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5.6.2 Equivalence of Fibrations

Let $\operatorname{\mathcal{C}}$ and $\operatorname{\mathcal{D}}$ be $\infty $-categories. Recall that a functor $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ is an equivalence of $\infty $-categories if there exists a functor $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ such that $G \circ F$ and $F \circ G$ are isomorphic to $\operatorname{id}_{\operatorname{\mathcal{C}}}$ and $\operatorname{id}_{\operatorname{\mathcal{D}}}$ as objects of the $\infty $-categories $\operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{C}})$ and $\operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{D}})$, respectively (Definition 4.5.1.10). In this section, we study a relative version of this notion, where $\operatorname{\mathcal{C}}$ and $\operatorname{\mathcal{D}}$ are simplicial sets equipped with inner fibrations $U: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}$ and $V: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$ over the same base simplicial set $\operatorname{\mathcal{E}}$ (which need not be an $\infty $-category). Recall that, in this case, the simplicial set

\[ \operatorname{Fun}_{ / \operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{D}}) = \{ U\} \times _{ \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{E}}) } \operatorname{Fun}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{E}}) \]

is also an $\infty $-category (Corollary 4.1.4.8).

Definition 5.6.2.1. Suppose we are given a commutative diagram of simplicial sets

\[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}\ar [rr]^{F} \ar [dr]_{U} & & \operatorname{\mathcal{D}}\ar [dl]^{V} \\ & \operatorname{\mathcal{E}}, & } \]

where $U$ and $V$ are inner fibrations. Let $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ be a morphism of simplicial sets. We say that $G$ is a homotopy inverse of $F$ relative to $\operatorname{\mathcal{E}}$ if the following conditions are satisfied:

  • The composition $U \circ G$ is equal to $V$: that is, the diagram

    \[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}\ar [dr]_{U} & & \operatorname{\mathcal{D}}\ar [ll]_{G} \ar [dl]^{V} \\ & \operatorname{\mathcal{E}}& } \]

    is commutative.

  • The composite morphisms $G \circ F $and $F \circ G$ are isomorphic to $\operatorname{id}_{\operatorname{\mathcal{C}}}$ and $\operatorname{id}_{\operatorname{\mathcal{D}}}$ as objects of the $\infty $-categories $\operatorname{Fun}_{ / \operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{C}})$ and $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{D}})$, respectively.

We say that $F$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$ if there exists a morphism of simplicial sets $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ which is a homotopy inverse of $F$ relative to $\operatorname{\mathcal{E}}$. We say that inner fibrations $U: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}$ and $V: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$ are equivalent if there exists a morphism of simplicial sets $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ which is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$ (so that, in particular, we have $U = V \circ F$).

Example 5.6.2.2. Let $\operatorname{\mathcal{C}}$ and $\operatorname{\mathcal{D}}$ be $\infty $-categories, so that the projection maps $U: \operatorname{\mathcal{C}}\rightarrow \Delta ^0$ and $V: \operatorname{\mathcal{D}}\rightarrow \Delta ^0$ are inner fibrations. Then a functor $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ is an equivalence of $\infty $-categories if and only if it is an equivalence of inner fibrations over $\Delta ^0$. In particular, the inner fibrations $U$ and $V$ are equivalent (in the sense of Definition 5.6.2.1) if and only if the $\infty $-categories $\operatorname{\mathcal{C}}$ and $\operatorname{\mathcal{D}}$ are equivalent (in the sense of Definition 4.5.1.10).

Remark 5.6.2.3 (Two-out-of-Three). Suppose we are given a commutative diagram of simplicial sets

\[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}\ar [dr] \ar [r]^-{F} & \operatorname{\mathcal{C}}' \ar [d] \ar [r]^-{F'} & \operatorname{\mathcal{C}}'' \ar [dl] \\ & \operatorname{\mathcal{E}}, & } \]

where the vertical maps are inner fibrations. If any two of the morphisms $F$, $F'$, and $F' \circ F$ are equivalences of inner fibrations over $\operatorname{\mathcal{E}}$, then so is the third. In particular, the collection of equivalences of inner fibrations over $\operatorname{\mathcal{E}}$ is closed under composition.

