# Kerodon

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### 7.4.1 Limits of $\infty$-Categories

Let $\operatorname{\mathcal{QC}}$ denote the $\infty$-category of (small) $\infty$-categories (Construction 5.5.4.1). Our goal in this section (and ยง7.4.2) is to show that the $\infty$-category $\operatorname{\mathcal{QC}}$ admits small limits (Corollary 7.4.1.11). In fact, we will prove something more precise: if $\operatorname{\mathcal{C}}$ is a small $\infty$-category, then the limit of any diagram $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{QC}}$ can be realized as explicitly as a full subcategory of the $\infty$-category of sections of the cocartesian fibration $U: \int _{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \operatorname{\mathcal{C}}$ of Proposition 5.6.2.2 (Corollary 7.4.1.10).

Recall that, if $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}$ and $U': \operatorname{\mathcal{E}}' \rightarrow \operatorname{\mathcal{C}}$ are cocartesian fibrations of simplicial sets, then $\operatorname{Fun}^{\operatorname{CCart}}_{/\operatorname{\mathcal{C}}}( \operatorname{\mathcal{E}}, \operatorname{\mathcal{E}}')$ denotes the full subcategory of $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}(\operatorname{\mathcal{E}}, \operatorname{\mathcal{E}}')$ spanned by those functors $F: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{E}}'$ which carry $U$-cocartesian edges of $\operatorname{\mathcal{E}}$ to $U'$-cocartesian edges of $\operatorname{\mathcal{E}}'$ (Notation 5.3.1.10). Our main result can be stated as follows:

Theorem 7.4.1.1 (Diffraction Criterion). Suppose we are given a pullback diagram of small simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}\ar [d]^{U} \ar [r] & \overline{\operatorname{\mathcal{E}}} \ar [d]^{ \overline{U} } \\ \operatorname{\mathcal{C}}\ar [r] & \operatorname{\mathcal{C}}^{\triangleleft }, }$

where $U$ and $\overline{U}$ are cocartesian fibrations. The following conditions are equivalent:

$(1)$

The restriction map

$\operatorname{Fun}_{ / \operatorname{\mathcal{C}}^{\triangleleft } }^{\operatorname{CCart}}( \operatorname{\mathcal{C}}^{\triangleleft }, \overline{\operatorname{\mathcal{E}}} ) \rightarrow \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$

is an equivalence of $\infty$-categories.

$(2)$

The covariant transport representation

$\operatorname{Tr}_{ \overline{\operatorname{\mathcal{E}}} / \operatorname{\mathcal{C}}^{\triangleleft } }: \operatorname{\mathcal{C}}^{\triangleleft } \rightarrow \operatorname{\mathcal{QC}}$

of Notation 5.6.5.14 is a limit diagram in the $\infty$-category $\operatorname{\mathcal{QC}}$.

Remark 7.4.1.2. In the situation of Theorem 7.4.1.1, the restriction map $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}^{\triangleleft } }^{\operatorname{CCart}}( \operatorname{\mathcal{C}}^{\triangleleft }, \overline{\operatorname{\mathcal{E}}} ) \rightarrow \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ is automatically an isofibration of $\infty$-categories (Remark 5.3.1.18). Using Proposition 4.5.5.20, we see that condition $(1)$ of Theorem 7.4.1.1 is equivalent to the following a priori stronger condition:

$(1')$

The restriction map

$\operatorname{Fun}_{ / \operatorname{\mathcal{C}}^{\triangleleft } }^{\operatorname{CCart}}( \operatorname{\mathcal{C}}^{\triangleleft }, \overline{\operatorname{\mathcal{E}}} ) \rightarrow \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$

is a trivial Kan fibration of simplicial sets.

