# Kerodon

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### 8.5.5 Idempotent Completion

Let $\operatorname{\mathcal{C}}$ be an $\infty$-category. It follows from Corollary 8.3.3.17 (together with the criterion of Proposition 8.5.4.7) that we can choose a fully faithful functor $H: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$, where $\widehat{\operatorname{\mathcal{C}}}$ is idempotent complete. Our goal in this section is to show that there is a canonical choice for the $\infty$-category $\widehat{\operatorname{\mathcal{C}}}$, which is characterized (up to equivalence) by the requirement that it is as small as possible.

Definition 8.5.5.1. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category. We say that a functor of $\infty$-categories $H: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ exhibits $\widehat{\operatorname{\mathcal{C}}}$ as an idempotent completion of $\operatorname{\mathcal{C}}$ if it satisfies the following conditions:

$(1)$

The functor $H$ is fully faithful.

$(2)$

The $\infty$-category category $\widehat{\operatorname{\mathcal{C}}}$ is idempotent complete.

$(3)$

For every object $Y \in \widehat{\operatorname{\mathcal{C}}}$, there exists an object $X \in \operatorname{\mathcal{C}}$ such that $Y$ is a retract of $H(X)$.

We will say that an $\infty$-category $\widehat{\operatorname{\mathcal{C}}}$ is an idempotent completion of $\operatorname{\mathcal{C}}$ if there exists a functor $H: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ which exhibits $\widehat{\operatorname{\mathcal{C}}}$ as an idempotent completion of $\operatorname{\mathcal{C}}$.

Remark 8.5.5.2. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category and let $\widehat{\operatorname{\mathcal{C}}}$ be an idempotent completion of $\operatorname{\mathcal{C}}$. Then any $\infty$-category $\widehat{\operatorname{\mathcal{C}}}'$ which is equivalent to $\widehat{\operatorname{\mathcal{C}}}$ is also an idempotent completion of $\operatorname{\mathcal{C}}$. More precisely, if $G: \widehat{\operatorname{\mathcal{C}}} \rightarrow \widehat{\operatorname{\mathcal{C}}}'$ is an equivalence of $\infty$-categories, then a functor $H: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ exhibits $\widehat{\operatorname{\mathcal{C}}}$ as an idempotent completion of $\operatorname{\mathcal{C}}$ if and only if the composite functor $(G \circ H): \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}'$ exhibits $\widehat{\operatorname{\mathcal{C}}}'$ as an idempotent completion of $\operatorname{\mathcal{C}}$.

Remark 8.5.5.3 (Isomorphism Invariance). Let $H: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ be a functor of $\infty$-categories, and let $H': \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ be another functor which is isomorphic to $H$ (as an object of the $\infty$-category $\operatorname{Fun}( \operatorname{\mathcal{C}}, \widehat{\operatorname{\mathcal{C}}} )$). Then $H$ exhibits $\widehat{\operatorname{\mathcal{C}}}$ as an idempotent completion of $\operatorname{\mathcal{C}}$ if and only if $H'$ exhibits $\widehat{\operatorname{\mathcal{C}}}$ as an idempotent completion of $\operatorname{\mathcal{C}}$.

We now use the Yoneda embedding of ยง8.3 to prove the existence of idempotent completions. For simplicity, let us assume first that $\operatorname{\mathcal{C}}$ is an essentially small $\infty$-category. We let $\operatorname{Fun}^{\mathrm{atm}}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}})$ denote the full subcategory of $\operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}})$ spanned by those functors $\mathscr {F}: \operatorname{\mathcal{C}}^{\operatorname{op}} \rightarrow \operatorname{\mathcal{S}}$ which are atomic, in the sense of Definition 8.4.5.1.