Remark 5.6.2.4 (Functoriality). Let $U: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}$ and $V: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$ be inner fibrations of simplicial sets, and let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$. For every morphism of simplicial sets $\operatorname{\mathcal{E}}' \rightarrow \operatorname{\mathcal{E}}$, the induced map $F': \operatorname{\mathcal{E}}' \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}' \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{D}}$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}'$. In particular, for every object $E \in \operatorname{\mathcal{E}}$, the induced map $F_{E}: \{ E\} \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{C}}\rightarrow \{ E\} \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{D}}$ is an equivalence of $\infty $-categories.

Proposition 5.6.2.5. Suppose we are given a commutative diagram of simplicial sets

\[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}\ar [rr]^{F} \ar [dr]_{U} & & \operatorname{\mathcal{D}}\ar [dl]^{V} \\ & \operatorname{\mathcal{E}}. & } \]

Then:

$(1)$

If $U$ and $V$ are inner fibrations and $F$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$, then $F$ is a categorical equivalence of simplicial sets.

$(2)$

If $U$ and $V$ are isofibrations and $F$ is a categorical equivalence of simplicial sets, then it is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$.

Proof. We first prove $(1)$. Assume that $U$ and $V$ are inner fibration and that $F$ is an categorical equivalence of simplicial sets. We wish to show that $F$ is a categorical equivalence of simplicial sets. Fix an $\infty $-category $\operatorname{\mathcal{K}}$, and let $\theta _{F}: \pi _0( \operatorname{Fun}( \operatorname{\mathcal{D}}, \operatorname{\mathcal{K}})^{\simeq } ) \rightarrow \pi _0( \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{K}})^{\simeq } )$ be the map given by precomposition with $F$. We wish to show that $\theta _{F}$ is a bijection. Let $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ be a homotopy inverse of $F$ relative to $\operatorname{\mathcal{E}}$, so that precomposition with $G$ determines a map $\theta _{G}: \pi _0( \operatorname{Fun}( \operatorname{\mathcal{D}}, \operatorname{\mathcal{K}})^{\simeq } ) \rightarrow \pi _0( \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{K}})^{\simeq } )$. We claim that $\theta _{G}$ is an inverse of $\theta _{F}$. We will show that $\theta _{G}$ is a left inverse of $\theta _{F}$; a similar argument will show that $\theta _{G}$ is a right inverse of $\theta _{F}$. Fix a morphism $H: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{K}}$; we wish to show that $H$ is isomorphic to $H \circ G \circ F$ as an object of the $\infty $-category $\operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{K}})$. This is clear, since postcomposition with $H$ determines a functor of $\infty $-categories $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{C}}) \rightarrow \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{K}})$.

We now prove $(2)$. Let $Q$ be a contractible Kan complex containing a pair of distinct vertices $x$ and $y$, and form a pushout diagram of simplicial sets

\[ \xymatrix@R =50pt@C=50pt{ \{ x \} \times \operatorname{\mathcal{C}}\ar [r]^-{F} \ar [d] & \operatorname{\mathcal{D}}\ar [d] \\ Q \times \operatorname{\mathcal{C}}\ar [r] & \operatorname{\mathcal{M}}. } \]

Since the vertical maps are monomorphisms, this diagram is also a categorical pushout square (Proposition 4.5.3.7). In particular, if $F$ is a categorical equivalence, then the map $Q \times \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{M}}$ is also a categorical equivalence (Proposition 4.5.3.6). Since $Q$ is contractible, the inclusion $\{ y\} \times \operatorname{\mathcal{C}}\hookrightarrow Q \times \operatorname{\mathcal{C}}$ is a categorical equivalence (Remark 4.5.2.6), so the inclusion $\{ y\} \times \operatorname{\mathcal{C}}\hookrightarrow \operatorname{\mathcal{M}}$ is also a categorical equivalence. If $U$ is an isofibration, then the lifting problem