Construction 7.4.1.3 (Covariant Diffraction). Suppose we are given a pullback diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}\ar [d]^{U} \ar [r] & \overline{\operatorname{\mathcal{E}}} \ar [d]^{ \overline{U} } \\ \operatorname{\mathcal{C}}\ar [r] & \operatorname{\mathcal{C}}^{\triangleleft }, }$

where $U$ and $\overline{U}$ are cocartesian fibrations. Let $\overline{\operatorname{\mathcal{E}}}_{ {\bf 0} }$ denote the fiber of $\overline{U}$ over the cone point ${\bf 0} \in \operatorname{\mathcal{C}}^{\triangleleft }$. We then have restriction maps

$\overline{\operatorname{\mathcal{E}}}_{ {\bf 0} } \xleftarrow {\operatorname{ev}} \operatorname{Fun}_{/ \operatorname{\mathcal{C}}^{\triangleleft } }^{\operatorname{CCart}}( \operatorname{\mathcal{C}}^{\triangleleft }, \overline{\operatorname{\mathcal{E}}} ) \xrightarrow {\theta } \operatorname{Fun}_{/ \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}),$

where $\operatorname{ev}$ is a trivial Kan fibration (Corollary 5.3.1.23). Composing $\theta$ with a section of $\operatorname{ev}$, we obtain a functor of $\infty$-categories $\mathrm{Df}: \overline{\operatorname{\mathcal{E}}}_{ {\bf 0} } \rightarrow \operatorname{Fun}_{/ \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ which is well-defined up to isomorphism. We will refer to $\mathrm{Df}$ as the covariant diffraction functor associated to the cocartesian fibration $\overline{U}$.

Remark 7.4.1.4. In the situation of Construction 7.4.1.3, let $C \in \operatorname{\mathcal{C}}$ be a vertex and let $\operatorname{ev}_{C}: \operatorname{Fun}_{/ \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \rightarrow \operatorname{\mathcal{E}}_{C}$ be the evaluation functor, given on objects by $\operatorname{ev}_{C}(F) = F(C)$. Then the composition

$\overline{\operatorname{\mathcal{E}}}_{ {\bf 0} } \xrightarrow {\mathrm{Df}} \operatorname{Fun}_{/ \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \xrightarrow {\operatorname{ev}_ C} \operatorname{\mathcal{E}}_{C}$

is given by covariant transport along the unique edge ${\bf 0} \rightarrow C$ of $\operatorname{\mathcal{C}}^{\triangleleft }$.

Remark 7.4.1.5. Suppose we are given a pullback diagram of small simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}\ar [d]^{U} \ar [r] & \overline{\operatorname{\mathcal{E}}} \ar [d]^{ \overline{U} } \\ \operatorname{\mathcal{C}}\ar [r] & \operatorname{\mathcal{C}}^{\triangleleft }. }$

Then the covariant diffraction functor $\mathrm{Df}: \overline{\operatorname{\mathcal{E}}}_{ {\bf 0} } \rightarrow \operatorname{Fun}_{/ \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ of Construction 7.4.1.3 is an equivalence of $\infty$-categories if and only if the covariant transport representation $\operatorname{Tr}_{ \overline{\operatorname{\mathcal{E}}} / \operatorname{\mathcal{C}}^{\triangleleft } }: \operatorname{\mathcal{C}}^{\triangleleft } \rightarrow \operatorname{\mathcal{QC}}$ is a limit diagram in the $\infty$-category $\operatorname{\mathcal{QC}}$ (this is a restatement of Theorem 7.4.1.1).

We now show that there exists a good supply of cocartesian fibrations which satisfy the hypotheses of Theorem 7.4.1.1.

Proposition 7.4.1.6. Let $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}$ be a cocartesian fibration of simplicial sets. Then there exists a pullback diagram

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}\ar [d]^{U} \ar [r] & \overline{\operatorname{\mathcal{E}}} \ar [d]^{ \overline{U} } \\ \operatorname{\mathcal{C}}\ar [r] & \operatorname{\mathcal{C}}^{\triangleleft }, }$

where $\overline{U}$ is a cocartesian fibration and the restriction map

$\operatorname{Fun}_{ / \operatorname{\mathcal{C}}^{\triangleleft } }^{\operatorname{CCart}}( \operatorname{\mathcal{C}}^{\triangleleft }, \overline{\operatorname{\mathcal{E}}} ) \rightarrow \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$

is an equivalence of $\infty$-categories.

