## 4.42 Representable 1-morphisms

Let $\mathcal{C}$ be a category. In this section we explain what it means for a $1$-morphism between categories fibred in groupoids over $\mathcal{C}$ to be representable.

Let $\mathcal{C}$ be a category. Let $\mathcal{X}$, $\mathcal{Y}$ be categories fibred in groupoids over $\mathcal{C}$. Let $U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$. Let $F : \mathcal{X} \to \mathcal{Y}$ and $G : \mathcal{C}/U \to \mathcal{Y}$ be $1$-morphisms of categories fibred in groupoids over $\mathcal{C}$. We want to describe the $2$-fibre product

$\xymatrix{ (\mathcal{C}/U) \times _\mathcal {Y} \mathcal{X} \ar[r] \ar[d] & \mathcal{X} \ar[d]^ F \\ \mathcal{C}/U \ar[r]^ G & \mathcal{Y} }$

Let $y = G(\text{id}_ U) \in \mathcal{Y}_ U$. Make a choice of pullbacks for $\mathcal{Y}$ (see Definition 4.33.6). Then $G$ is isomorphic to the functor $(f : V \to U) \mapsto f^*y$, see Lemma 4.41.2 and its proof. We may think of an object of $(\mathcal{C}/U) \times _\mathcal {Y} \mathcal{X}$ as a quadruple $(V, f : V \to U, x, \phi )$, see Lemma 4.32.3. Using the description of $G$ above we may think of $\phi$ as an isomorphism $\phi : f^*y \to F(x)$ in $\mathcal{Y}_ V$.

Lemma 4.42.1. In the situation above the fibre category of $(\mathcal{C}/U) \times _\mathcal {Y} \mathcal{X}$ over an object $f : V \to U$ of $\mathcal{C}/U$ is the category described as follows:

1. objects are pairs $(x, \phi )$, where $x \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{X}_ V)$, and $\phi : f^*y \to F(x)$ is a morphism in $\mathcal{Y}_ V$,

2. the set of morphisms between $(x, \phi )$ and $(x', \phi ')$ is the set of morphisms $\psi : x \to x'$ in $\mathcal{X}_ V$ such that $F(\psi ) = \phi ' \circ \phi ^{-1}$.

Proof. See discussion above. $\square$

Lemma 4.42.2. Let $\mathcal{C}$ be a category. Let $\mathcal{X}$, $\mathcal{Y}$ be categories fibred in groupoids over $\mathcal{C}$. Let $F : \mathcal{X} \to \mathcal{Y}$ be a $1$-morphism. Let $G : \mathcal{C}/U \to \mathcal{Y}$ be a $1$-morphism. Then

$(\mathcal{C}/U) \times _\mathcal {Y} \mathcal{X} \longrightarrow \mathcal{C}/U$

is a category fibred in groupoids.

Proof. We have already seen in Lemma 4.35.7 that the composition

$(\mathcal{C}/U) \times _\mathcal {Y} \mathcal{X} \longrightarrow \mathcal{C}/U \longrightarrow \mathcal{C}$

is a category fibred in groupoids. Then the lemma follows from Lemma 4.35.12. $\square$

Definition 4.42.3. Let $\mathcal{C}$ be a category. Let $\mathcal{X}$, $\mathcal{Y}$ be categories fibred in groupoids over $\mathcal{C}$. Let $F : \mathcal{X} \to \mathcal{Y}$ be a $1$-morphism. We say $F$ is representable, or that $\mathcal{X}$ is relatively representable over $\mathcal{Y}$, if for every $U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$ and any $G : \mathcal{C}/U \to \mathcal{Y}$ the category fibred in groupoids

$(\mathcal{C}/U) \times _\mathcal {Y} \mathcal{X} \longrightarrow \mathcal{C}/U$

is representable.

Lemma 4.42.4. Let $\mathcal{C}$ be a category. Let $\mathcal{X}$, $\mathcal{Y}$ be categories fibred in groupoids over $\mathcal{C}$. Let $F : \mathcal{X} \to \mathcal{Y}$ be a $1$-morphism. If $F$ is representable then every one of the functors

$F_ U : \mathcal{X}_ U \longrightarrow \mathcal{Y}_ U$

between fibre categories is faithful.

Proof. Clear from the description of fibre categories in Lemma 4.42.1 and the characterization of representable fibred categories in Lemma 4.40.2. $\square$

Lemma 4.42.5. Let $\mathcal{C}$ be a category. Let $\mathcal{X}$, $\mathcal{Y}$ be categories fibred in groupoids over $\mathcal{C}$. Let $F : \mathcal{X} \to \mathcal{Y}$ be a $1$-morphism. Make a choice of pullbacks for $\mathcal{Y}$. Assume

1. each functor $F_ U : \mathcal{X}_ U \longrightarrow \mathcal{Y}_ U$ between fibre categories is faithful, and

2. for each $U$ and each $y \in \mathcal{Y}_ U$ the presheaf

$(f : V \to U) \longmapsto \{ (x, \phi ) \mid x \in \mathcal{X}_ V, \phi : f^*y \to F(x)\} /\cong$

is a representable presheaf on $\mathcal{C}/U$.

