# Kerodon

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### 9.10.7 Relative Homotopy Equivalences

Recall that a morphism of simplicial sets $f: X \rightarrow Y$ is said to be a homotopy equivalence if there exists a morphism $g: Y \rightarrow X$ such that $f \circ g$ and $g \circ f$ are homotopic to the identity morphisms $\operatorname{id}_{Y}$ and $\operatorname{id}_{X}$, respectively (Definition 3.1.6.1). We now introduce a relative version of this condition.

Definition 9.10.7.1. Suppose we are given a commutative diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ X \ar [rr]^{f} \ar [dr]_{q_ X} & & Y \ar [dl]^{q_ Y} \\ & S, & }$

so that we can regard $f$ as a vertex of the simplicial set $\operatorname{Fun}_{/S}(X,Y)$ of Construction 3.1.3.7. We say that $f$ is a homotopy equivalence relative to $S$ if there exists a morphism $g \in \operatorname{Fun}_{/S}(Y,X)$ such that $g \circ f$ and $\operatorname{id}_{X}$ belong to the same connected component of $\operatorname{Fun}_{/S}(X,X)$, and $f \circ g$ and $\operatorname{id}_{Y}$ belong to the same connected component of $\operatorname{Fun}_{/S}(Y,Y)$. In this case, we will refer to $g$ as a homotopy inverse of $f$ relative to $S$.

Remark 9.10.7.2. Suppose we are given a commutative diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ X \ar [rr]^{f} \ar [dr]_{q_ X} & & Y \ar [dl]^{q_ Y} \\ & S. & }$

The following conditions are equivalent:

• The morphism $f$ is a homotopy equivalence relative to $S$ (Definition 9.10.7.1).

• For every object $B$ of the slice category $(\operatorname{Set_{\Delta }})_{/S}$, postcomposition with $f$ induces a homotopy equivalence of simplicial sets $\operatorname{Fun}_{/S}(B,X) \rightarrow \operatorname{Fun}_{/S}(B,Y)$.

• For every object $B$ of the slice category $(\operatorname{Set_{\Delta }})_{/S}$, postcomposition with $f$ induces a bijection $\pi _0( \operatorname{Fun}_{/S}(B,X) ) \rightarrow \pi _0( \operatorname{Fun}_{/S}(B,Y) )$.

• For every object $Z$ of the slice category $(\operatorname{Set_{\Delta }})_{/S}$, precomposition with $f$ induces a homotopy equivalence of simplicial sets $\operatorname{Fun}_{/S}(Y,Z) \rightarrow \operatorname{Fun}_{/S}(X,Z)$.

• For every object $Z$ of the slice category $(\operatorname{Set_{\Delta }})_{/S}$, precomposition with $f$ induces a bijection $\pi _0( \operatorname{Fun}_{/S}(Y,Z) )\rightarrow \pi _0( \operatorname{Fun}_{/S}(X,Z) )$.

In the special case where $q_{X}$ and $q_{Y}$ are both left fibrations (or both right fibrations), then we can improve upon Remark 9.10.7.2:

Theorem 9.10.7.3. Suppose we are given a commutative diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ X \ar [rr]^{f} \ar [dr]_{q_ X} & & Y \ar [dl]^{q_ Y} \\ & S. & }$

Assume either that $q_{X}$ and $q_{Y}$ are both left fibrations, or that they are both right fibrations. Then the following conditions are equivalent:

$(1)$

The morphism $f$ is a homotopy equivalence relative to $S$ (Definition 9.10.7.1).

$(2)$

For every object $B$ of the slice category $(\operatorname{Set_{\Delta }})_{/S}$, the induced map $f_{B}: \operatorname{Fun}_{/S}(B,X) \rightarrow \operatorname{Fun}_{/S}(B,Y)$ is a homotopy equivalence.

$(3)$

For each vertex $s \in S$, the induced map $f_{s}: X_{s} \rightarrow Y_{s}$ is a homotopy equivalence of Kan complexes.

The proof of Theorem 9.10.7.3 will require some preliminaries.

Proposition 9.10.7.4. Let $q_{X}: X \rightarrow S$ and $q_{Y}: Y \rightarrow S$ be morphisms of simplicial sets. If $q_{Y}$ is either a left fibration or a right fibration, then the simplicial set $\operatorname{Fun}_{/S}(X,Y)$ is a Kan complex.