Proposition 8.5.5.4. Let $\operatorname{\mathcal{C}}$ be an essentially small $\infty$-category and let $h_{\bullet }: \operatorname{\mathcal{C}}\rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}})$ be a covariant Yoneda embedding for $\operatorname{\mathcal{C}}$ (Definition 8.3.3.9). Then the functor $h_{\bullet }$ exhibits $\operatorname{Fun}^{\mathrm{atm}}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}})$ as an idempotent completion of $\operatorname{\mathcal{C}}$.

Following the convention of Remark 5.4.0.5, we can regard Proposition 8.3.3.14 as a special case of the following more general assertion:

Proposition 8.5.5.5. Let $\kappa$ be an uncountable regular cardinal, let $\operatorname{\mathcal{C}}$ be an $\infty$-category which is essentially $\kappa$-small, and let $\widehat{\operatorname{\mathcal{C}}} \subseteq \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa } )$ be the full subcategory spanned by those functors $\mathscr {F}$ for which the corepresentable functor $\operatorname{Hom}_{ \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa } ) }( \mathscr {F}, \bullet )$ commutes with $\kappa$-small colimits. Then the covariant Yoneda embedding $h_{\bullet }: \operatorname{\mathcal{C}}\rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa } )$ exhibits $\widehat{\operatorname{\mathcal{C}}}$ as an idempotent completion of $\operatorname{\mathcal{C}}$.

Proof. To simplify the notation, set $\operatorname{\mathcal{D}}= \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa } )$. For each object $C \in \operatorname{\mathcal{C}}$, the representable functor $h_{C} \in \operatorname{\mathcal{D}}$ corepresents the functor

$\operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{S}}^{< \kappa } \quad \quad \mathscr {F} \mapsto \mathscr {F}(C)$

given by evaluation at $C$, which preserves $\kappa$-small colimits by virtue of Proposition 7.1.6.1. It follows that the covariant Yoneda embedding $h_{\bullet }$ factors through the subcategory $\widehat{\operatorname{\mathcal{C}}} \subseteq \operatorname{\mathcal{D}}$. Moreover, the functor $h_{\bullet }$ is fully faithful (Theorem 8.3.3.13).

The $\infty$-category $\operatorname{\mathcal{D}}$ admits $\kappa$-small colimits, and is therefore idempotent complete by virtue of Proposition 8.5.4.7. It follows from Corollary 8.5.1.11 that the full category $\widehat{\operatorname{\mathcal{C}}} \subseteq \operatorname{\mathcal{D}}$ is closed under the formation of retracts, and is therefore also idempotent complete (Proposition 8.5.4.6).

To complete the proof, it will suffice to show that every object $\mathscr {F} \in \widehat{\operatorname{\mathcal{C}}}$ is a retract of $h_{C}$ for some object $C \in \operatorname{\mathcal{C}}$. Applying Corollary 8.4.3.12, we deduce that $\mathscr {F}$ can be realized as the colimit of a diagram

$\operatorname{\mathcal{K}}\xrightarrow { T } \operatorname{\mathcal{C}}\xrightarrow { h_{\bullet } } \operatorname{\mathcal{D}},$

where $\operatorname{\mathcal{K}}$ is an essentially $\kappa$-small $\infty$-category. Since the functor $\operatorname{Hom}_{\operatorname{\mathcal{D}}}( \mathscr {F}, \bullet )$ preserves $\kappa$-small colimits, it follows that the identity map $\operatorname{id}_{\mathscr {F}} \in \operatorname{Hom}_{\operatorname{\mathcal{D}}}( \mathscr {F}, \mathscr {F} )$ factors (up to homotopy) through $h_{ T(B) }$ for some $B$. In particular, $\mathscr {F}$ is a retract of the representable functor $h_{ T(B) }$. $\square$

Corollary 8.5.5.6 (Existence). Let $\operatorname{\mathcal{C}}$ be an $\infty$-category. Then there exists a functor $H: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ which exhibits $\widehat{\operatorname{\mathcal{C}}}$ as an idempotent completion of $\operatorname{\mathcal{C}}$.

