Definition 59.9.1. Let $\mathcal{C}$ be a category. A *presheaf of sets* (respectively, an *abelian presheaf*) on $\mathcal{C}$ is a functor $\mathcal{C}^{opp} \to \textit{Sets}$ (resp. $\textit{Ab}$).

## 59.9 Presheaves

A reference for this section is Sites, Section 7.2.

**Terminology.** If $U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$, then elements of $\mathcal{F}(U)$ are called *sections* of $\mathcal{F}$ over $U$. For $\varphi : V \to U$ in $\mathcal{C}$, the map $\mathcal{F}(\varphi ) : \mathcal{F}(U) \to \mathcal{F}(V)$ is called the *restriction map* and is often denoted $s \mapsto s|_ V$ or sometimes $s \mapsto \varphi ^*s$. The notation $s|_ V$ is ambiguous since the restriction map depends on $\varphi $, but it is a standard abuse of notation. We also use the notation $\Gamma (U, \mathcal{F}) = \mathcal{F}(U)$.

Saying that $\mathcal{F}$ is a functor means that if $W \to V \to U$ are morphisms in $\mathcal{C}$ and $s \in \Gamma (U, \mathcal{F})$ then $(s|_ V)|_ W = s |_ W$, with the abuse of notation just seen. Moreover, the restriction mappings corresponding to the identity morphisms $\text{id}_ U : U \to U$ are the identity.

The category of presheaves of sets (respectively of abelian presheaves) on $\mathcal{C}$ is denoted $\textit{PSh} (\mathcal{C})$ (resp. $\textit{PAb} (\mathcal{C})$). It is the category of functors from $\mathcal{C}^{opp}$ to $\textit{Sets}$ (resp. $\textit{Ab}$), which is to say that the morphisms of presheaves are natural transformations of functors. We only consider the categories $\textit{PSh}(\mathcal{C})$ and $\textit{PAb}(\mathcal{C})$ when the category $\mathcal{C}$ is small. (Our convention is that a category is small unless otherwise mentioned, and if it isn't small it should be listed in Categories, Remark 4.2.2.)

Example 59.9.2. Given an object $X \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$, we consider the rule

This defines a functor $h_ X : \mathcal{C}^{opp} \to \textit{Sets}$ and hence a presheaf. This is called the *representable presheaf associated to $X$.* It is not true that representable presheaves are sheaves in every topology on every site.

Lemma 59.9.3 (Yoneda). Let $\mathcal{C}$ be a category, and $X, Y \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C})$. There is a natural bijection

**Proof.**
See Categories, Lemma 4.3.5.
$\square$

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