Proof. Let $\operatorname{ev}: \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}$ denote the evaluation morphism (given on vertices by the formula $\operatorname{ev}( F, C) = F(C)$), and let

$\operatorname{\mathcal{E}}' = (\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}) \star _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{E}}$

denote the relative join of Construction 5.2.3.1. Note that we have a canonical map

$U': \operatorname{\mathcal{E}}' = (\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}) \star _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}\star _{\operatorname{\mathcal{C}}} \operatorname{\mathcal{C}}\simeq \Delta ^{1} \times \operatorname{\mathcal{C}}.$

Let $\pi : \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{C}}$ be given by projection onto the second factor. Note that $\pi$ is a cocartesian fibration, and that an edge of the product $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}$ is $\pi$-cocartesian if and only if its image in $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ is an isomorphism. It follows that the $\operatorname{ev}$ carries $\pi$-cocartesian edges of $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}$ to $U$-cocartesian edges of $\operatorname{\mathcal{E}}$. Applying Lemma 5.2.3.17, we deduce that $U'$ is a cocartesian fibration. By construction, we can identify $\operatorname{\mathcal{E}}$ with the inverse image of $\{ 1\} \times \operatorname{\mathcal{C}}$ under $U'$.

Let $\operatorname{\mathcal{E}}''$ denote the pushout

$( \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}^{\triangleleft } ) {\coprod }_{ ( \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}^{\triangleleft } ) } \operatorname{\mathcal{E}}'.$

Amalgamating $U'$ with the projection map $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}^{\triangleleft } \rightarrow \operatorname{\mathcal{C}}^{\triangleleft }$, we obtain a morphism of simplicial sets $U'': \operatorname{\mathcal{E}}'' \rightarrow K$, where $K$ denotes the pushout $( \{ 0 \} \times \operatorname{\mathcal{C}})^{\triangleleft } {\coprod }_{ ( \{ 0\} \times \operatorname{\mathcal{C}}) } ( \Delta ^1 \times \operatorname{\mathcal{C}})$. It follows from Proposition 5.1.4.7 that $U''$ is also a cocartesian fibration.

Let us abuse notation by identifying $K$ with its image in the simplicial set $( \Delta ^1 \times \operatorname{\mathcal{C}})^{\triangleleft }$. Since the inclusion map $\{ 0\} \times \operatorname{\mathcal{C}}\hookrightarrow \Delta ^1 \times \operatorname{\mathcal{C}}$ is left anodyne (Proposition 4.2.5.3), the inclusion $K \hookrightarrow (\Delta ^1 \times \operatorname{\mathcal{C}})^{\triangleleft }$ is inner anodyne (Example 4.3.6.5). Applying Proposition 5.6.7.2, we can write $U''$ as the pullback of a cocartesian fibration $U''': \operatorname{\mathcal{E}}''' \rightarrow (\Delta ^1 \times \operatorname{\mathcal{C}})^{\triangleleft }$. We then have a commutative diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}\ar [r] \ar [d]^{U} & \operatorname{\mathcal{E}}' \ar [r] \ar [d]^{U'} & \operatorname{\mathcal{E}}'' \ar [r] \ar [d]^{U''} & \operatorname{\mathcal{E}}''' \ar [d]^{U'''} \\ \{ 1\} \times \operatorname{\mathcal{C}}\ar [r] & \Delta ^1 \times \operatorname{\mathcal{C}}\ar [r] & K \ar [r] & ( \Delta ^1 \times \operatorname{\mathcal{C}})^{\triangleleft }, }$

where each square is a pullback and each vertical map is a cocartesian fibration. Let $\overline{\operatorname{\mathcal{E}}}$ denote the pullback $( \{ 1\} \times \operatorname{\mathcal{C}})^{\triangleleft } \times _{ (\Delta ^1 \times \operatorname{\mathcal{C}})^{\triangleleft } } \operatorname{\mathcal{E}}'''$, so that $U'''$ restricts to a cocartesian fibration $\overline{U}: \overline{\operatorname{\mathcal{E}}} \rightarrow ( \{ 1\} \times \operatorname{\mathcal{C}})^{\triangleleft }$. We will complete the proof by showing that the commutative diagram

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}\ar [d]^{U} \ar [r] & \overline{\operatorname{\mathcal{E}}} \ar [d]^{ \overline{U} } \\ \{ 1\} \times \operatorname{\mathcal{C}}\ar [r] & (\{ 1\} \times \operatorname{\mathcal{C}})^{\triangleleft } }$

satisfies the requirements of Proposition 7.4.1.6.