Note that our proof of Corollary 8.5.5.6 is somewhat transcendental in nature: it locates an idempotent completion of an $\infty$-category $\operatorname{\mathcal{C}}$ as a full subcategory of an $\infty$-category which is much larger than $\operatorname{\mathcal{C}}$. Let us remark that this is not necessary: the $\infty$-category $\widehat{\operatorname{\mathcal{C}}}$ is essentially of the same size as $\operatorname{\mathcal{C}}$ itself.

Proposition 8.5.5.7. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category, and let $\widehat{\operatorname{\mathcal{C}}}$ be an idempotent completion of $\operatorname{\mathcal{C}}$, and let $\kappa$ be an uncountable cardinal. Then:

$(1)$

The $\infty$-category $\operatorname{\mathcal{C}}$ is locally $\kappa$-small if and only if $\widehat{\operatorname{\mathcal{C}}}$ is locally $\kappa$-small.

$(2)$

The $\infty$-category $\operatorname{\mathcal{C}}$ is essentially $\kappa$-small if and only if $\widehat{\operatorname{\mathcal{C}}}$ is essentially $\kappa$-small.

Proof. Choose a functor $F: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ which exhibits $\widehat{\operatorname{\mathcal{C}}}$ as an idempotent completion of $\operatorname{\mathcal{C}}$. We first prove $(1)$. Assume that $\operatorname{\mathcal{C}}$ is locally $\kappa$-small; we wish to show that $\widehat{\operatorname{\mathcal{C}}}$ is locally $\kappa$-small (the reverse implication follows immediately from the definition). Fix a pair of objects $Y,Y' \in \widehat{\operatorname{\mathcal{C}}}$; we wish to show that the morphism space $\operatorname{Hom}_{\widehat{\operatorname{\mathcal{C}}}}(Y, Y' )$ is essentially $\kappa$-small. By assumption, the object $Y \in \widehat{\operatorname{\mathcal{C}}}$ is a retract of $F(X)$ for some object $X \in \operatorname{\mathcal{C}}$. It follows that $\operatorname{Hom}_{\widehat{\operatorname{\mathcal{C}}}}(Y, Y' )$ is a retract of $\operatorname{Hom}_{ \widehat{\operatorname{\mathcal{C}}} }( F(X), Y' )$ in the homotopy category $\mathrm{h} \mathit{\operatorname{Kan}}$. By virtue of Corollary 8.5.1.14, it will suffice to show that the Kan complex $\operatorname{Hom}_{ \widehat{\operatorname{\mathcal{C}}} }( F(X), Y' )$ is essentially $\kappa$-small. Applying the same argument to $Y'$, we are reduced to showing that the mapping space $\operatorname{Hom}_{ \widehat{\operatorname{\mathcal{C}}} }( F(X), F(X') )$ is essentially $\kappa$-small for every pair of objects $X,X' \in \operatorname{\mathcal{C}}$. Since the functor $F$ is fully faithful, the canonical map $\operatorname{Hom}_{ \operatorname{\mathcal{C}}}( X, X' ) \rightarrow \operatorname{Hom}_{ \widehat{\operatorname{\mathcal{C}}} }( F(X), F(X') )$ is a homotopy equivalence. The desired result now follows from our assumption that the $\infty$-category $\operatorname{\mathcal{C}}$ is essentially $\kappa$-small.

