### 1.5.4 Digression: Lifting Properties

We now review some categorical terminology which will be useful in the proof of Theorem 1.5.3.7, and in several other parts of this book.

Definition 1.5.4.1. Let $\operatorname{\mathcal{C}}$ be a category. A *lifting problem* in $\operatorname{\mathcal{C}}$ is a commutative diagram $\sigma :$

\[ \xymatrix@C =40pt@R=40pt{ A \ar [d]^{f} \ar [r]^-{u} & X \ar [d]^{g} \\ B \ar [r]^-{v} & Y } \]

in $\operatorname{\mathcal{C}}$. A *solution to the lifting problem $\sigma $* is a morphism $h: B \rightarrow X$ in $\operatorname{\mathcal{C}}$ satisfying $g \circ h = v$ and $h \circ f = u$, as indicated in the diagram

\[ \xymatrix@C =40pt@R=40pt{ A \ar [d]^{f} \ar [r]^-{u} & X \ar [d]^{g} \\ B \ar [r]^-{v} \ar [ur]^{h} & Y. } \]

Definition 1.5.4.3. Let $\operatorname{\mathcal{C}}$ be a category and suppose we are given a morphism $f: A \rightarrow B$ and $g: X \rightarrow Y$ in $\operatorname{\mathcal{C}}$. We will say that *$f$ is weakly left orthogonal to $g$* if, for every pair of morphisms $u: A \rightarrow X$ and $v: B \rightarrow Y$ satisfying $g \circ u= v \circ f$, the associated lifting problem

\[ \xymatrix@C =40pt@R=40pt{ A \ar [d]^{f} \ar [r]^-{u} & X \ar [d]^{g} \\ B \ar [r]^-{v} \ar@ {-->}[ur] & Y } \]

admits a solution (that is, there exists a map $h: B \rightarrow X$ satisfying $g \circ h = v$ and $h \circ f = u$). In this case, we will also say that *$g$ is weakly right orthogonal to $f$*.

If $S$ and $T$ are collections of morphisms of $\operatorname{\mathcal{C}}$, we say that *$S$ is weakly left orthogonal to $T$* if every morphism $f \in S$ is weakly left orthogonal to every morphism $g \in T$. In this case, we also say that *$T$ is weakly right orthogonal to $S$*. In the special case where $S = \{ f\} $ is a singleton, we abbreviate this condition by saying that *$f$ is weakly left orthogonal to $T$*, or *$T$ is weakly right orthogonal to $f$*. In the special case $T = \{ g\} $ is a singleton, we abbreviate this condition by saying that *$g$ is weakly right orthogonal to $S$*, or *$S$ is weakly left orthogonal to $g$*.

Let $T$ be a collection of morphisms in a category $\operatorname{\mathcal{C}}$. We now summarize some closure properties enjoyed by the collection of morphisms which are weakly left orthogonal to $T$.

Definition 1.5.4.4. Let $\operatorname{\mathcal{C}}$ be a category which admits pushouts and let $S$ be a collection of morphisms of $\operatorname{\mathcal{C}}$. We will say that $S$ is *closed under pushouts* if, for every pushout diagram

\[ \xymatrix@C =40pt@R=40pt{ A \ar [d]^{f} \ar [r] & A' \ar [d]^{f'} \\ B \ar [r] & B' } \]

in the category $\operatorname{\mathcal{C}}$ where the morphism $f$ belongs to $S$, the morphism $f'$ also belongs to $S$.

Proposition 1.5.4.5. Let $\operatorname{\mathcal{C}}$ be a category which admits pushouts, let $T$ be a collection of morphisms of $\operatorname{\mathcal{C}}$, and let $S$ be the collection of all morphisms of $\operatorname{\mathcal{C}}$ which are weakly left orthogonal to $T$. Then $S$ is closed under pushouts.

**Proof.**
Suppose we are given a pushout diagram $\sigma :$

\[ \xymatrix@C =40pt@R=40pt{ A \ar [d]^{f} \ar [r]^-{s} & A' \ar [d]^{f'} \\ B \ar [r]^-{t} & B' } \]

where $f$ belongs to $S$. We wish to show that $f'$ also belongs to $S$. For this, we must show that every lifting problem

\[ \xymatrix@C =40pt@R=40pt{ A' \ar [d]^{f'} \ar [r]^-{u} & X \ar [d]^{g} \\ B' \ar [r]^-{v} \ar@ {-->}[ur] & Y } \]

admits a solution, provided that the morphism $g$ belongs to $T$. Using our assumption that $\sigma $ is a pushout square, we are reduced to solving the associated lifting problem

\[ \xymatrix@C =40pt@R=40pt{ A \ar [d]^{f} \ar [r]^-{u \circ s} & X \ar [d]^{g} \\ B \ar [r]^-{v \circ t} \ar@ {-->}[ur] & Y, } \]

which is possible by virtue of our assumption that $f$ is weakly left orthogonal to $g$.
$\square$

Definition 1.5.4.6. Let $\operatorname{\mathcal{C}}$ be a category containing a pair of objects $C$ and $C'$. We will say that *$C$ is a retract of $C'$* if there exist maps $i: C \rightarrow C'$ and $r: C' \rightarrow C$ such that $r \circ i = \operatorname{id}_{C}$.