\[ \xymatrix@R =50pt@C=50pt{ \{ y\} \times \operatorname{\mathcal{C}}\ar [r]^-{\operatorname{id}} \ar [d] & \operatorname{\mathcal{C}}\ar [d]^{U} \\ \operatorname{\mathcal{M}}\ar [r] \ar@ {-->}[ur] & \operatorname{\mathcal{E}}} \]

admits a solution, which we can identify with a pair of morphisms $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ and $u: Q \rightarrow \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{C}})$ satisfying $u(x) = G \circ F$ and $u(y) = \operatorname{id}_{\operatorname{\mathcal{C}}}$. It follows that $G \circ F$ is isomorphic to $\operatorname{id}_{\operatorname{\mathcal{C}}}$ as an object of the $\infty $-category $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}},\operatorname{\mathcal{C}})$.

Repeating the above argument with $F$ replaced by $G$, we conclude that there exists a morphism $H: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ in $(\operatorname{Set_{\Delta }})_{/S}$ such that $H \circ G$ is isomorphic to $\operatorname{id}_{\operatorname{\mathcal{D}}}$ as an object of the $\infty $-category $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{D}},\operatorname{\mathcal{D}})$. Then $F$ and $H$ are both isomorphic to $H \circ G \circ F$ as objects of the $\infty $-category $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})$, and are therefore isomorphic to each other. We may therefore assume without loss of generality that $H = F$, so that $G$ is a homotopy inverse of $F$ relative to $\operatorname{\mathcal{E}}$. In particular, $F$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$. $\square$

Warning 5.6.2.6. Assertion $(2)$ of Proposition 5.6.2.5 need not be true if $U$ and $V$ are only assumed to be inner fibrations. For example, let $\operatorname{\mathcal{E}}$ be an $\infty $-category and let $\operatorname{\mathcal{E}}' \subseteq \operatorname{\mathcal{E}}$ be a full subcategory for which the inclusion map $\iota : \operatorname{\mathcal{E}}' \hookrightarrow \operatorname{\mathcal{E}}$ is an equivalence. Then we have a commutative diagram of $\infty $-categories

\[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{E}}' \ar [rr]^{\iota } \ar [dr]_{\iota } & & \operatorname{\mathcal{E}}\ar [dl]^{\operatorname{id}} \\ & \operatorname{\mathcal{E}}, & } \]

where the vertical maps are inner fibrations. However, $\iota $ is not an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$ unless $\operatorname{\mathcal{E}}' = \operatorname{\mathcal{E}}$.

Corollary 5.6.2.7. Suppose we are given a commutative diagram of simplicial sets

\[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}\ar [rr]^{F} \ar [dr]_{U} & & \operatorname{\mathcal{D}}\ar [dl]^{V} \\ & \operatorname{\mathcal{E}}, & } \]

where $U$ and $V$ are inner fibrations and $\operatorname{\mathcal{E}}= \Delta ^ n$ is a standard simplex. Then $F$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$ if and only if it an equivalence of $\infty $-categories.

Proof. Our assumption that $\operatorname{\mathcal{E}}= \Delta ^ n$ is a standard simplex guarantees that the inner fibrations $U$ and $V$ are isofibrations (Example 4.4.1.6), so the desired result follows from Proposition 5.6.2.5. $\square$

Proposition 5.6.2.8. Suppose we are given a commutative diagram of simplicial sets

\[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}\ar [rr]^{F} \ar [dr]_{U} & & \operatorname{\mathcal{D}}\ar [dl]^{V} \\ & \operatorname{\mathcal{E}}, & } \]

where $U$ and $V$ are inner fibrations. The following conditions are equivalent:

$(1)$

For every morphism of simplicial sets $B \rightarrow \operatorname{\mathcal{E}}$, postcomposition with $F$ induces a homotopy equivalence of Kan complexes $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B, \operatorname{\mathcal{C}})^{\simeq } \rightarrow \operatorname{Fun}_{/\operatorname{\mathcal{E}}}( B, \operatorname{\mathcal{D}})^{\simeq }$.