We now prove $(2)$. Assume that $\operatorname{\mathcal{C}}$ is essentially $\kappa$-small; we wish to show that $\widehat{\operatorname{\mathcal{C}}}$ is also essentially $\kappa$-small (again, the reverse implication follows immediately from the definitions). Without loss of generality, we may assume that $\kappa$ is the smallest cardinal for which $\operatorname{\mathcal{C}}$ is essentially $\kappa$-small, and is therefore regular (Corollary 5.4.6.14). By virtue of the criterion of Proposition 5.4.8.8, it will suffice to show that the set of isomorphism classes $S = \pi _0( \widehat{\operatorname{\mathcal{C}}}^{\simeq } )$ is $\kappa$-small. For each object $X \in \operatorname{\mathcal{C}}$, let $S_{X} \subseteq S$ be the collection of isomorphism classes of objects $Y \in \widehat{\operatorname{\mathcal{C}}}$ which can be realized as a retract of $F(X)$. Note that we can write $S$ as a union of the subsets $S_{X}$, where the $X$ ranges over a set of representatives for the isomorphism classes in $\operatorname{\mathcal{C}}$. Since $\kappa$ is regular, and the set $\pi _0( \operatorname{\mathcal{C}}^{\simeq } )$ is $\kappa$-small, it will suffice to show that each of the sets $S_{X}$ is $\kappa$-small. Let us henceforth regard the object $X \in \operatorname{\mathcal{C}}$ as fixed, and let $Y$ be any retract of $F(X)$ in the $\infty$-category $\widehat{\operatorname{\mathcal{C}}}$. It follows from Proposition 8.5.2.4 that, as an object of the homotopy category $\mathrm{h} \mathit{\operatorname{\mathcal{C}}}$, $Y$ can be identified with the equalizer of a pair of morphisms $(\operatorname{id}, e): F(X) \rightarrow F(X)$. It follows that the cardinality of the set of isomorphism classes $S_{X}$ is bounded above by the cardinality of the set $\operatorname{Hom}_{\mathrm{h} \mathit{ \widehat{\operatorname{\mathcal{C}}}} }( F(X), F(X) )$ of morphisms $e: F(X) \rightarrow F(X)$ in $\mathrm{h} \mathit{\operatorname{\mathcal{C}}}$, which we can identify with the $\kappa$-small set $\operatorname{Hom}_{\mathrm{h} \mathit{\operatorname{\mathcal{C}}}}(X,X)$. $\square$

Our next goal is to show that the idempotent completion $\widehat{\operatorname{\mathcal{C}}}$ of an $\infty$-category $\operatorname{\mathcal{C}}$ is uniquely determined up to equivalence. To prove this, we will show that $\widehat{\operatorname{\mathcal{C}}}$ can be characterized by a universal mapping property.

Proposition 8.5.5.8 (Uniqueness). Let $H: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ be a functor of $\infty$-categories, where $\widehat{\operatorname{\mathcal{C}}}$ is idempotent complete. The following conditions are equivalent:

$(a)$

The functor $H$ exhibits $\widehat{\operatorname{\mathcal{C}}}$ as an idempotent completion of $\operatorname{\mathcal{C}}$, in the sense of Definition 8.5.5.1.

$(b)$

For every idempotent complete $\infty$-category $\operatorname{\mathcal{D}}$, precomposition with $H$ induces an equivalence of $\infty$-categories $\operatorname{Fun}( \widehat{\operatorname{\mathcal{C}}}, \operatorname{\mathcal{D}}) \rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})$.

$(c)$

For every idempotent complete $\infty$-category $\operatorname{\mathcal{D}}$, precomposition with $H$ induces a homotopy equivalence of Kan complexes $\operatorname{Fun}( \widehat{\operatorname{\mathcal{C}}}, \operatorname{\mathcal{D}})^{\simeq } \rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})^{\simeq }$.

$(d)$

For every idempotent complete $\infty$-category $\operatorname{\mathcal{D}}$, precomposition with $H$ induces a bijection $\pi _0( \operatorname{Fun}( \widehat{\operatorname{\mathcal{C}}}, \operatorname{\mathcal{D}})^{\simeq } ) \rightarrow \pi _0( \operatorname{Fun}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})^{\simeq } )$.