For every simplicial subset $A \subseteq (\Delta ^1 \times \operatorname{\mathcal{C}})^{\triangleleft }$, let $\operatorname{\mathcal{D}}(A)$ denote the $\infty$-category

$\operatorname{Fun}_{ / A}^{\operatorname{CCart}}( A, A \times _{ ( \Delta ^1 \times \operatorname{\mathcal{C}})^{\triangleleft } } \operatorname{\mathcal{E}}''').$

Let ${\bf 0}$ denote the cone point of $(\Delta ^1 \times \operatorname{\mathcal{C}})^{\triangleleft }$. Note that we have a commutative diagram of restriction functors

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{D}}( ( \Delta ^1 \times \operatorname{\mathcal{C}})^{\triangleleft } ) \ar [r]^-{\alpha '} \ar [d]^{\beta } & \operatorname{\mathcal{D}}( (\{ 1\} \times \operatorname{\mathcal{C}})^{\triangleleft } ) \ar [d]^{\alpha } \\ \operatorname{\mathcal{D}}(K) \ar [r]^-{\beta '} \ar [d]^{\gamma } & \operatorname{\mathcal{D}}( \{ 1\} \times \operatorname{\mathcal{C}}) \\ \operatorname{\mathcal{D}}( \{ {\bf 0} \} ). & }$

We wish to show that $\alpha$ is an equivalence of $\infty$-categories. Since the inclusion $K \hookrightarrow ( \Delta ^1 \times \operatorname{\mathcal{C}})^{\triangleleft }$ is inner anodyne (as noted above) and the inclusion $( \{ 1\} \times \operatorname{\mathcal{C}})^{\triangleleft } \hookrightarrow ( \Delta ^1 \times \operatorname{\mathcal{C}})^{\triangleleft }$ is left anodyne (Lemma 4.3.7.8), the morphisms $\alpha '$ and $\beta$ are trivial Kan fibrations (Proposition 5.3.1.21). It will therefore suffice to show that $\beta '$ is an equivalence of $\infty$-categories.

Amalgamating the map

\begin{eqnarray*} \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \Delta ^1 \times \operatorname{\mathcal{C}}& \simeq & (\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}) \star _{ (\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}) } (\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}) \\ & \rightarrow & (\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}) \star _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{E}}\\ & = & \operatorname{\mathcal{E}}' \end{eqnarray*}

with the identity on $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times \operatorname{\mathcal{C}}^{\triangleleft }$, we obtain a morphism of simplicial sets $F: \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times K \rightarrow \operatorname{\mathcal{E}}''$. If $e$ is an edge of the product $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \times K$ whose image in $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ is an isomorphism, then $F(e)$ is a $U''$-cocartesian edge of $\operatorname{\mathcal{E}}''$. We can therefore identify $F$ with a morphism of simplicial sets $f: \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \rightarrow \operatorname{\mathcal{D}}(K)$. Unwinding the definitions, we see that $\beta ' \circ f$ is an isomorphism of simplicial sets. Consequently, to show that $\beta '$ is an equivalence of $\infty$-categories, it will suffice to show that $f$ is an equivalence of $\infty$-categories. Similarly, the composite map $\gamma \circ f$ is an isomorphism, so we are reduced to proving that $\gamma$ is an equivalence of $\infty$-categories. Since $\beta$ is a trivial Kan fibration, this is equivalent to the assertion that $\gamma \circ \beta$ is an equivalence of $\infty$-categories, which is a special case of Corollary 5.3.1.23. $\square$

Remark 7.4.1.7. If $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}$ is a cocartesian fibration of small simplicial sets, then the simplicial set $\overline{\operatorname{\mathcal{E}}}$ constructed in the proof of Proposition 7.4.1.6 will also be small.

