### 3.3.7 Application: Characterizations of Weak Homotopy Equivalences

Let $f: X \rightarrow S$ be a Kan fibration between Kan complexes. In §3.2.6, we proved that $f$ is a homotopy equivalence if and only if it is a trivial Kan fibration (Corollary 3.2.6.9). We now apply the machinery of §3.3.6 to extend this result to the case where $S$ is an arbitrary simplicial set. First, we establish a generalization of Proposition 3.2.7.1:

Proposition 3.3.7.1. Suppose we are given a commutative diagram of simplicial sets

\[ \xymatrix { X \ar [r]^-{f} \ar [d] & X' \ar [d] \\ S \ar [r]^-{g} & S', } \]

where the vertical maps are Kan fibrations and $g$ is a weak homotopy equivalence. The following conditions are equivalent:

- $(1)$
The morphism $f$ is a weak homotopy equivalence.

- $(2)$
For every vertex $s \in S$, the induced map of fibers $f_{s}: X_{s} \rightarrow X'_{ g(s)}$ is a homotopy equivalence of Kan complexes.

**Proof.**
Using Corollaries 3.3.6.8 and 3.3.6.5, we can replace $(1)$ and $(2)$ by the following assertions:

- $(1')$
The morphism $\operatorname{Ex}^{\infty }(f): \operatorname{Ex}^{\infty }(X) \rightarrow \operatorname{Ex}^{\infty }(X')$ is a weak homotopy equivalence.

- $(2')$
For every vertex $s \in S$, the induced map of fibers $f_{s}: \operatorname{Ex}^{\infty }(X)_{s} \rightarrow \operatorname{Ex}^{\infty }(X')_{g(s)}$ is a homotopy equivalence of Kan complexes.

The equivalence of $(1')$ and $(2')$ follows by applying Proposition 3.2.7.1 to the diagram

\[ \xymatrix { \operatorname{Ex}^{\infty }(X) \ar [r]^-{\operatorname{Ex}^{\infty }(f) } \ar [d] & \operatorname{Ex}^{\infty }(X') \ar [d] \\ \operatorname{Ex}^{\infty }(S) \ar [r]^-{\operatorname{Ex}^{\infty }(g)} & \operatorname{Ex}^{\infty }(S'); } \]

note that every simplicial set appearing in this diagram is a Kan complex (Proposition 3.3.6.9), the vertical maps are Kan fibrations (Proposition 3.3.6.6) and $\operatorname{Ex}^{\infty }(g)$ is a homotopy equivalence by virtue of Corollary 3.3.6.8.
$\square$

Proposition 3.3.7.2. Let $f: X \rightarrow S$ be a Kan fibration of simplicial sets. The following conditions are equivalent:

- $(1)$
For every vertex $s \in S$, the fiber $X_{s} = \{ s\} \times _{S} X$ is a contractible Kan complex.

- $(2)$
The morphism $f$ is a trivial Kan fibration.

- $(3)$
The morphism $f$ is a homotopy equivalence.

- $(4)$
The morphism $f$ is a weak homotopy equivalence.

**Proof.**
The equivalence $(1) \Leftrightarrow (2)$ follows from Proposition 3.2.6.8 and the equivalence $(1) \Leftrightarrow (4)$ follows from Proposition 3.3.7.1 (by taking $X' = S = S'$). The implication $(2) \Rightarrow (3)$ follows from Proposition 3.1.5.9, and the implication $(3) \Rightarrow (4)$ from Proposition 3.1.5.11,
$\square$

Corollary 3.3.7.3. Let $f: X \rightarrow Y$ be a morphism of simplicial sets. The following conditions are equivalent:

- $(1)$
The morphism $f$ is anodyne.

- $(2)$
The morphism $f$ is both a monomorphism and a weak homotopy equivalence.

**Proof.**
The implication $(1) \Rightarrow (2)$ follows from Proposition 3.1.6.7 and Remark 3.1.6.3. To prove the converse, assume that $f$ is a weak homotopy equivalence and apply Proposition 3.1.7.1 to write $f$ as a composition $X \xrightarrow {f'} Q \xrightarrow {f''} Y$, where $f'$ is anodyne and $f''$ is a Kan fibration. Then $f'$ is a weak homotopy equivalence (Proposition 3.1.6.7), so $f''$ is a weak homotopy equivalence (Remark 3.1.5.13). Invoking Proposition 3.3.7.2, we conclude that $f''$ is a trivial Kan fibration. If $f$ is a monomorphism, then the lifting problem

\[ \xymatrix { X \ar [d]^{f} \ar [r]^-{ f' } & Q \ar [d]^{f''} \\ Y \ar [r]^-{\operatorname{id}_ Y} \ar@ {-->}[ur] & Y } \]

admits a solution. It follows that $f$ is a retract of $f'$ (in the arrow category $\operatorname{Fun}( [1], \operatorname{Set_{\Delta }})$). Since the collection of anodyne morphisms is closed under retracts, we conclude that $f$ is anodyne.
$\square$