The implication $(a) \Rightarrow (b)$ of Proposition 8.5.5.8 is a special case of the following more precise result:

Lemma 8.5.5.9. Let $H: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ be a fully faithful functor of $\infty$-categories. Suppose that every object $Y \in \widehat{\operatorname{\mathcal{C}}}$ is a retract of $H(X)$, for some object $X \in \operatorname{\mathcal{C}}$. Then, for any $\infty$-category $\operatorname{\mathcal{D}}$, precomposition with $H$ determines a fully faithful functor $\theta : \operatorname{Fun}( \widehat{\operatorname{\mathcal{C}}}, \operatorname{\mathcal{D}}) \rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})$. Moreover, the essential image of $\theta$ consists of those functors $G: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ which satisfy the following condition:

$(\ast )$

For every functor $F: \operatorname{N}_{\bullet }( \operatorname{Idem}) \rightarrow \operatorname{\mathcal{C}}$, if $H \circ F$ is a split idempotent in $\widehat{\operatorname{\mathcal{C}}}$, then $G \circ F$ is a split idempotent in $\operatorname{\mathcal{D}}$.

Proof. Without loss of generality, we may assume that $\operatorname{\mathcal{C}}$ is a full subcategory of $\widehat{\operatorname{\mathcal{C}}}$ and that $H$ is the inclusion functor. Let $\operatorname{Fun}'( \operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})$ denote the full subcategory of $\operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})$ spanned by those functors $G: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ which admit an extension $\widehat{G}: \widehat{\operatorname{\mathcal{C}}} \rightarrow \operatorname{\mathcal{D}}$. It follows from Proposition 8.5.1.7 that, in this case, the functor $\widehat{G}$ is automatically left (and right) Kan extended from $\operatorname{\mathcal{C}}$. Applying Corollary 7.3.6.16, we deduce that the restriction functor $\theta : \operatorname{Fun}( \widehat{\operatorname{\mathcal{C}}}, \operatorname{\mathcal{D}}) \rightarrow \operatorname{Fun}'(\operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})$ is a trivial Kan extension. Note that any functor $\widehat{G}: \widehat{\operatorname{\mathcal{C}}} \rightarrow \operatorname{\mathcal{D}}$ carries split idempotents in $\widehat{\operatorname{\mathcal{C}}}$ to split idempotents in $\operatorname{\mathcal{D}}$, so that $G = \widehat{G}|_{\operatorname{\mathcal{C}}}$ satisfies condition $(\ast )$. To complete the proof, it will suffice to prove the reverse implication. Fix a functor $G: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ which satisfies condition $(\ast )$; we wish to show that $G$ admits an extension $\widehat{G}: \widehat{\operatorname{\mathcal{C}}} \rightarrow \operatorname{\mathcal{D}}$.

Choose an uncountable regular cardinal $\kappa$ for which $\operatorname{\mathcal{C}}$ is essentially $\kappa$-small. Using Proposition 8.5.4.7, we can choose a fully faithful functor $H': \operatorname{\mathcal{D}}\rightarrow \widehat{\operatorname{\mathcal{D}}}$, where the $\infty$-category $\widehat{\operatorname{\mathcal{D}}}$ admits $\kappa$-small colimits. Replacing $\operatorname{\mathcal{D}}$ by the essential image of $H'$, we may assume without loss of generality that $\operatorname{\mathcal{D}}$ is a replete full subcategory of $\widehat{\operatorname{\mathcal{D}}}$. Invoking Proposition 7.6.7.12, we deduce that the functor $G: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}\subseteq \widehat{\operatorname{\mathcal{D}}}$ admits a left Kan extension $\widehat{G}: \widehat{\operatorname{\mathcal{C}}} \rightarrow \widehat{\operatorname{\mathcal{D}}}$. We will complete the proof by showing that $\widehat{G}$ factors through $\operatorname{\mathcal{D}}$.

