}}$-Valued Functors). Let $\operatorname{\mathcal{S}}$ denote the $\infty $-category of spaces (Construction 5.4.1.1), which we view as a full simplicial subset of $\operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }})$, and let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{S}}$ be a morphism of simplicial sets. Then the simplicial set $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ fits into pullback diagram

\[ \xymatrix@R =50pt@C=50pt{ \int _{\operatorname{\mathcal{C}}} \mathscr {F} \ar [r] \ar [d]^-{\pi } & \operatorname{\mathcal{S}}_{\ast } \ar [d] \\ \operatorname{\mathcal{C}}\ar [r]^-{\mathscr {F}} & \operatorname{\mathcal{S}}, } \]

where $\operatorname{\mathcal{S}}_{\ast }$ is the $\infty $-category of pointed spaces (Construction 5.4.3.1). In this case, Proposition 5.4.3.2 guarantees that the projection map $\pi : \int _{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \operatorname{\mathcal{C}}$ is a left fibration of simplicial sets.

Example 5.5.4.10 ($\operatorname{ \pmb {\mathcal{QC}} }$-Valued Functors). Let $\operatorname{ \pmb {\mathcal{QC}} }$ denote the $(\infty ,2)$-category of $\infty $-categories (Construction 5.4.5.1), which we view as a full simplicial subset of $\operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }})$, and let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{ \pmb {\mathcal{QC}} }$ be a morphism of simplicial sets. Then the Grothendieck construction $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ fits into pullback diagram

\[ \xymatrix@R =50pt@C=50pt{ \int _{\operatorname{\mathcal{C}}} \mathscr {F} \ar [r] \ar [d]^-{\pi } & \operatorname{ \pmb {\mathcal{QC}} }_{\operatorname{Obj}} \ar [d] \\ \operatorname{\mathcal{C}}\ar [r]^-{\mathscr {F}} & \operatorname{ \pmb {\mathcal{QC}} }, } \]

where $\operatorname{ \pmb {\mathcal{QC}} }_{\operatorname{Obj}}$ is the $(\infty ,2)$-category of Construction 5.4.6.10 (Construction 5.4.6.8). If $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{ \pmb {\mathcal{QC}} }$ is a functor of $(\infty ,2)$-categories, then Proposition 5.4.6.9 and Remark 5.3.2.4 guarantee that $\pi : \int _{\operatorname{\mathcal{C}}} \mathscr {F}$ is an interior fibration; in particular, $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ is also an $(\infty ,2)$-category.

Warning 5.5.4.11. Let $\operatorname{\mathcal{C}}$ be an $(\infty ,2)$-category and let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{ \pmb {\mathcal{QC}} }$ be a morphism of simplicial sets. If $\mathscr {F}$ is not a functor, then $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ need not be an $(\infty ,2)$-category (this phenomenon arises already in the case $\operatorname{\mathcal{C}}= \Delta ^2$).

Example 5.5.4.12 (Objects of the $\infty $-Category of Elements). Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }})$ be a morphism of simplicial sets. Then vertices of the simplicial set $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ can be identified with pairs $(C, X)$, where $C$ is a vertex of $\operatorname{\mathcal{C}}$ and $X$ is a vertex of the simplicial set $\mathscr {F}(C)$ (see Example 5.4.6.12). Moreover, the projection map $U: \int _{\operatorname{\mathcal{C}}} \mathscr {F}$ is given on vertices by the construction $U(C,X) = C$.

Example 5.5.4.13 (Morphisms of the $\infty $-Category of Elements). Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }})$ be a morphism of simplicial sets. Let $(C,X)$ and $(D,Y)$ be vertices of the Grothendieck construction $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$. Edges of $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ from $(C,X)$ to $(D,Y)$ can be identified with pairs $(f,u)$, where $f: C \rightarrow D$ is an edge of the simplicial set $\operatorname{\mathcal{C}}$ and $u: \mathscr {F}(f)(X) \rightarrow Y$ is an edge of the simplicial set $\mathscr {F}(D)$ (see Example 5.4.6.12). Moreover, the projection map $U: \int _{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \operatorname{\mathcal{C}}$ is given on edges by the construction $U(f,u) = f$.

Remark 5.5.4.14 (Cocartesian Morphisms of the $\infty $-Category of Elements). Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{QC}}$ be a morphism of simplicial sets, so that the projection map $U: \int _{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \operatorname{\mathcal{C}}$ is a cocartesian fibration of simplicial sets (Proposition 5.5.4.2). Then an edge $(f,u): (C,X) \rightarrow (D,Y)$ of $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ is $U$-cocartesian if and only if $u: \mathscr {F}(f)(X) \rightarrow Y$ is an isomorphism in the $\infty $-category $\mathscr {F}(D)$ (see Example 5.4.6.12).

