Kerodon

$\Newextarrow{\xRightarrow}{5,5}{0x21D2}$ $\newcommand\empty{}$
$\Newextarrow{\xhookrightarrow}{10,10}{0x21AA}$

8.3.5 Characterization of Yoneda Embeddings

Let $\operatorname{\mathcal{C}}$ be an essentially small $\infty $-category, let $\widehat{\operatorname{\mathcal{C}}}$ denote the $\infty $-category $\operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}})$, and let $h: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ be a covariant Yoneda embedding for $\operatorname{\mathcal{C}}$. Then:

$(1)$

The functor $h$ is fully faithful (Theorem 8.2.5.5).

$(2)$

For each object $X \in \operatorname{\mathcal{C}}$, the functor

\[ \operatorname{Hom}_{ \widehat{\operatorname{\mathcal{C}}} }( h(X), \bullet ): \widehat{\operatorname{\mathcal{C}}} \rightarrow \operatorname{\mathcal{S}} \]

preserves small colimits (see Example 8.3.5.2 below).

$(3)$

The $\infty $-category $\widehat{\operatorname{\mathcal{C}}}$ is generated (under small colimits) by the essential image of $h$ (in fact, the functor $h$ is dense: see Theorem 8.3.2.1).

Our goal in this section is to show that these properties characterize the $\infty $-category $\widehat{\operatorname{\mathcal{C}}}$ (and the functor $h$) up to equivalence. First, let us introduce a bit of terminology.

Definition 8.3.5.1. Let $\operatorname{\mathcal{D}}$ be a locally small $\infty $-category which admits small colimits. We say that an object $X \in \operatorname{\mathcal{D}}$ is atomic if the corepresentable functor

\[ h^{X}: \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{S}}\quad \quad Y \mapsto \operatorname{Hom}_{\operatorname{\mathcal{D}}}(X,Y) \]

preserves small colimits.

Example 8.3.5.2 (Representable Functors are Atomic). Let $\operatorname{\mathcal{C}}$ be an essentially small $\infty $-category. Then every representable functor $\mathscr {F}: \operatorname{\mathcal{C}}^{\operatorname{op}} \rightarrow \operatorname{\mathcal{S}}$ is atomic when regarded as an object of the $\infty $-category $\widehat{\operatorname{\mathcal{C}}} = \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}})$. To see this, suppose that $\mathscr {F}$ is representable by an object $C \in \operatorname{\mathcal{C}}$. Using Remark 8.2.1.5, we see that $\mathscr {F}$ corepresents the evaluation functor

\[ \operatorname{ev}_{C}: \widehat{\operatorname{\mathcal{C}}} = \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}) \rightarrow \operatorname{\mathcal{S}}\quad \quad \mathscr {G} \mapsto \mathscr {G}(C), \]

and therefore preserves small colimits by virtue of Proposition 7.1.6.1.

Definition 8.3.5.3. Let $\widehat{\operatorname{\mathcal{C}}}$ be an $\infty $-category. We say that a full subcategory $\operatorname{\mathcal{C}}\subseteq \widehat{\operatorname{\mathcal{C}}}$ is weakly dense if the following condition is satisfied:

  • Let $f: Y \rightarrow Z$ be a morphism of $\widehat{\operatorname{\mathcal{C}}}$ such that, for every object $X \in \operatorname{\mathcal{C}}$, the induced map

    \[ \operatorname{Hom}_{\widehat{\operatorname{\mathcal{C}}}}( X, Y) \xrightarrow { [f] \circ } \operatorname{Hom}_{\widehat{\operatorname{\mathcal{C}}}}(X, Z) \]

    is a homotopy equivalence of Kan complexes. Then $f$ is an isomorphism.

We say that a collection of objects $\{ X_ i \} _{i \in I}$ of $\widehat{\operatorname{\mathcal{C}}}$ is weakly dense if it spans a weakly dense full subcategory of $\widehat{\operatorname{\mathcal{C}}}$.

Remark 8.3.5.4. Let $\widehat{\operatorname{\mathcal{C}}}$ be a locally small $\infty $-category. A full subcategory $\operatorname{\mathcal{C}}\subseteq \widehat{\operatorname{\mathcal{C}}}$ is weakly dense if and only if the restricted Yoneda embedding $\widehat{\operatorname{\mathcal{C}}} \rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}})$ is conservative.

Example 8.3.5.5. Let $\widehat{\operatorname{\mathcal{C}}}$ be an $\infty $-category and let $\operatorname{\mathcal{C}}\subseteq \widehat{\operatorname{\mathcal{C}}}$ be a full subcategory which generates $\widehat{\operatorname{\mathcal{C}}}$ under colimits (see Warning 8.3.1.10). Then the full subcategory $\operatorname{\mathcal{C}}$ is weakly dense. In particular, every dense subcategory of $\widehat{\operatorname{\mathcal{C}}}$ is weakly dense.