Remark 7.4.1.8. In the situation of Proposition 7.4.1.6, suppose that $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}$ is a left fibration. Then the extension $\overline{U}: \overline{\operatorname{\mathcal{E}}} \rightarrow \operatorname{\mathcal{C}}^{\triangleleft }$ is also a left fibration. To prove this, it will suffice to show that the fiber $\overline{\operatorname{\mathcal{E}}}_{ {\bf 0} }$ is a Kan complex (Proposition 5.1.4.14). This follows from the fact that the covariant diffraction functor

$\mathrm{Df}: \overline{\operatorname{\mathcal{E}}}_{ {\bf 0} } \rightarrow \operatorname{Fun}_{/ \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) = \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$

is an equivalence of $\infty$-categories, since the simplicial set $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ is a Kan complex by (Corollary 4.4.2.5).

Corollary 7.4.1.9. Let $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}$ be a cocartesian fibration of small simplicial sets and let $\operatorname{Tr}_{ \operatorname{\mathcal{E}}/ \operatorname{\mathcal{C}}}: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{QC}}$ be a covariant transport representation for $U$. Then the diagram $\operatorname{Tr}_{ \operatorname{\mathcal{E}}/ \operatorname{\mathcal{C}}}$ has a limit in the $\infty$-category $\operatorname{\mathcal{QC}}$, given by the $\infty$-category $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ of cocartesian sections of $U$.

Proof. Using Proposition 7.4.1.6 (and Remark 7.4.1.7), we see that there exists a pullback diagram of small simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}\ar [d]^{U} \ar [r] & \overline{\operatorname{\mathcal{E}}} \ar [d]^{ \overline{U} } \\ \operatorname{\mathcal{C}}\ar [r] & \operatorname{\mathcal{C}}^{\triangleleft }, }$

where $\overline{U}$ is a cocartesian fibration and the restriction map $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}^{\triangleleft } }^{\operatorname{CCart}}( \operatorname{\mathcal{C}}^{\triangleleft }, \overline{\operatorname{\mathcal{E}}} ) \rightarrow \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ is a trivial Kan fibration. Using Corollary 5.6.5.11, we can extend $\operatorname{Tr}_{ \operatorname{\mathcal{E}}/ \operatorname{\mathcal{C}}}$ to a diagram $\operatorname{Tr}_{ \overline{\operatorname{\mathcal{E}}} / \operatorname{\mathcal{C}}^{\triangleleft } }: \operatorname{\mathcal{C}}^{\triangleleft } \rightarrow \operatorname{\mathcal{QC}}$ which is a covariant transport representation for $\overline{U}$. Let ${\bf 0}$ denote the cone point of $\operatorname{\mathcal{C}}^{\triangleleft }$. It follows from Theorem 7.4.1.1 that $\operatorname{Tr}_{ \overline{\operatorname{\mathcal{E}}} / \operatorname{\mathcal{C}}^{\triangleleft } }$ is a limit diagram in the $\infty$-category $\operatorname{\mathcal{QC}}$, and therefore exhibits the $\infty$-category $\operatorname{Tr}_{ \overline{\operatorname{\mathcal{E}}} / \operatorname{\mathcal{C}}^{\triangleleft } }( {\bf 0} ) \simeq \overline{\operatorname{\mathcal{E}}}_{ {\bf 0} }$ as a limit of the diagram $\operatorname{Tr}_{\operatorname{\mathcal{E}}/\operatorname{\mathcal{C}}}$. Using Remark 7.4.1.5, we see that covariant diffraction supplies an equivalence of $\infty$-categories $\overline{\operatorname{\mathcal{E}}}_{ {\bf 0} } \rightarrow \operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$, so that $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ is also a limit of the diagram $\operatorname{Tr}_{\operatorname{\mathcal{E}}/\operatorname{\mathcal{C}}}$ (Proposition 7.1.1.12). $\square$

Corollary 7.4.1.10. Let $\operatorname{\mathcal{C}}$ be a small simplicial set and let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{QC}}$ be a diagram in the $\infty$-category $\operatorname{\mathcal{QC}}$. Then the $\infty$-category of cocartesian sections $\operatorname{Fun}_{ /\operatorname{\mathcal{C}}}^{\operatorname{CCart}}( \operatorname{\mathcal{C}}, \int _{\operatorname{\mathcal{C}}} \mathscr {F} )$ is a limit of the diagram $\mathscr {F}$.