Remark 5.5.4.15 (Functoriality). Let $\operatorname{\mathcal{C}}$ be a simplicial set and let $\operatorname{Path}[\operatorname{\mathcal{C}}]_{\bullet }$ denote the simplicial path category of $\operatorname{\mathcal{C}}$ (see Definition 2.4.4.1). Suppose we are given a pair of simplicial functors $\mathscr {F}, \mathscr {G}: \operatorname{Path}[\operatorname{\mathcal{C}}]_{\bullet } \rightarrow \operatorname{Set_{\Delta }}$, which (by a slight abuse of notation) we will identify with morphisms of simplicial sets from $\operatorname{\mathcal{C}}$ to the homotopy coherent nerve $\operatorname{N}_{\bullet }^{\operatorname{hc}}(\operatorname{Set_{\Delta }})$. Every natural transformation of simplicial functors $\alpha : \mathscr {F} \rightarrow \mathscr {G}$ then induces a morphism of simplicial sets $\int _{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \int _{\operatorname{\mathcal{C}}} \mathscr {G}$, which we will denote by $\int _{\operatorname{\mathcal{C}}} \alpha $.

Suppose that $\mathscr {F}$ and $\mathscr {G}$ take values in the simplicial subcategory $\operatorname{QCat}\subseteq \operatorname{Set_{\Delta }}$ of Construction 5.4.4.1, so that the projection maps $U: \int _{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \operatorname{\mathcal{C}}$ and $V: \int _{\operatorname{\mathcal{C}}} \mathscr {G} \rightarrow \operatorname{\mathcal{C}}$ are cocartesian fibrations (Proposition 5.5.4.2). It follows from Remark 5.5.4.14 that the morphism $\int _{\operatorname{\mathcal{C}}} \alpha $ carries $U$-cocartesian edges of $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ to $V$-cocartesian edges of $\int _{\operatorname{\mathcal{C}}} \mathscr {G}$.

Example 5.5.4.16 ($2$-Simplices of the $\infty $-Category of Elements). Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }})$ be a morphism of simplicial sets and let $\sigma _0: \operatorname{\partial \Delta }^2 \rightarrow \int _{\operatorname{\mathcal{C}}} \mathscr {F}$ be a morphism of simplicial sets, which we depict informally as a diagram

\[ \xymatrix@R =50pt@C=50pt{ & (D,Y) \ar [dr]^{ (g,v) } & \\ (C,X) \ar [ur]^{(f,u)} \ar [rr]^{ (h,w) } & & (E,Z). } \]

Extensions of $\sigma _0$ to a $2$-simplex of $\int _{\operatorname{\mathcal{C}}} \mathscr {F}$ can be identified with pairs $(\mu , \theta )$, where $\mu : \mathscr {F}(g) \circ \mathscr {F}(f) \rightarrow \mathscr {F}(h)$ is an edge of the simplicial set $\operatorname{Fun}( \mathscr {F}(C), \mathscr {F}(E) )$, and $\theta : \operatorname{\raise {0.1ex}{\square }}^{2} \rightarrow \mathscr {F}(E)$ is a morphism of simplicial sets whose restriction to the boundary $\operatorname{\partial \raise {0.1ex}{\square }}^2$ is indicated in the diagram

\[ \xymatrix@R =50pt@C=50pt{ (\mathscr {F}(g) \circ \mathscr {F}(f))(X) \ar [r]^-{\mu (X)} \ar [d]^-{ \mathscr {F}(g)(u)} & \mathscr {F}(h)(X) \ar [d]^-{w} \\ \mathscr {F}(g)(Y) \ar [r]^-{v} & Z } \]

(see Example 5.4.6.14). Moreover, the projection map $U: \int _{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \mathscr {C}$ is given on $2$-simplices by the construction $U( \mu , \theta ) = \mu $.