Variant 1.5.4.7. Let $\operatorname{\mathcal{C}}$ be a category. We will say that a morphism $f: C \rightarrow D$ of $\operatorname{\mathcal{C}}$ is a *retract* of another morphism $f': C' \rightarrow D'$ if it is a retract of $f'$ when viewed as an object of the functor category $\operatorname{Fun}( [1], \operatorname{\mathcal{C}})$. In other words, we say that $f$ is a retract of $f'$ if there exists a commutative diagram

\[ \xymatrix@C =40pt@R=40pt{ C \ar [r]^-{i} \ar [d]^{f} & C' \ar [d]^{f'} \ar [r]^-{r} & C \ar [d]^{f} \\ D \ar [r]^-{\overline{i}} & D' \ar [r]^-{\overline{r}} & D } \]

in the category $\operatorname{\mathcal{C}}$, where $r \circ i = \operatorname{id}_{C}$ and $\overline{r} \circ \overline{i} = \operatorname{id}_{D}$.

We say that a collection of morphisms $T$ of $\operatorname{\mathcal{C}}$ is *closed under retracts* if, for every pair of morphisms $f,f'$ in $\operatorname{\mathcal{C}}$, if $f$ is a retract of $f'$ and $f'$ belongs to $T$, then $f$ also belongs to $T$.

Exercise 1.5.4.8. Let $\operatorname{\mathcal{C}}$ be a category and let $S$ be the collection of all monomorphisms in $\operatorname{\mathcal{C}}$. Show that $S$ is closed under retracts.

Proposition 1.5.4.9. Let $\operatorname{\mathcal{C}}$ be a category, let $T$ be a collection of morphisms of $\operatorname{\mathcal{C}}$, and let $S$ be the collection of all morphisms of $\operatorname{\mathcal{C}}$ which are weakly left orthogonal to $S$. Then $S$ is closed under retracts.

**Proof.**
Let $f'$ be a morphism of $\operatorname{\mathcal{C}}$ which belongs to $S$ and let $f$ be a retract of $f'$, so that there exists a commutative diagram

\[ \xymatrix@C =40pt@R=40pt{ C \ar [r]^-{i} \ar [d]^{f} & C' \ar [d]^{f'} \ar [r]^-{r} & C \ar [d]^{f} \\ D \ar [r]^-{\overline{i}} & D' \ar [r]^-{\overline{r}} & D } \]

with $r \circ i = \operatorname{id}_{C}$ and $\overline{r} \circ \overline{i} = \operatorname{id}_{D}$. We wish to show that $f$ also belongs to $S$. Consider a lifting problem $\sigma :$

\[ \xymatrix@C =40pt@R=40pt{ C \ar [d]^{f} \ar [r]^-{u} & X \ar [d]^{g} \\ D \ar [r]^-{v} \ar@ {-->}[ur]^{h} & Y, } \]

where $g$ belongs to $T$. Our assumption $f' \in S$ ensures that the associated lifting problem

\[ \xymatrix@C =40pt@R=40pt{ C' \ar [d]^{f'} \ar [r]^-{u \circ r} & X \ar [d]^{g} \\ D' \ar [r]^-{v \circ \overline{r}} \ar@ {-->}[ur] & Y } \]

admits a solution: that is, we can choose a morphism $h': D' \rightarrow X$ satisfying $g \circ h' = v \circ \overline{r}$ and $h' \circ f' = u \circ r$. Then the morphism $h = h' \circ \overline{i}$ is a solution to the lifting problem $\sigma $, by virtue of the calculations

\[ g \circ h = g \circ h' \circ \overline{i} = v \circ \overline{r} \circ \overline{i} = v \]

\[ h \circ f = h' \circ \overline{i} \circ f = h' \circ f' \circ i = u \circ r \circ i = u. \]

$\square$
In what follows, we assume that the reader is familiar with the theory of ordinals (see ยง4.7.1 for a quick review).

Definition 1.5.4.10. For every ordinal $\alpha $, let $\mathrm{Ord}_{\leq \alpha } = \{ \beta : \beta \leq \alpha \} $ denote the collection of all ordinal numbers which are less than or equal to $\alpha $, regarded as a linearly ordered set.

Let $\operatorname{\mathcal{C}}$ be a category and let $S$ be a collection of morphisms of $\operatorname{\mathcal{C}}$. We will say that a morphism $f$ of $\operatorname{\mathcal{C}}$ is a *transfinite composition of morphisms of $S$* if there exists an ordinal $\alpha $ and a functor $F: \mathrm{Ord}_{\leq \alpha } \rightarrow \operatorname{\mathcal{C}}$, given by a collection of objects $\{ C_{\beta } \} _{ \beta \leq \alpha }$ and morphisms $\{ f_{\gamma , \beta }: C_{\beta } \rightarrow C_{\gamma } \} _{\beta \leq \gamma }$ with the following properties:

- $(a)$
For every nonzero limit ordinal $\lambda \leq \alpha $, the functor $F$ exhibits $C_{\lambda }$ as a colimit of the diagram $( \{ C_{\beta } \} _{\beta < \lambda }, \{ f_{\gamma , \beta } \} _{\beta \leq \gamma < \lambda } )$.