Proof. Apply Corollary 7.4.1.9 to the cocartesian fibration $U: \int _{\operatorname{\mathcal{C}}} \mathscr {F} \rightarrow \operatorname{\mathcal{C}}$. $\square$

Corollary 7.4.1.11. The $\infty$-category $\operatorname{\mathcal{QC}}$ is complete: that is, it admits small limits.

By inspecting the proof of Corollary 7.4.1.11, we can obtain more precise information.

Corollary 7.4.1.12. Let $n$ be an integer, let $\operatorname{\mathcal{C}}$ be a simplicial set and let $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{QC}}$ be a diagram. Suppose that, for every vertex $C \in \operatorname{\mathcal{C}}$, the $\infty$-category $\mathscr {F}(C)$ is locally $n$-truncated. Then the limit $\varprojlim ( \mathscr {F} )$ is a locally $n$-truncated $\infty$-category.

Proof. Without loss of generality, we may assume that $\operatorname{\mathcal{C}}$ is an $\infty$-category and that $n \geq -2$. Let $\operatorname{\mathcal{E}}= \int _{\operatorname{\mathcal{C}}} \mathscr {F}$ denote the $\infty$-category of elements of $\mathscr {F}$. It follows from Variant 5.1.5.17 that the projection map $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{C}}$ is an essentially $(n+1)$-categorical cocartesian fibration. Applying Corollary 4.8.6.22, we see that the $\infty$-category of sections $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ is locally $n$-truncated. Since $\varprojlim (\mathscr {F} )$ can be identified with a full subcategory of $\operatorname{Fun}_{ / \operatorname{\mathcal{C}}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ (Corollary 7.4.1.10), it is also locally $n$-truncated (Remark 4.8.2.3). $\square$

Corollary 7.4.1.13. Let $\lambda$ be an uncountable cardinal and let $\kappa = \mathrm{ecf}(\lambda )$ be the exponential cofinality of $\lambda$. Suppose we are given a diagram $\mathscr {F}: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{QC}}$, where $\operatorname{\mathcal{C}}$ is a $\kappa$-small simplicial set. If the $\infty$-category $\mathscr {F}(C)$ is essentially $\lambda$-small for each $C \in \operatorname{\mathcal{C}}$, then the limit $\varprojlim (\mathscr {F} )$ is also essentially $\lambda$-small.

Proof. Using Proposition 4.7.5.5, we can choose a categorical equivalence $G: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$, where $\operatorname{\mathcal{D}}$ is a $\lambda$-small $\infty$-category (if $\kappa$ is uncountable, we can even arrange that $\operatorname{\mathcal{D}}$ is $\kappa$-small). Without loss of generality, we may assume that $\mathscr {F}$ is obtained as the restriction of the covariant transport representation of some cocartesian fibration $U: \operatorname{\mathcal{E}}\rightarrow \operatorname{\mathcal{D}}$. Using Corollary 7.4.1.9, we can identify $\varprojlim ( \mathscr {F} )$ with a full subcategory of the $\infty$-category $\operatorname{Fun}_{ / \operatorname{\mathcal{D}}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$. It will therefore suffice to show that the $\infty$-category $\operatorname{Fun}_{ / \operatorname{\mathcal{D}}}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ is essentially $\lambda$-small (Corollary 4.7.5.13). By construction, we have a pullback diagram of simplicial sets

7.47
$$\begin{gathered}\label{equation:limit-essential-smallness} \xymatrix@C =50pt@R=50pt{ \operatorname{Fun}_{ / \operatorname{\mathcal{D}}} ( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \ar [r] \ar [d] & \operatorname{Fun}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) \ar [d]^{U \circ } \\ \{ G \} \ar [r] & \operatorname{Fun}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{D}}) } \end{gathered}$$

where the vertical maps are cocartesian fibrations (Theorem 5.2.1.1), and therefore isofibrations (Proposition 5.1.4.8). It follows that (7.47) is also a categorical pullback square (Corollary 4.5.2.27). Using Corollary 4.7.5.16, we are reduced to proving that the $\infty$-categories $\operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}})$ and $\operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})$ are essentially $\lambda$-small, which follows from Remark 4.7.5.10. $\square$