Proof. Without loss of generality, we may assume that $q_{Y}$ is a left fibration. Then postcomposition with $q_{Y}$ induces a left fibration $Q_{Y}: \operatorname{Fun}(X,Y) \rightarrow \operatorname{Fun}(X,S)$ (Corollary 4.2.5.2). By construction, $\operatorname{Fun}_{/S}(X,Y)$ is a fiber of the morphism $Q_{Y}$, and is therefore a Kan complex by virtue of Corollary 4.4.2.3. $\square$

Proposition 9.10.7.5. Let $q_{X}: X \rightarrow S$ and $q_{Y}: Y \rightarrow S$ be morphisms of simplicial sets, and let $X' \subseteq X$ be a simplicial subset. If $q_{Y}$ is either a left fibration or a right fibration, then the restriction map $\theta : \operatorname{Fun}_{/S}(X, Y) \rightarrow \operatorname{Fun}_{/S}(X', Y)$ is a Kan fibration.

Proof. Without loss of generality, we may assume that $q_{Y}$ is a left fibration. By construction, we have a pullback diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ \operatorname{Fun}_{/S}(X,Y) \ar [r] \ar [d]^{\theta } & \operatorname{Fun}(X,Y) \ar [d]^{\theta '} \\ \operatorname{Fun}_{/S}(X', Y) \ar [r] & \operatorname{Fun}(X',Y) \times _{ \operatorname{Fun}(X',S)} \operatorname{Fun}(X,S). }$

Proposition 4.2.5.1 guarantees that $\theta '$ is a left fibration, so that $\theta$ is also a left fibration (Remark 4.2.1.8). Since $\operatorname{Fun}_{/S}(X',Y)$ is a Kan complex (Proposition 9.10.7.4), it follows that $\theta$ is also a Kan fibration (Corollary 4.4.3.8). $\square$

Remark 9.10.7.6. In the situation of Proposition 9.10.7.5, suppose that $q_{Y}$ is a left fibration and the inclusion $X' \hookrightarrow X$ is left anodyne. Then the restriction map $\theta : \operatorname{Fun}_{/S}(X, Y) \rightarrow \operatorname{Fun}_{/S}(X', Y)$ is a trivial Kan fibration (this follows immediately from Proposition 4.2.5.4). Similarly, if $q_{Y}$ is a right fibration and the inclusion $X' \hookrightarrow X$ is right anodyne, then $\theta$ is a trivial Kan fibration.

Remark 9.10.7.7. Let $q: Y \rightarrow S$ be a morphism of simplicial sets. Then the construction $X \mapsto \operatorname{Fun}_{/S}(X,Y)$ carries colimits in the slice category $( \operatorname{Set_{\Delta }})_{/S}$ to limits in the category of simplicial sets. In particular, $\operatorname{Fun}_{/S}( \emptyset , Y)$ can be identified with the $0$-simplex $\Delta ^{0}$.

Corollary 9.10.7.8. Let $q: Y \rightarrow S$ be a morphism of simplicial sets, and suppose we are given a pushout diagram

$\xymatrix@R =50pt@C=50pt{ A \ar [r]^-{i} \ar [d] & A' \ar [d] \\ B \ar [r] & B' }$

in the slice category $(\operatorname{Set_{\Delta }})_{/S}$. If $i$ is a monomorphism and $q$ is either a left or right fibration, then the induced diagram

9.12
$$\begin{gathered}\label{equation:mapping-space-homotopy-pullback} \xymatrix@R =50pt@C=50pt{ \operatorname{Fun}_{/S}(A', Y) & \operatorname{Fun}_{/S}(A, Y) \ar [l] \\ \operatorname{Fun}_{/S}(B',Y) \ar [u] & \operatorname{Fun}_{/S}(B, Y) \ar [u] \ar [l] } \end{gathered}$$

is a homotopy pullback square of Kan complexes.

Proof. Diagram (9.12) is automatically a pullback square (Remark 9.10.7.7). If $q$ is a right fibration and $i$ is a monomorphism, then the horizontal maps in (9.12) are Kan fibrations (Proposition 9.10.7.5). Invoking Example 3.4.1.3, we deduce that (9.12) is also a homotopy pullback square. $\square$

Proof of Theorem 9.10.7.3. The equivalence $(1) \Leftrightarrow (2)$ follows from Remark 9.10.7.2, and the implication $(2) \Rightarrow (3)$ is immediate (Example 3.1.3.12). We will complete the proof by showing that $(3) \Rightarrow (2)$. Assume that we are given a commutative diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ X \ar [rr]^{f} \ar [dr]_{q_ X} & & Y \ar [dl]^{q_ Y} \\ & S, & }$

