nLab internal sheaf

Idea

The notions of presheaf, site and sheaf can be formulated internal to any topos. The ordinary such notions are recovered by internalization into Set.

More precisely, the direct internalization of these notions is into the very large 2-topos $\hat Sh_2(\mathcal{S},can)$ of the given ambient topos $\mathcal{S}$, since an internal presheaf is to be an $\mathcal{S}$-valued internal functor, but $\mathcal{S}$ does not quite sit inside itself. It does, however, sit inside $\hat Sh_2(\mathcal{S},can)$, incarnated as the 2-sheaf $\bar \mathcal{S}$ corresponding to its codomain fibration.

Therefore, regarding $\hat Sh_2(\mathcal{S}, can)$ as the 2-category of internal categories in $\mathcal{S}$, an internal site in $\mathcal{S}$ is an object $\bar \mathbb{C}$ of $\hat Sh_2(\mathcal{S}, can)$ and an internal presheaf is a morphism $F : \bar \mathbb{C}^{op} \to \bar \mathcal{S}$.

Definition

While it is straightforward to define an internal site, hence the domain of an internal (pre)sheaf, the definition of the codomain is slightly more subtle, for that needs to be a copy of the ambient universe internalized into itself. One way to naturally say this is by passing to the external 2-sheaves 2-topos. This version of the definition we state in

• In terms of external 2-sheaves.

But the resulting notion can of course be expressed entirely in terms of data in the ambient topos. This we spell out in

• Explicit definition.

In terms of external 2-sheaves

Let $\mathcal{S}$ be a topos and let $\mathbb{C}$ be an internal category in $\mathcal{S}$:

$$\mathbb{C} = \left( C_1 \stackrel{\overset{s}{\to}}{\underset{t}{\to}} C_0 \right) \in \mathcal{S} \,.$$

Definition

Write $\bar \mathbb{C}$ for the 2-sheaf on $\mathcal{S}$

$$\bar \mathbb{C} : \mathcal{S}^{op} \to Cat$$

that is represented by $\mathbb{C}$. More explicitly, this is the pseudofunctor which to an object $X \in \mathcal{S}$ assigns

$$\bar \mathbb{C} : X \mapsto \left( \mathcal{S}(X,C_1) \stackrel{\overset{\mathcal{S}(X,s)}{\to}}{\underset{\mathcal{S}(X,t)}{\to}} \mathcal{S}(X,C_0) \right) \,.$$

Definition

Write

$$\bar \mathcal{S} : \mathcal{S}^{op} \to Cat$$

for the 2-sheaf that classifies the codomain fibration of $\mathcal{S}$, the pseudofunctor which sends an object to the corresponding slice topos and morphisms to base change

$$\bar \mathcal{S} : X \mapsto \mathcal{S}_{/X}$$

(also called the “self-indexing of $\mathcal{S}$”).

Remark

This is the incarnation of $\mathcal{S}$ itself, regard internal to its 2-sheaf 2-topos $\hat Sh_{2}(\mathcal{T}, can)$.

Definition

An internal presheaf on $\mathbb{C}$ (internal to $\mathcal{T}$) is a morphism

$$F : \bar \mathbb{C}^{op} \to \bar \mathcal{S}$$

of 2-sheaves on $\mathcal{S}$. (Also called an “indexed functor” between indexed categories“.)

Suppose moreover that $\mathbb{C}$ is equipped with the structure of an internal site. Then $F$ above is an internal sheaf on $\mathbb{C}$ if it satisfies the evident descent condition.

A morphism of internal presheaves is simply a 2-morphism in $\hat Sh_2(\mathcal{S}, can)$ (also called an “indexed natural transformation”). This yields a category

$$PSh(\mathbb{C})$$

of internal presheaves. Accordingly we have the full subcategory

$$Sh(\mathbb{C}, \mathcal{S}) \hookrightarrow PSh(\mathbb{C}, \mathcal{S})$$

of internal sheaves.

Explicit definition

We unwind what the above amounts to more explicitly.

Let $(\mathbb{C},J)$ be an internal site in $\mathcal{S}$, i.e. an internal category $\mathbb{C}$ equipped with an internal coverage $J$. Let $\mathcal{S}^{\mathbb{C}^{op}}$ be the topos of internal diagrams on $\mathbb{C}^{op}$.

Definition

1. An internal presheaf on $\mathbb{C}$ is an internal diagram $F \in \mathcal{S}^{\mathbb{C}^{op}}$.

2. An internal sheaf on $\mathbb{C}$ (with respect to $J$) is an internal presheaf on $\mathbb{C}$ satisfying one of the following equivalent conditions:

   1. $F$ satisfies the usual sheaf condition interpreted in the internal language of $\mathcal{S}$.
   2. $F$ is a $j$-sheaf for the Lawvere-Tierney topology on $\mathcal{S}^{\mathbb{C}^{op}}$ induced by $J$. (The equivalence is because the usual proof for $\mathcal{S} =$Set is constructive and can thus be internalized in an arbitrary topos.)

Properties

Let $\mathcal{S}$ and $\mathbb{C}$ be as above.

Proposition

The category of internal presheaves $PSh(\mathbb{C}, \mathcal{S})$ is a topos.

This appears as (Johnstone, cor. B.2.3.17).

Proposition

Let $f : \mathbb{C} \to \mathbb{D}$ be an internal functor. Write $\bar f : \bar \mathbb{C} \to \bar \mathbb{D}$ for the corresponding morphism in $\hat Sh_2(\mathcal{S}, can)$. Precomposition with this morphism induces a functor of internal presheaf catgeories

$$f^* \colon PSh(\mathbb{D}, \mathcal{S}) \to PSh(\mathbb{C}, \mathcal{S}) \,.$$

This is the inverse image of a geometric morphism of toposes

$$f \colon PSh(\mathbb{C}, \mathcal{S}) \to PSh(\mathbb{D}, \mathcal{S}) \,.$$

This appears as (Johnstone, cor. B.2.3.22).

The global section functors of internal sheaf toposes in $\mathcal{S}$ are bounded geometric morphisms over $\mathcal{S}$.

Related concepts

• internal diagram

References

On internal presheaves in elementary toposes:

• Marta Bunge, Internal Presheaf Toposes, Cahiers,Cahiers de topologie et géométrie différentielle catégoriques? 18 3 (1977) 291-330 [numdam:CTGDC_1977__18_3_291_0]

On internal presheaves in a Grothendieck toposes:

• Saunders MacLane, Ieke Moerdijk, Section V.7 of Sheaves in Geometry and Logic (1992)

and in section B2.3 of

• Peter Johnstone, Sketches of an Elephant

In these references internal presheaves are introduced in components as in the explicit definition above. The equivalence to the abstract formulation above, in terms of morphisms between 2-sheaves, follows for instance with (Johnstone, lemma B.2.3.13).

category: sheaf theory