Then $F$ is representable.

Proof. Clear from the description of fibre categories in Lemma 4.42.1 and the characterization of representable fibred categories in Lemma 4.40.2. $\square$

Before we state the next lemma we point out that the $2$-category of categories fibred in groupoids is a $(2, 1)$-category, and hence we know what it means to say that it has a final object (see Definition 4.31.1). And it has a final object namely $\text{id} : \mathcal{C} \to \mathcal{C}$. Thus we define $2$-products of categories fibred in groupoids over $\mathcal{C}$ as the $2$-fibre products

$\mathcal{X} \times \mathcal{Y} := \mathcal{X} \times _\mathcal {C} \mathcal{Y}.$

With this definition in place the following lemma makes sense.

Lemma 4.42.6. Let $\mathcal{C}$ be a category. Let $\mathcal{S} \to \mathcal{C}$ be a category fibred in groupoids. Assume $\mathcal{C}$ has products of pairs of objects and fibre products. The following are equivalent:

1. The diagonal $\mathcal{S} \to \mathcal{S} \times \mathcal{S}$ is representable.

2. For every $U$ in $\mathcal{C}$, any $G : \mathcal{C}/U \to \mathcal{S}$ is representable.

Proof. Suppose the diagonal is representable, and let $U, G$ be given. Consider any $V \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$ and any $G' : \mathcal{C}/V \to \mathcal{S}$. Note that $\mathcal{C}/U \times \mathcal{C}/V = \mathcal{C}/U \times V$ is representable. Hence the fibre product

$\xymatrix{ (\mathcal{C}/U \times V) \times _{(\mathcal{S} \times \mathcal{S})} \mathcal{S} \ar[r] \ar[d] & \mathcal{S} \ar[d] \\ \mathcal{C}/U \times V \ar[r]^{(G, G')} & \mathcal{S} \times \mathcal{S} }$

is representable by assumption. This means there exists $W \to U \times V$ in $\mathcal{C}$, such that

$\xymatrix{ \mathcal{C}/W \ar[d] \ar[r] & \mathcal{S} \ar[d] \\ \mathcal{C}/U \times \mathcal{C}/V \ar[r] & \mathcal{S} \times \mathcal{S} }$

is cartesian. This implies that $\mathcal{C}/W \cong \mathcal{C}/U \times _\mathcal {S} \mathcal{C}/V$ (see Lemma 4.31.11) as desired.

Assume (2) holds. Consider any $V \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$ and any $(G, G') : \mathcal{C}/V \to \mathcal{S} \times \mathcal{S}$. We have to show that $\mathcal{C}/V \times _{\mathcal{S} \times \mathcal{S}} \mathcal{S}$ is representable. What we know is that $\mathcal{C}/V \times _{G, \mathcal{S}, G'} \mathcal{C}/V$ is representable, say by $a : W \to V$ in $\mathcal{C}/V$. The equivalence

$\mathcal{C}/W \to \mathcal{C}/V \times _{G, \mathcal{S}, G'} \mathcal{C}/V$

followed by the second projection to $\mathcal{C}/V$ gives a second morphism $a' : W \to V$. Consider $W' = W \times _{(a, a'), V \times V} V$. There exists an equivalence

$\mathcal{C}/W' \cong \mathcal{C}/V \times _{\mathcal{S} \times \mathcal{S}} \mathcal{S}$

namely

\begin{eqnarray*} \mathcal{C}/W' & \cong & \mathcal{C}/W \times _{(\mathcal{C}/V \times \mathcal{C}/V)} \mathcal{C}/V \\ & \cong & \left(\mathcal{C}/V \times _{(G, \mathcal{S}, G')} \mathcal{C}/V\right) \times _{(\mathcal{C}/V \times \mathcal{C}/V)} \mathcal{C}/V \\ & \cong & \mathcal{C}/V \times _{(\mathcal{S} \times \mathcal{S})} \mathcal{S} \end{eqnarray*}

(for the last isomorphism see Lemma 4.31.12) which proves the lemma. $\square$

Bibliographic notes: Parts of this have been taken from Vistoli's notes [Vis2].

Comment #1779 by Kiran Kedlaya on

Is "Biographical notes" perhaps a typo for "Bibliographic notes"? (It being a reference to Vistoli's book rather than his life story.)

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