$(2)$

For every morphism of simplicial sets $B \rightarrow \operatorname{\mathcal{E}}$, postcomposition with $F$ induces an equivalence of $\infty $-categories $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B, \operatorname{\mathcal{C}}) \rightarrow \operatorname{Fun}_{/\operatorname{\mathcal{E}}}( B, \operatorname{\mathcal{D}})$.

$(3)$

The morphism $F$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$.

$(4)$

For every simplex $\sigma : \Delta ^ n \rightarrow \operatorname{\mathcal{E}}$, the induced map $F_{\sigma }: \Delta ^ n \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{C}}\rightarrow \Delta ^ n \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{D}}$ is an equivalence of $\infty $-categories.

Proof. We first show that $(1)$ implies $(2)$. Assume that $(1)$ is satisfied and let $B \rightarrow \operatorname{\mathcal{E}}$ be a morphism of simplicial sets; we wish to show that the induced map $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B, \operatorname{\mathcal{C}}) \rightarrow \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B,\operatorname{\mathcal{D}})$ is an equivalence of $\infty $-categories. By virtue of Theorem 4.5.4.1, it will suffice to show that for every simplicial set $A$, the induced map

\[ \operatorname{Fun}( B', \operatorname{Fun}_{/\operatorname{\mathcal{E}}}( B, \operatorname{\mathcal{C}}) )^{\simeq } \rightarrow \operatorname{Fun}( B', \operatorname{Fun}_{/\operatorname{\mathcal{E}}}( B, \operatorname{\mathcal{D}}) )^{\simeq } \]

is a homotopy equivalence of Kan complexes. This follows by applying $(1)$ to the composite map $B' \times B \rightarrow B \rightarrow \operatorname{\mathcal{E}}$.

We now prove that $(2)$ implies $(3)$. Assume that condition $(2)$ is satisfied. Setting $B=\operatorname{\mathcal{D}}$, we deduce that composition with $F$ induces an equivalence of $\infty $-categories $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{D}},\operatorname{\mathcal{C}}) \rightarrow \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{D}},\operatorname{\mathcal{D}})$. In particular, there exists a morphism $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ in $(\operatorname{Set_{\Delta }})_{/\operatorname{\mathcal{E}}}$ such that $F \circ G$ is isomorphic to $\operatorname{id}_{\operatorname{\mathcal{D}}}$ as an object of the $\infty $-category $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{D}},\operatorname{\mathcal{D}})$. It follows that $F \circ G \circ F$ is isomorphic to $F$ as an object of the $\infty $-category $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}},\operatorname{\mathcal{D}})$. Applying condition $(2)$ in the case $B = \operatorname{\mathcal{C}}$, we see that postcomposition with $F$ induces an equivalence of $\infty $-categories $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}},\operatorname{\mathcal{C}}) \rightarrow \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}},\operatorname{\mathcal{D}})$, so that $G \circ F$ is isomorphic to $\operatorname{id}_{\operatorname{\mathcal{C}}}$ as an object of $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{C}})$. It follows that $G$ is a homotopy inverse of $F$ relative to $\operatorname{\mathcal{E}}$. In particular, $F$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$.

The implication $(3) \Rightarrow (4)$ follows by combining Remark 5.6.2.4 with Corollary 5.6.2.7. We will complete the proof by showing that $(4)$ implies $(1)$. Assume that condition $(4)$ is satisfied, and let $B$ be a simplicial set equipped with a morphism $B \rightarrow \operatorname{\mathcal{E}}$. We wish to show that composition with $F$ induces a homotopy equivalence of Kan complexes $\theta _{B}: \operatorname{Fun}_{/\operatorname{\mathcal{E}}}( B, \operatorname{\mathcal{C}})^{\simeq } \rightarrow \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B,\operatorname{\mathcal{D}})^{\simeq }$. Assume first that the simplicial set $B$ has dimension $\leq n$, for some integer $n \geq -1$. Our proof proceeds by induction on $n$. If $n=-1$, then $B$ is empty and there is nothing to prove. We may therefore assume without loss of generality that $n \geq 0$. Let $A$ be the $(n-1)$-skeleton of $B$. Our inductive hypothesis guarantees that $\theta _{A}$ is a homotopy equivalence. By virtue of Proposition 3.2.7.1, it will suffice to verify the following:

$(\ast )$

The restriction maps

\[ \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B,\operatorname{\mathcal{C}}) \rightarrow \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(A,\operatorname{\mathcal{C}}) \quad \quad \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B,\operatorname{\mathcal{D}}) \rightarrow \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(A,\operatorname{\mathcal{D}}) \]

are isofibrations of $\infty $-categories, and therefore induce Kan fibrations

\[ \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B,\operatorname{\mathcal{C}})^{\simeq } \rightarrow \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(A,\operatorname{\mathcal{C}})^{\simeq } \quad \quad \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B,\operatorname{\mathcal{D}})^{\simeq } \rightarrow \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(A,\operatorname{\mathcal{D}})^{\simeq }; \]

see Proposition 4.4.3.7.

$(\ast ')$

For every object $T \in \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(A,\operatorname{\mathcal{C}})$, the induced map of fibers

\[ \{ T\} \times _{ \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(A,\operatorname{\mathcal{C}})} \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B,\operatorname{\mathcal{C}}) \rightarrow \{ F \circ T\} \times _{ \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(A,\operatorname{\mathcal{D}})} \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B,\operatorname{\mathcal{D}}) \]

is an equivalence of $\infty $-categories, and therefore induces a homotopy equivalence of Kan complexes

\[ \{ T\} \times _{ \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(A,\operatorname{\mathcal{C}})^{\simeq }} \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B,\operatorname{\mathcal{C}})^{\simeq } \rightarrow \{ F \circ T\} \times _{ \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(A,\operatorname{\mathcal{D}})^{\simeq }} \operatorname{Fun}_{/\operatorname{\mathcal{E}}}(B,\operatorname{\mathcal{D}})^{\simeq } \]

(see Remark 4.5.1.18).

Let $J$ denote the set of all nondegenerate $n$-simplices of $B$. Proposition 1.1.3.13 supplies a pushout diagram of simplicial sets

\[ \xymatrix@R =50pt@C=50pt{ \coprod _{\sigma \in J} \operatorname{\partial \Delta }^ n \ar [r] \ar [d] & \coprod _{\sigma \in J} \Delta ^ n \ar [d] \\ A \ar [r] & B. } \]

Consequently, to verify $(\ast )$ and $(\ast ')$, we can assume without loss of generality that $B = \Delta ^ n$ is a standard simplex and that $A = \operatorname{\partial \Delta }^ n$ is its boundary. Replacing $\operatorname{\mathcal{C}}$ and $\operatorname{\mathcal{D}}$ by the fiber products $\Delta ^ n \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{C}}$ and $\Delta ^ n \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{D}}$, we can reduce further to the case where $\operatorname{\mathcal{E}}= \Delta ^ n$ is a standard simplex. Applying Example 4.4.1.6, we deduce that $U$ and $V$ are isofibrations, so that assertion $(\ast )$ follows from Proposition 4.4.5.1. Invoking assumption $(4)$, we deduce that $F$ is an equivalence of $\infty $-categories, and therefore induces equivalences

\[ \operatorname{Fun}(A, \operatorname{\mathcal{C}}) \rightarrow \operatorname{Fun}(A, \operatorname{\mathcal{D}}) \quad \quad \operatorname{Fun}(B, \operatorname{\mathcal{C}}) \rightarrow \operatorname{Fun}(B, \operatorname{\mathcal{D}}). \]

Assertion $(\ast ')$ now follows from Corollary 4.5.4.6.