Fix an object $Y \in \widehat{\operatorname{\mathcal{C}}}$; we wish to show that $\widehat{G}(Y)$ belongs to $\operatorname{\mathcal{D}}$. By assumption, there exists a retraction diagram

8.50
$$\begin{gathered}\label{equation:universal-mapping-property-of-idempotent-completion} \xymatrix@R =50pt@C=50pt{ & Y \ar [dr]^{i} & \\ X \ar [ur]^{r} \ar [rr]^{ \operatorname{id}_{X} } & & X } \end{gathered}$$

in $\widehat{\operatorname{\mathcal{C}}}$, where the object $X$ belongs to $\operatorname{\mathcal{C}}$. Using Corollary 8.5.1.24, we can extend (8.50) to a functor $\overline{F}: \operatorname{N}_{\bullet }( \operatorname{Ret}) \rightarrow \overline{\operatorname{\mathcal{C}}}$. Then $F = \overline{F}|_{ \operatorname{N}_{\bullet }( \operatorname{Idem}) }$ is an idempotent in $\operatorname{\mathcal{C}}$ which splits in $\widehat{\operatorname{\mathcal{C}}}$. Invoking assumption $(\ast )$, we deduce that $G \circ F$ is a split idempotent in $\operatorname{\mathcal{D}}$. That is, there exists a functor $\overline{F}': \operatorname{N}_{\bullet }( \operatorname{Ret}) \rightarrow \operatorname{\mathcal{D}}$ satisfying $\overline{F}' |_{ \operatorname{N}_{\bullet }( \operatorname{Idem}) } = G \circ F$. Applying Corollary 8.5.3.10, we deduce that $\widehat{G} \circ \overline{F}$ is isomorphic to $\overline{F}'$ as an object of the $\infty$-category $\operatorname{Fun}( \operatorname{N}_{\bullet }( \operatorname{Ret}), \widehat{\operatorname{\mathcal{D}}} )$. Evaluating on the final object of $\operatorname{Ret}$, we deduce that $\widehat{G}(Y)$ is isomorphic to an object of $\operatorname{\mathcal{D}}$ and therefore belongs to $\operatorname{\mathcal{D}}$ (since the full subcategory $\operatorname{\mathcal{D}}\subseteq \widehat{\operatorname{\mathcal{D}}}$ was assumed to be replete). $\square$

Proof of Proposition 8.5.5.8. Let $H: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ be a functor of $\infty$-categories, where $\widehat{\operatorname{\mathcal{C}}}$ is idempotent complete. The implication $(a) \Rightarrow (b)$ follows from Lemma 8.5.5.9, the implication $(b) \Rightarrow (c)$ from Remark 4.5.1.19, and the implication $(c) \Rightarrow (d)$ from Remark 3.1.6.5. We will complete the proof by showing that $(d)$ implies $(a)$.

Using Corollary 8.5.5.6, we can choose an $\infty$-category $\widehat{\operatorname{\mathcal{C}}}'$ and a functor $H': \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}'$ which exhibits $\widehat{\operatorname{\mathcal{C}}'}$ as an idempotent completion of $\operatorname{\mathcal{C}}$. Since the $\infty$-category $\widehat{\operatorname{\mathcal{C}}}'$ is idempotent complete, assumption $(d)$ guarantees that there exists a functor $G: \widehat{\operatorname{\mathcal{C}}} \rightarrow \widehat{\operatorname{\mathcal{C}}}'$ such that $H'$ is isomorphic to $G \circ H$ (as an object of the $\infty$-category $\operatorname{Fun}( \operatorname{\mathcal{C}}, \widehat{\operatorname{\mathcal{C}}}' )$). By virtue of Remark 8.5.5.3, the functor $G \circ H$ also exhibits $\widehat{\operatorname{\mathcal{C}}}'$ as an idempotent completion of $\operatorname{\mathcal{C}}$. To show that $H$ exhibits $\widehat{\operatorname{\mathcal{C}}}$ as an idempotent completion of $\operatorname{\mathcal{C}}$, it will suffice to show that the functor $G$ is an equivalence of $\infty$-categories (Remark 8.5.5.2). This is equivalent to the assertion that the homotopy class $[G]$ is an isomorphism when regarded as a morphism in the homotopy category of idempotent complete $\infty$-categories. Fix an idempotent complete $\infty$-category $\operatorname{\mathcal{D}}$; we wish to show that precomposition with $[G]$ induces a bijection