where $q_{X}$ and $q_{Y}$ are left fibrations and, for every vertex $s \in S$, the induced map of fibers $f_{s}: X_{s} \rightarrow Y_{s}$ is a homotopy equivalence of Kan complexes. We wish to show that, for each object $B \in (\operatorname{Set_{\Delta }})_{/S}$, the induced map $f_{B}: \operatorname{Fun}_{/S}(B,X) \rightarrow \operatorname{Fun}_{/S}(B,Y)$ is a homotopy equivalence. For each $n \geq 0$, let $\operatorname{sk}_{n}(B)$ denote the $n$-skeleton of $B$ (Construction 1.1.3.5). Then $f_{A}$ can be realized as the inverse limit of a tower of morphisms $\{ f_{ \operatorname{sk}_ n(B) }: \operatorname{Fun}_{/S}( \operatorname{sk}_ n(B), X) \rightarrow \operatorname{Fun}_{/S}( \operatorname{sk}_ n(B), Y) \} _{n \geq 0}$. Using Proposition 9.10.7.5, we see that the transition morphisms $\operatorname{Fun}_{/S}( \operatorname{sk}_{n+1}(B), X) \rightarrow \operatorname{Fun}_{/S}( \operatorname{sk}_ n(B), X)$ and $\operatorname{Fun}_{/S}( \operatorname{sk}_{n+1}(B), Y) \rightarrow \operatorname{Fun}_{/S}( \operatorname{sk}_ n(B), Y)$ are Kan fibrations. Consequently, to show that $f_{B}$ is a homotopy equivalence, it will suffice to show that each of the morphisms $f_{ \operatorname{sk}_{n}(B) }$ is a homotopy equivalence (Proposition 9.9.1.6). We may therefore replace $B$ by $\operatorname{sk}_{n}(B)$ and thereby reduce to the case where the simplicial set $B$ has dimension $\leq n$, for some $n \geq -1$.

The proof now proceeds by induction on $n$. In the case $n=-1$, the simplicial set $B$ is empty, and the morphism $f_{B}$ is an isomorphism (see Remark 9.10.7.7). Assume that $n \geq 0$, let $B'$ denote the $(n-1)$-skeleton of $B$, and let $T$ denote the collection of all nondegenerate $n$-simplices of $B$. Then Proposition 1.1.3.13 supplies a pushout diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ A' \ar [r] \ar [d] & A \ar [d] \\ B' \ar [r] & B, }$

where $A = \coprod _{ \tau \in T} \Delta ^{n}$ and $A' = \coprod _{\tau \in T} \operatorname{\partial \Delta }^ n$. This pushout square determines a commutative diagram of Kan complexes

$\xymatrix@R =50pt@C=50pt{ \operatorname{Fun}_{/S}(B, X) \ar [dd]^(.5){ f_{B} } \ar [rr] \ar [dr] & & \operatorname{Fun}_{/S}( B', X) \ar [dd]^(.6){ f_{B'} } \ar [dr] & \\ & \operatorname{Fun}_{/S}(A, X) \ar [rr] \ar [dd]^(.6){ f_{A} } & & \operatorname{Fun}_{/S}(A', X) \ar [dd]^(.5){ f_{A'} } \\ \operatorname{Fun}_{/S}(B, Y) \ar [rr] \ar [dr] & & \operatorname{Fun}_{/S}(B', Y) \ar [dr] & \\ & \operatorname{Fun}_{/S}(A, Y) \ar [rr] & & \operatorname{Fun}_{/S}(A', Y), }$

where the upper and lower squares are homotopy Cartesian by virtue of Corollary 9.10.7.8. Consequently, to show that $f_{B}$ is a homotopy equivalence of Kan complexes, it will suffice to show that $f_{B'}$, $f_{A'}$, and $f_{A}$ are homotopy equivalences (Corollary 3.4.1.12). In the first two cases, this follows from our inductive hypothesis. We may therefore replace $B$ by $A$, and thereby reduce to the case where is a coproduct of simplices. Using Remarks 9.10.7.7 and 3.1.6.8, we can further reduce to the case where $B = \Delta ^ n$ is a simplex of dimension $n$. In this case, the inclusion of the initial vertex $0 \in \Delta ^ n$ is left anodyne (Example 4.3.7.10). Let $s \in S$ denote the image of the vertex $0 \in \Delta ^ n$, so that we have a commutative diagram of Kan complexes

$\xymatrix@R =50pt@C=50pt{ \operatorname{Fun}_{/S}( B, X) \ar [r]^-{ f_{B} } \ar [d] & \operatorname{Fun}_{/S}(B, Y) \ar [d] \\ \operatorname{Fun}_{/S}( \{ s\} , X) \ar [r]^-{f_ s} & \operatorname{Fun}_{/S}( \{ s\} , Y). }$

Invoking Remark 9.10.7.6, we conclude that the vertical maps in this diagram are trivial Kan fibrations. Since $f_{s}$ is a homotopy equivalence by assumption, it follows that $f_{B}$ is also a homotopy equivalence. $\square$