Let $h: \operatorname{\mathcal{C}}\rightarrow \widehat{\operatorname{\mathcal{C}}}$ be a functor of $\infty $-categories. In what follows, we will say that another functor $f: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ is equivalent to $h$ if there is a diagram of $\infty $-categories

\[ \xymatrix@R =50pt@C=50pt{ & \widehat{\operatorname{\mathcal{C}}} \ar [dr]^{F} & \\ \operatorname{\mathcal{C}}\ar [ur]^{h} \ar [rr]^{f} & & \operatorname{\mathcal{D}}} \]

which commutes up to isomorphism, where $F$ is an equivalence of $\infty $-categories. We will be primarily interested in the special case where $\widehat{\operatorname{\mathcal{C}}} = \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}})$ and $h$ is a covariant Yoneda embedding for $\operatorname{\mathcal{C}}$.

Proposition 8.3.5.6. Let $f: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a functor of $\infty $-categories, where $\operatorname{\mathcal{C}}$ is essentially small. Then $f$ is equivalent to the covariant Yoneda embedding $h_{\bullet }: \operatorname{\mathcal{C}}\rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}})$ if and only if it satisfies the following conditions:

$(0)$

The $\infty $-category $\operatorname{\mathcal{D}}$ is locally small and admits small colimits.

$(1)$

The functor $f$ is fully faithful.

$(2)$

For each object $C \in \operatorname{\mathcal{C}}$, the image $f(C) \in \operatorname{\mathcal{D}}$ is atomic.

$(3)$

The collection of objects $\{ f(C) \} _{C \in \operatorname{\mathcal{C}}}$ is weakly dense in $\operatorname{\mathcal{D}}$.

Corollary 8.3.5.7. Let $\operatorname{\mathcal{D}}$ be an $\infty $-category. The following conditions are equivalent:

$(a)$

There exists an essentially small $\infty $-category $\operatorname{\mathcal{C}}$ and an equivalence of $\infty $-categories $F: \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}) \rightarrow \operatorname{\mathcal{D}}$.

$(b)$

The $\infty $-category $\operatorname{\mathcal{D}}$ is locally small and admits small colimits. Moreover, it contains a small collection of atomic objects $\{ X_ i \} _{i \in I}$ which is weakly dense.

Proof. We first show that $(a)$ implies $(b)$. Without loss of generality, we may assume that $\operatorname{\mathcal{D}}= \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}})$, where $\operatorname{\mathcal{C}}$ is a small $\infty $-category. Since the $\infty $-category $\operatorname{\mathcal{S}}$ admits small colimits (Corollary 7.4.5.6), the $\infty $-category $\operatorname{\mathcal{D}}$ also admits small colimits (Proposition 7.1.6.1). Moreover, Corollary 5.4.8.10 guarantees that $\operatorname{\mathcal{D}}$ is locally small. For each object $X \in \operatorname{\mathcal{C}}$, let $h_{X} \in \operatorname{\mathcal{D}}$ be a functor represented by $X$. Then the collection of objects $\{ h_ X \} _{X \in \operatorname{\mathcal{C}}}$ span a full subcategory of $\operatorname{\mathcal{D}}$ which is dense (Corollary 8.3.2.2), and therefore weakly dense (Example 8.3.5.5). We conclude by observing that each of the representable functors $h_{X}$ is a atomic object of $\operatorname{\mathcal{D}}$ (Example 8.3.5.2).

We now show that $(b)$ implies $(a)$. Assume that $\operatorname{\mathcal{D}}$ is locally small and admits small colimits. Let $\{ X_ i \} _{i \in I}$ be a small collection of atomic objects of $\operatorname{\mathcal{D}}$, and let $\operatorname{\mathcal{D}}_0 \subseteq \operatorname{\mathcal{D}}$ be the full subcategory that they span. It follows from Proposition 5.4.8.8 that the $\infty $-category $\operatorname{\mathcal{D}}_0$ is essentially small. If $\operatorname{\mathcal{D}}_0$ is weakly dense in $\operatorname{\mathcal{D}}$, then the inclusion map $\operatorname{\mathcal{D}}_0 \hookrightarrow \operatorname{\mathcal{D}}$ satisfies the hypotheses of Proposition 8.3.5.6 and therefore induces an equivalence of $\infty $-categories $\operatorname{Fun}( \operatorname{\mathcal{D}}_0^{\operatorname{op}}, \operatorname{\mathcal{S}}) \rightarrow \operatorname{\mathcal{D}}$. $\square$

Following the convention of Remark 5.4.0.5, we will deduce Proposition 8.3.5.6 from the following more precise assertion.