- $(b)$
For every ordinal $\beta < \alpha $, the morphism $f_{\beta +1, \beta }$ belongs to $S$.

- $(c)$
The morphism $f$ is equal to $f_{\alpha ,0}: C_0 \rightarrow C_{\alpha }$.

We will say that $S$ is *closed under transfinite composition* if, for every morphism $f$ which is a transfinite composition of morphisms of $S$, we have $f \in S$.

Proposition 1.5.4.11. Let $\operatorname{\mathcal{C}}$ be a category, let $T$ be a collection of morphisms in $\operatorname{\mathcal{C}}$, and let $S$ be the collection of all morphisms of $\operatorname{\mathcal{C}}$ which are weakly left orthogonal to $T$. Then $S$ is closed under transfinite composition.

**Proof.**
Let $\alpha $ be an ordinal and suppose we are given a functor $ \mathrm{Ord}_{\leq \alpha } \rightarrow \operatorname{\mathcal{C}}$, given by a pair

\[ ( \{ C_{\beta } \} _{\beta \leq \alpha }, \{ f_{\gamma ,\beta } \} _{\beta \leq \gamma \leq \alpha } ) \]

which satisfies condition $(a)$ of Definition 1.5.4.10. Assume that each of the morphisms $f_{\beta +1,\beta }$ belongs to $S$. We wish to show that the morphism $f_{\alpha ,0}$ also belongs to $S$. For this, we must show that every lifting problem $\sigma :$

\[ \xymatrix@C =40pt@R=40pt{ C_0 \ar [d]_{ f_{\alpha ,0} } \ar [r]^-{u} & X \ar [d]^{g} \\ C_{\alpha } \ar@ {-->}[ur] \ar [r]^-{v} & Y } \]

admits a solution, provided that $g$ belongs to $T$. We construct a collection of morphisms $\{ u_{\beta }: C_{\beta } \rightarrow X \} _{\beta \leq \alpha }$, satisfying the requirements $g \circ u_{\beta } = v \circ f_{\alpha ,\beta }$ and $u_{\beta } = u_{\gamma } \circ f_{\gamma ,\beta }$ for $\beta \leq \gamma $, using transfinite recursion. Fix an ordinal $\gamma \leq \alpha $, and assume that the morphisms $\{ u_{\beta } \} _{ \beta < \gamma }$ have been constructed. We consider three cases:

If $\gamma = 0$, we set $u_{\gamma } = u$.

If $\gamma $ is a nonzero limit ordinal, then our hypothesis that $C_{\gamma }$ is the colimit of the diagram $\{ C_{\beta } \} _{\beta < \gamma }$ guarantees that there is a unique morphism $u_{\gamma }: C_{\gamma } \rightarrow X$ satisfying $u_{\beta } = u_{\gamma } \circ f_{\gamma ,\beta }$ for $\beta < \gamma $. Moreover, our assumption that the equality $g \circ u_{\beta } = v \circ f_{\alpha ,\beta }$ holds for $\beta < \gamma $ guarantees that it also holds for $\beta = \gamma $.

Suppose that $\gamma = \beta +1$ is a successor ordinal. In this case, we take $u_{\gamma }$ to be any solution to the lifting problem

\[ \xymatrix@C =70pt@R=70pt{ C_{\beta } \ar [d]_{ f_{\beta +1,\beta } } \ar [r]^-{u_{\beta }} & X \ar [d]^{g} \\ C_{\beta +1} \ar@ {-->}[ur] \ar [r]^-{v \circ f_{\alpha , \beta +1}} & Y, } \]

which exists by virtue of our assumption that $f_{\beta +1,\beta }$ belongs to $S$.

We now complete the proof by observing that $u_{\alpha }$ is a solution to the lifting problem $\sigma $.
$\square$

Motivated by the preceding discussion, we introduce the following:

Definition 1.5.4.12. Let $\operatorname{\mathcal{C}}$ be a category which admits small colimits and let $S$ be a collection of morphisms of $\operatorname{\mathcal{C}}$. We will say that $S$ is *weakly saturated* if it is closed under pushouts (Definition 1.5.4.4), retracts (Variant 1.5.4.7), and transfinite composition (Definition 1.5.4.10).

Proposition 1.5.4.13. Let $\operatorname{\mathcal{C}}$ be a category which admits small colimits, let $T$ be a collection of morphisms of $\operatorname{\mathcal{C}}$, and let $S$ be the collection of all morphisms of $\operatorname{\mathcal{C}}$ which are weakly left orthogonal to $T$. Then $S$ is weakly saturated.

**Proof.**
Combine Propositions 1.5.4.5, 1.5.4.9, and 1.5.4.11.
$\square$