We now treat the case where $B$ is a general simplicial set. For each $n \geq 0$, let $\operatorname{sk}_{n}(B)$ denote the $n$-skeleton of $B$ (Construction 1.1.3.5). Using $(\ast )$ and Corollary 4.4.3.18, we see that $\theta _{B}$ can be realized as the inverse limit of a tower

\[ \xymatrix@R =50pt@C=25pt{ \cdots \ar [r] & \operatorname{Fun}_{/S}( \operatorname{sk}_2(B), X)^{\simeq } \ar [d]^{ \theta _{\operatorname{sk}_2(B)}} \ar [r] & \operatorname{Fun}_{/S}( \operatorname{sk}_1(B), X)^{\simeq } \ar [d]^{ \theta _{\operatorname{sk}_1(B)} } \ar [r] & \operatorname{Fun}_{/S}( \operatorname{sk}_0(B), X)^{\simeq } \ar [d]^{ \theta _{\operatorname{sk}_0(B)} } \\ \cdots \ar [r] & \operatorname{Fun}_{/S}( \operatorname{sk}_2(B), X')^{\simeq } \ar [r] & \operatorname{Fun}_{/S}( \operatorname{sk}_1(B), X')^{\simeq } \ar [r] & \operatorname{Fun}_{/S}( \operatorname{sk}_0(B), X')^{\simeq }, } \]

where each of the transition maps is a Kan fibration. The preceding arguments show that each of the vertical maps $\theta _{ \operatorname{sk}_ n(B) }$ is a homotopy equivalence of Kan complexes. Applying Proposition 3.3.9.6, we conclude that $\theta _{B}$ is a homotopy equivalence of Kan complexes. $\square$

We now study properties of inner fibrations that are invariant under equivalence.

Lemma 5.6.2.9. Let $U: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$ be an isofibration of simplicial sets and $F: \operatorname{\mathcal{C}}\hookrightarrow \operatorname{\mathcal{D}}$ be a monomorphism of simplicial sets. The following conditions are equivalent:

$(1)$

The restriction $(U \circ F): \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}$ is an inner fibration and $F$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$.

$(2)$

There exists a morphism $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ in $(\operatorname{Set_{\Delta }})_{/\operatorname{\mathcal{E}}}$ satisfying $G \circ F = \operatorname{id}_{\operatorname{\mathcal{C}}}$ and an isomorphism $u: \operatorname{id}_{\operatorname{\mathcal{D}}} \rightarrow F \circ G$ in the $\infty $-category $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{D}})$ whose image in $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})$ is the identity morphism $\operatorname{id}_{F}: F \rightarrow F \circ G \circ F = F$.

Proof. We first show that $(2)$ implies $(1)$. Suppose that there exists a morphism $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ satisfying $G \circ F = \operatorname{id}_{\operatorname{\mathcal{C}}}$. Then $F$ and $G$ exhibit $\operatorname{\mathcal{C}}$ as a retract of $\operatorname{\mathcal{D}}$ in the category $(\operatorname{Set_{\Delta }})_{/\operatorname{\mathcal{E}}}$. Since $U: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$ is an isofibration, it follows that $(U \circ F): \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}$ is an isofibration (Remark 4.5.7.6). In particular, $U \circ F$ is an inner fibration (Remark 4.5.7.3). If there exists an isomorphism $u: \operatorname{id}_{\operatorname{\mathcal{D}}} \rightarrow F \circ G$ in the $\infty $-category $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{D}})$, then $G$ is a homotopy inverse of $F$ relative to $\operatorname{\mathcal{E}}$, so that $F$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$.