Variant 8.3.5.8. Let $\kappa $ be an uncountable regular cardinal and let $f: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a functor of $\infty $-categories, where $\operatorname{\mathcal{C}}$ is essentially $\kappa $-small. Then $f$ is equivalent to the covariant Yoneda embedding $h_{\bullet }: \operatorname{\mathcal{C}}\rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{<\kappa })$ if and only if it satisfies the following conditions:

$(0)$

The $\infty $-category $\operatorname{\mathcal{D}}$ admits $\kappa $-small colimits.

$(1)$

The functor $f$ is fully faithful.

$(2)$

Let $C$ be an object of $\operatorname{\mathcal{C}}$. Then the image $f(C) \in \operatorname{\mathcal{D}}$ corepresents a functor

\[ \operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{S}}^{< \kappa } \quad \quad Y \mapsto \operatorname{Hom}_{\operatorname{\mathcal{D}}}( f(C), Y) \]

which preserves $\kappa $-small colimits.

$(3)$

The collection of objects $\{ f(X) \} _{X \in \operatorname{\mathcal{C}}}$ is weakly dense in $\operatorname{\mathcal{D}}$.

Warning 8.3.5.9. In the situation of Variant 8.3.5.8, it is not necessarily true that the $\infty $-category $\operatorname{\mathcal{D}}$ is locally $\kappa $-small. However, condition $(2)$ requires that the morphism space $\operatorname{Hom}_{\operatorname{\mathcal{D}}}( X, Y)$ is essentially $\kappa $-small whenever the object $X$ belongs to the essential image of the functor $f$.

Proof of Proposition 8.3.5.6. By virtue of Variant 8.3.5.8, the only nontrivial point is to verify that if $\operatorname{\mathcal{C}}$ is an essentially small $\infty $-category, then the $\infty $-category $\operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}})$ is locally small. This follows from Corollary 5.4.8.10. $\square$

We first prove a weak version of Variant 8.3.5.8.

Lemma 8.3.5.10. Let $\kappa $ be an uncountable regular cardinal and let $\lambda $ be an uncountable cardinal of exponential cofinality $\geq \kappa $. Let $\operatorname{\mathcal{C}}$ be an essentially $\kappa $-small $\infty $-category, let $\operatorname{\mathcal{D}}$ be a locally $\lambda $-small $\infty $-category, and let $f: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a functor which satisfies the following conditions:

$(0)$

The $\infty $-category $\operatorname{\mathcal{D}}$ admits $\kappa $-small colimits.

$(1)$

The functor $f$ is fully faithful.

$(2)$

Let $C$ be an object of $\operatorname{\mathcal{C}}$. Then the image $f(C) \in \operatorname{\mathcal{D}}$ corepresents a functor $\operatorname{\mathcal{D}}\rightarrow \operatorname{\mathcal{S}}^{<\lambda }$ which preserves $\kappa $-small colimits.

Then $f$ is isomorphic to the composition $T \circ h_{\bullet }$, for some fully faithful functor

\[ T: \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa } ) \rightarrow \operatorname{\mathcal{D}} \]

which preserves $\kappa $-small colimits. Moreover, the functor $T$ is uniquely determined up to isomorphism.

Proof. It follows from Variant 8.3.3.6 that $f$ is isomorphic to a composition $F \circ h_{\bullet }$, for some functor $F: \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa } ) \rightarrow \operatorname{\mathcal{D}}$ which preserves $\kappa $-small colimits (and that $F$ is uniquely determined up to isomorphism). To complete the proof, it will suffice to show that the functor $F$ is fully faithful. Since $\lambda $ has exponential cofinality $\geq \kappa $, the functor $\infty $-category $\operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa } )$ is locally $\lambda $-small (Corollary 5.4.8.9). For every pair of functors $\mathscr {G}, \mathscr {G}': \operatorname{\mathcal{C}}^{\operatorname{op}} \rightarrow \operatorname{\mathcal{S}}^{< \kappa }$, the functor $F$ induces a morphism of Kan complexes

\[ \theta _{\mathscr {G}, \mathscr {G}'}: \operatorname{Hom}_{ \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa }) }( \mathscr {G}, \mathscr {G}' ) \rightarrow \operatorname{Hom}_{ \operatorname{\mathcal{D}}}( F( \mathscr {G}), F( \mathscr {G}' ) ). \]

By virtue of Proposition 8.2.7.2 (and Remark 8.2.7.3), we can promote the construction $(\mathscr {G}, \mathscr {G}') \mapsto \theta _{ \mathscr {G}, \mathscr {G}' }$ to a functor of $\infty $-categories

\[ \theta : \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa })^{\operatorname{op}} \times \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa }) \rightarrow \operatorname{Fun}( \Delta ^1, \operatorname{\mathcal{S}}^{< \lambda } ). \]