Example 5.5.4.17. Let $\operatorname{\mathcal{E}}$ be a simplicial set, which we identify with the morphism of simplicial sets $\Delta ^{0} \rightarrow \operatorname{N}_{\bullet }^{\operatorname{hc}}(\operatorname{Set_{\Delta }})$ taking the value $\operatorname{\mathcal{E}}$. Then the simplicial set $\int _{\Delta ^{0}} \operatorname{\mathcal{E}}$ can be identified with the left-pinched morphism space $\operatorname{Hom}^{\mathrm{L}}_{ \operatorname{N}_{\bullet }^{\operatorname{hc}}(\operatorname{Set_{\Delta }}) }( \Delta ^{0}, \operatorname{\mathcal{E}})$. In particular, Construction 4.6.6.3 supplies a comparison morphism

\[ \rho : \operatorname{\mathcal{E}}= \operatorname{Hom}_{\operatorname{Set_{\Delta }}}( \Delta ^{0}, \operatorname{\mathcal{E}})_{\bullet } \rightarrow \operatorname{Hom}^{\mathrm{L}}_{ \operatorname{N}_{\bullet }^{\operatorname{hc}}(\operatorname{Set_{\Delta }}) }( \Delta ^{0}, \operatorname{\mathcal{E}}) = \int _{\Delta ^{0}} \operatorname{\mathcal{E}}. \]

If $\operatorname{\mathcal{E}}$ is an $\infty $-category, then $\operatorname{Hom}^{\mathrm{L}}_{ \operatorname{N}_{\bullet }^{\operatorname{hc}}(\operatorname{Set_{\Delta }}) }( \Delta ^{0}, \operatorname{\mathcal{E}})$ is also an $\infty $-category, and the comparison morphism $\rho $ is an equivalence of $\infty $-categories (Theorem 4.6.6.9). Beware that $\rho $ is generally not an isomorphism (though it is always a monomorphism which is bijective on simplices of dimension $\leq 1$). For example, Example 5.5.4.16 implies that $2$-simplices of $\int _{\Delta ^0} \operatorname{\mathcal{E}}$ can be identified with morphisms of simplicial sets $\theta : \Delta ^1 \times \Delta ^1 \rightarrow \operatorname{\mathcal{E}}$ for which the restriction $\theta |_{ \Delta ^1 \times \{ 0\} }$ is a degenerate edge of $\operatorname{\mathcal{E}}$, as indicated in the diagram

\[ \xymatrix@R =10pt@C=10pt{ X \ar [rrrr]^{\operatorname{id}_ X} \ar [dddd]_{u} \ar [ddddrrrr] & & & & X \ar [dddd]^{w} \\ & & & \sigma & \\ & & & & \\ & \tau & & & \\ Y \ar [rrrr]_{v} & & & & Z. } \]

The corresponding $2$-simplex of $\int _{\Delta ^{0}} \operatorname{\mathcal{E}}$ belongs to the image of $\rho $ if and only if $\sigma $ is a left-degenerate $2$-simplex of $\operatorname{\mathcal{E}}$ (in which case it is given by $\rho (\tau )$).

Remark 5.5.4.18. Let $U: \operatorname{\mathcal{C}}' \rightarrow \operatorname{\mathcal{C}}$ and $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }})$ be morphisms of simplicial sets, and let $\mathscr {F}'$ denote the composition $(\mathscr {F}' \circ U): \operatorname{\mathcal{C}}' \rightarrow \operatorname{N}_{\bullet }^{\operatorname{hc}}( \operatorname{Set_{\Delta }})$. Then the simplicial set $\int _{\operatorname{\mathcal{C}}'} \mathscr {F}'$ can be identified with the fiber product $\operatorname{\mathcal{C}}' \times _{\operatorname{\mathcal{C}}} \int _{\operatorname{\mathcal{C}}} \mathscr {F}$.

Example 5.5.4.19 (Fibers of the $\infty $-Category of Elements). Let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{ \pmb {\mathcal{QC}} }$ be a morphism of simplicial sets. For each vertex $C \in \operatorname{\mathcal{C}}$, Remark 5.5.4.18 and Example 5.5.4.17 supply a canonical isomorphism

\[ \{ C\} \times _{\operatorname{\mathcal{C}}} \int _{\operatorname{\mathcal{C}}} \mathscr {F} \simeq \operatorname{Hom}^{\mathrm{L}}_{\operatorname{ \pmb {\mathcal{QC}} }}( \Delta ^{0}, \mathscr {F}(C) ). \]

In particular, Construction 4.6.6.3 supplies a comparison functor $\theta : \mathscr {F}(C) \rightarrow \{ C \} \times _{\operatorname{\mathcal{C}}} \int _{\operatorname{\mathcal{C}}} \mathscr {F}$ which is an equivalence of $\infty $-categories (Theorem 4.6.6.9), but generally not an isomorphism of simplicial sets.