We now show that $(1)$ implies $(2)$. Assume that $U \circ F$ is an inner fibration and that $F$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$. Let $G': \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ be a homotopy inverse of $F$ relative to $\operatorname{\mathcal{E}}$, so that there exists an isomorphism $e: \operatorname{id}_{\operatorname{\mathcal{C}}} \rightarrow G' \circ F$ in the $\infty $-category $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{C}})$. Applying Proposition 4.4.5.6, we can lift $e$ to an isomorphism $\widetilde{e}: G \rightarrow G'$ in the $\infty $-category $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{C}})$, where $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{C}}$ satisfies $G \circ F = \operatorname{id}_{\operatorname{\mathcal{C}}}$. Note that $F$ is a categorical equivalence of simplicial sets (Proposition 5.6.2.5), and therefore induces a categorical equivalence

\[ (\Delta ^1 \times \operatorname{\mathcal{C}}) \coprod _{ (\operatorname{\partial \Delta }^1 \times \operatorname{\mathcal{C}})} (\operatorname{\partial \Delta }^1 \times \operatorname{\mathcal{D}}) \hookrightarrow \Delta ^1 \times \operatorname{\mathcal{D}}. \]

Since $U$ is an isofibration, every lifting problem

\[ \xymatrix@R =50pt@C=50pt{ (\Delta ^1 \times \operatorname{\mathcal{C}}) \coprod _{ (\operatorname{\partial \Delta }^1 \times \operatorname{\mathcal{C}})} (\operatorname{\partial \Delta }^1 \times \operatorname{\mathcal{D}}) \ar [r] \ar [d] & \operatorname{\mathcal{D}}\ar [d]^{U} \\ \Delta ^1 \times \operatorname{\mathcal{D}}\ar [r] \ar@ {-->}[ur] & \operatorname{\mathcal{E}}} \]

admits a solution. In particular, there exists a morphism $u: \operatorname{id}_{\operatorname{\mathcal{D}}} \rightarrow F \circ G$ in the $\infty $-category $\operatorname{Fun}_{ / \operatorname{\mathcal{E}}}( \operatorname{\mathcal{D}}, \operatorname{\mathcal{D}})$ whose image in $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})$ is the identity map $\operatorname{id}_{F}$. We will complete the proof by showing that $u$ is an isomorphism in the $\infty $-category $\operatorname{Fun}_{/\operatorname{\mathcal{E}}}(\operatorname{\mathcal{D}}, \operatorname{\mathcal{D}})$. Using the criterion of Proposition 4.4.4.9, we are reduced to checking that, for each vertex $D \in \operatorname{\mathcal{D}}$ having image $E = U(D) \in \operatorname{\mathcal{E}}$, the induced map $u_{D}: D \rightarrow (F \circ G)(D)$ is an isomorphism in the $\infty $-category $\operatorname{\mathcal{D}}_{E} = \{ E\} \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{D}}$. This is clear, since $D$ is isomorphic (in the $\infty $-category $\operatorname{\mathcal{D}}_{E}$) to an object of the form $F(C)$ for $C \in \operatorname{\mathcal{C}}_{E}$, and the morphism $u_{F(C)}$ is equal to the identity $\operatorname{id}_{F(C)}$. $\square$

Proposition 5.6.2.10. Let $U: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{E}}$ and $V: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{E}}$ be inner fibrations of simplicial sets which are equivalent to one another. Then:

$(1)$

The morphism $U$ is an isofibration if and only if $V$ is an isofibration.

$(2)$

The morphism $U$ is a cartesian fibration if and only if $V$ is a cartesian fibration.

$(3)$

The morphism $U$ is a right fibration if and only if $V$ is a right fibration.

$(4)$

The morphism $U$ is a cocartesian fibration if and only if $V$ is a cocartesian fibration.

$(5)$

The morphism $U$ is a left fibration if and only if $V$ is a left fibration.

$(6)$

The morphism $U$ is a Kan fibration if and only if $V$ is a Kan fibration.

