Corollary 5.3.7.4 (02VW)

Corollary 5.3.7.4. Let $\operatorname{\mathcal{C}}$ be an $\infty $-category and let $K$ be a simplicial set. Then:

$(1)$: The restriction map $U: \operatorname{Fun}(K^{\triangleleft }, \operatorname{\mathcal{C}}) \rightarrow \operatorname{Fun}(K, \operatorname{\mathcal{C}})$ is a cocartesian fibration. Moreover, a morphism $e$ of $\operatorname{Fun}(K^{\triangleleft }, \operatorname{\mathcal{C}})$ is $U$-cocartesian if and only if it carries the cone point ${\bf 0} \in K^{\triangleleft }$ to an isomorphism in $\operatorname{\mathcal{C}}$.
$(2)$: The restriction map $V: \operatorname{Fun}(K^{\triangleright }, \operatorname{\mathcal{C}}) \rightarrow \operatorname{Fun}(K, \operatorname{\mathcal{C}})$ is a cartesian fibration. Moreover, a morphism $e$ of $\operatorname{Fun}(K^{\triangleright }, \operatorname{\mathcal{C}})$ is $U$-cartesian if and only if it carries the cone point ${\bf 1} \in K^{\triangleright }$ to an isomorphism in $\operatorname{\mathcal{C}}$.

Proof. We will prove $(1)$; the proof of $(2)$ is similar. Let $\Delta ^0 \diamond K$ denote the blunt join of Notation 4.5.8.3, and let $c: \Delta ^0 \diamond K \rightarrow \Delta ^{0} \star K = K^{\triangleleft }$ be the categorical equivalence of Theorem 4.5.8.8. We have a commutative diagram of $\infty $-categories

\[ \xymatrix@R =50pt@C=50pt{ \operatorname{Fun}( K^{\triangleleft }, \operatorname{\mathcal{C}}) \ar [dr]^{U} \ar [rr]^-{ \circ c} & & \operatorname{Fun}( \Delta ^0 \diamond K, \operatorname{\mathcal{C}}) \ar [dl]^{U'} \\ & \operatorname{Fun}(K, \operatorname{\mathcal{C}}) & } \]

where the horizontal map is an equivalence of $\infty $-categories (Proposition 4.5.3.8) and the vertical maps are isofibrations (Corollary 4.4.5.3). Unwinding the definitions, we can identify $\operatorname{Fun}( \Delta ^0 \diamond K, \operatorname{\mathcal{C}})$ with the oriented fiber product $\operatorname{\mathcal{C}}\operatorname{\vec{\times }}_{ \operatorname{Fun}(K, \operatorname{\mathcal{C}}) } \operatorname{Fun}(K, \operatorname{\mathcal{C}})$. Under this identification, the functor $U'$ is given by projection onto the second factor, and is therefore a cocartesian fibration (Corollaryt 5.3.7.3). Applying Corollary 5.1.6.2, we deduce that $U$ is also a cocartesian fibration. Moreover, a morphism $e$ of $\operatorname{Fun}(K^{\triangleleft }, \operatorname{\mathcal{C}})$ is $U$-cocartesian if and only if its image in $\operatorname{Fun}( \Delta ^0 \diamond K, \operatorname{\mathcal{C}})$ is $U'$-cocartesian (Proposition 5.1.6.6). Using the criterion of Corollary 5.3.7.3, we see that this is equivalent to the requirement that $e$ carries the cone point ${\bf 0} \in K^{\triangleleft }$ to an isomorphism in $\operatorname{\mathcal{C}}$. $\square$