We wish to show that, for every pair of functors $\mathscr {G}, \mathscr {G}': \operatorname{\mathcal{C}}^{\operatorname{op}} \rightarrow \operatorname{\mathcal{S}}^{< \kappa }$, the morphism $\theta _{\mathscr {G}, \mathscr {G}'}$ is a homotopy equivalence. Let us first regard the functor $\mathscr {G}'$ as fixed. Since $T$ preserves $\kappa $-small colimits, it follows from Remark 7.4.5.15 that the functor

\[ \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa })^{\operatorname{op}} \rightarrow \operatorname{Fun}( \Delta ^1, \operatorname{\mathcal{S}}^{< \lambda } ) \quad \quad \mathscr {G} \mapsto \theta _{ \mathscr {G}, \mathscr {G}'} \]

preserves $\kappa $-small limits. By virtue of Corollary 8.3.3.12, it will suffice to show that $\theta _{ \mathscr {G}, \mathscr {G}' }$ is a homotopy equivalence in the special case where $\mathscr {G} = h_{C}$ for some object $C \in \operatorname{\mathcal{C}}$. Let us now regard $\mathscr {G} = h_ C$ as fixed. Combining Example 8.3.5.2 with assumption $(2)$, we deduce that the functor

\[ \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa }) \rightarrow \operatorname{Fun}( \Delta ^1, \operatorname{\mathcal{S}}^{< \lambda } ) \quad \quad \mathscr {G}' \mapsto \theta _{ \mathscr {G}, \mathscr {G}'} \]

preserves $\kappa $-small colimits. Invoking Corollary 8.3.3.12 again, we are reduced to proving that $\theta _{ \mathscr {G}, \mathscr {G}' }$ is a homotopy equivalence in the special case where $\mathscr {G}' = h_{C'}$ for some object $C' \in \operatorname{\mathcal{C}}$. In this case, we have a commutative diagram of Kan complexes

\[ \xymatrix@R =50pt@C=50pt{ & \operatorname{Hom}_{\operatorname{\mathcal{C}}}(C,C') \ar [dl] \ar [dr] & \\ \operatorname{Hom}_{ \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa } )}( \mathscr {G}, \mathscr {G}' ) \ar [rr]^{ \theta _{\mathscr {G}, \mathscr {G}'} } & & \operatorname{Hom}_{\operatorname{\mathcal{D}}}( F(\mathscr {G}), F( \mathscr {G}') ), } \]

where the left vertical map is a homotopy equivalence by virtue of Yoneda's lemma (Theorem 8.2.5.5). It will therefore suffice to show that the right vertical map is a homtoopy equivalence. Since $F \circ h_{\bullet }$ is isomorphic to $f$, this is equivalent to the assertion that the natural map $\operatorname{Hom}_{\operatorname{\mathcal{C}}}(C,C') \rightarrow \operatorname{Hom}_{\operatorname{\mathcal{D}}}( f(C), f(C') )$ is a homotopy equivalence, which follows from assumption $(1)$. $\square$

Proof of Variant 8.3.5.8. Let $\kappa $ be an uncountable regular cardinal, let $\operatorname{\mathcal{C}}$ be an essentially $\kappa $-small $\infty $-category, and let $f: \operatorname{\mathcal{C}}\rightarrow \operatorname{\mathcal{D}}$ be a functor of $\infty $-categories which satisfies the hypotheses of Variant 8.3.5.8. By virtue of Lemma 8.3.5.10, there exists a fully faithful functor $F: \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa } ) \rightarrow \operatorname{\mathcal{D}}$ which preserves $\kappa $-small colimits such that $f$ is isomorphic to the composite functor

\[ \operatorname{\mathcal{C}}\xrightarrow { h_{\bullet } } \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa } ) \xrightarrow { F} \operatorname{\mathcal{D}}. \]

To complete the proof, it will suffice to show that $F$ is an equivalence of $\infty $-categories. Using Variant 8.3.4.2, we see that the functor $F$ admits a right adjoint $G: \operatorname{\mathcal{D}}\rightarrow \operatorname{Fun}( \operatorname{\mathcal{C}}^{\operatorname{op}}, \operatorname{\mathcal{S}}^{< \kappa } )$, given on objects by the formula $G(D)(C) = \operatorname{Hom}_{\operatorname{\mathcal{D}}}( f(C), D)$. By virtue of Corollary 6.3.3.14, it will suffice to show that the functor $G$ is conservative. This follows from our assumption that the collection of objects $\{ f(C) \} _{C \in \operatorname{\mathcal{C}}}$ is weakly dense in $\operatorname{\mathcal{D}}$ (Remark 8.3.5.4). $\square$