Proof. Let $F: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$. We first prove $(1)$. Assume that $V$ is an isofibration; we will show that $U$ is also an isofibration. Choose a monomorphism of simplicial sets $\operatorname{\mathcal{C}}\hookrightarrow \operatorname{\mathcal{Q}}$, where $\operatorname{\mathcal{Q}}$ is a contractible Kan complex (Exercise 3.1.7.11). Replacing $\operatorname{\mathcal{D}}$ by the product $\operatorname{\mathcal{D}}\times \operatorname{\mathcal{Q}}$, we can assume that $F$ is a monomorphism of simplicial sets. In this case, Lemma 5.6.2.9 guarantees that $F$ exhibits $\operatorname{\mathcal{C}}$ as a retract of $\operatorname{\mathcal{D}}$ in the category $(\operatorname{Set_{\Delta }})_{/\operatorname{\mathcal{E}}}$, so that $U$ is an isofibration by virtue of Remark 4.5.7.6.

To prove $(2)$, we may assume without loss of generality that $\operatorname{\mathcal{E}}= \Delta ^ n$ is a standard simplex (Proposition 5.1.4.7). In this case, $U$ and $V$ are isofibrations (Example 4.4.1.6) and $F$ is an equivalence of $\infty $-categories (Corollary 5.6.2.7). It follows from Corollary 5.1.5.2 that $U$ is a cartesian fibration if and only if $V$ is a cartesian fibration.

To prove $(3)$, suppose that $U$ is a right fibration; we will show that $V$ is a right fibration. It follows from $(2)$ that $V$ is a cartesian fibration. It will therefore suffice to show that, for each vertex $E \in \operatorname{\mathcal{E}}$, the $\infty $-category $\{ E \} \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{D}}$ is a Kan complex (Proposition 5.1.4.14). By virtue of Remark 5.6.2.4, the morphism $F$ induces an equivalence of $\infty $-categories $F_{E}: \{ E\} \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{C}}\rightarrow \{ E\} \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{D}}$. It will therefore suffice to show that $\{ E \} \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{C}}$ is a Kan complex (Remark 4.5.1.20), which follows from our assumption that $U$ is a right fibration.

Assertions $(4)$ and $(5)$ follow by similar arguments. Assertion $(6)$ follows by combining $(3)$ and $(5)$ (see Example 4.2.1.5). $\square$

Proposition 5.6.2.11. Suppose we are given a commutative diagram of simplicial sets

\[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}\ar [rr]^{F} \ar [dr]_{U} & & \operatorname{\mathcal{D}}\ar [dl]^{V} \\ & \operatorname{\mathcal{E}}, & } \]

where $U$ and $V$ are cartesian fibrations. Then $F$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$ if and only if the following conditions are satisfied:

$(1)$

For every vertex $E \in \operatorname{\mathcal{E}}$, the induced map $F_{E}: \{ E\} \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{C}}\rightarrow \{ E \} \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{D}}$ is an equivalence of $\infty $-categories.

$(2)$

The morphism $F$ carries $U$-cartesian edges of $\operatorname{\mathcal{C}}$ to $V$-cartesian edges of $\operatorname{\mathcal{D}}$.

Proof. By virtue of Proposition 5.6.2.8, we may assume without loss of generality that $\operatorname{\mathcal{E}}= \Delta ^ n$ is a standard simplex, so that $F$ is an equivalence of inner fibrations over $\operatorname{\mathcal{E}}$ if and only if it is an equivalence of $\infty $-categories (Corollary 5.6.2.7). Since $U$ and $V$ are isofibrations (Example 4.4.1.6), the desired result follows from Theorem 5.1.5.1. $\square$

Corollary 5.6.2.12. Suppose we are given a commutative diagram of simplicial sets

\[ \xymatrix@R =50pt@C=50pt{ \operatorname{\mathcal{C}}\ar [rr]^{F} \ar [dr]_{U} & & \operatorname{\mathcal{D}}\ar [dl]^{V} \\ & \operatorname{\mathcal{E}}, & } \]

where $U$ and $V$ are right fibrations. Then $F$ is an equivalence of inner fibrations if and only if, for every vertex $E \in \operatorname{\mathcal{E}}$, the induced map $F_{E}: \{ E\} \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{C}}\rightarrow \{ E \} \times _{\operatorname{\mathcal{E}}} \operatorname{\mathcal{D}}$ is a homotopy equivalence of Kan complexes.