$\theta : \pi _0( \operatorname{Fun}( \widehat{\operatorname{\mathcal{C}}}', \operatorname{\mathcal{D}})^{\simeq }) \rightarrow \pi _0( \operatorname{Fun}( \widehat{\operatorname{\mathcal{C}}}, \operatorname{\mathcal{D}})^{\simeq } ).$

Invoking assumption $(d)$, this is equivalent to the bijectivity of the composite map

$\pi _0( \operatorname{Fun}( \widehat{\operatorname{\mathcal{C}}}', \operatorname{\mathcal{D}})^{\simeq } ) \xrightarrow { \circ [G]} \pi _0( \operatorname{Fun}( \widehat{\operatorname{\mathcal{C}}}, \operatorname{\mathcal{D}})^{\simeq } ) \xrightarrow { \circ [H]} \pi _0( \operatorname{Fun}(\operatorname{\mathcal{C}}, \operatorname{\mathcal{D}})^{\simeq } ),$

which follows from Lemma 8.5.5.9. $\square$

Let $\operatorname{\mathcal{QC}}$ denote the $\infty$-category of (small) $\infty$-categories (Construction 5.6.4.1), and let $\operatorname{\mathcal{QC}}^{ \mathrm{ic} }$ denote the full subcategory of $\operatorname{\mathcal{QC}}$ spanned by the idempotent complete $\infty$-categories. Proposition 8.5.5.8 asserts that a functor $F: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ exhibits $\widehat{\operatorname{\mathcal{C}}}$ as an idempotent completion of $\operatorname{\mathcal{C}}$ if and only if exhibits $\widehat{\operatorname{\mathcal{C}}}$ as a $\operatorname{\mathcal{QC}}^{\mathrm{ic}}$-reflection of $\operatorname{\mathcal{C}}$, in the sense of Definition 6.2.2.1. Consequently, Proposition 8.5.5.7 is equivalent to the assertion that $\operatorname{\mathcal{QC}}^{\mathrm{ic} } \subseteq \operatorname{\mathcal{QC}}$ is reflective. Combining this observation with Proposition 6.2.2.8, we obtain the following:

Corollary 8.5.5.10. Then the inclusion functor $\operatorname{\mathcal{QC}}^{\mathrm{ic} } \hookrightarrow \operatorname{\mathcal{QC}}$ admits a left adjoint, which carries each $\infty$-category $\operatorname{\mathcal{C}}$ to an idempotent completion $\widehat{\operatorname{\mathcal{C}}}$.

Corollary 8.5.5.11. Let $\operatorname{\mathcal{C}}$ be an $\infty$-category which can be realized as the limit of a small diagram $\mathscr {F}: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{QC}}$. Suppose that, for each vertex $D \in \operatorname{\mathcal{D}}$, the $\infty$-category $\mathscr {F}(D)$ is idempotent complete. Then $\operatorname{\mathcal{C}}$ is idempotent complete.

Corollary 8.5.5.12. Let $\operatorname{\mathcal{X}}$ and $\operatorname{\mathcal{Y}}$ be $\infty$-categories. Suppose that $\operatorname{\mathcal{Y}}$ is a retract of $\operatorname{\mathcal{X}}$ in the homotopy category $\mathrm{h} \mathit{\operatorname{QCat}}$. If $\operatorname{\mathcal{X}}$ is idempotent complete, then $\operatorname{\mathcal{Y}}$ is also idempotent complete.

Proof. By virtue of Remark 8.5.3.9, we can identify $\operatorname{\mathcal{Y}}$ with the limit of a diagram $\operatorname{N}_{\bullet }( \operatorname{Idem}) \rightarrow \operatorname{\mathcal{QC}}$ carrying the unique object of $\operatorname{Idem}$ to the idempotent complete $\infty$-category $\operatorname{\mathcal{X}}$. The desired result is now a special case of Corollary 8.5.5.11. $\square$