## Tag `00YQ`

Chapter 7: Sites and Sheaves > Section 7.38: Criterion for existence of points

Proposition 7.38.3. Let $\mathcal{C}$ be a site. Assume that

- finite limits exist in $\mathcal{C}$, and
- every covering $\{U_i \to U\}_{i \in I}$ has a refinement by a finite covering of $\mathcal{C}$.
Then $\mathcal{C}$ has enough points.

Proof.We have to show that given any sheaf $\mathcal{F}$ on $\mathcal{C}$, any $U \in \mathop{\rm Ob}\nolimits(\mathcal{C})$, and any distinct sections $s, s' \in \mathcal{F}(U)$, there exists a point $p$ such that $s, s'$ have distinct image in $\mathcal{F}_p$. See Lemma 7.37.3. Consider the system $(J, \geq, V_j, g_{jj'})$ with $J = \{1\}$, $V_1 = U$, $g_{11} = \text{id}_U$. Apply Lemma 7.38.2. By the result of that lemma we get a system $(I, \geq, U_i, f_{ii'})$ refining our system such that $s_p \not = s'_p$ and such that moreover for every finite covering $\{W_k \to W\}$ of the site $\mathcal{C}$ the map $\coprod_k u(W_k) \to u(W)$ is surjective. Since every covering of $\mathcal{C}$ can be refined by a finite covering we conclude that $\coprod_k u(W_k) \to u(W)$ is surjective foranycovering $\{W_k \to W\}$ of the site $\mathcal{C}$. This implies that $u = p$ is a point, see Proposition 7.32.2 (and the discussion at the beginning of this section which guarantees that $u$ commutes with finite limits). $\square$

The code snippet corresponding to this tag is a part of the file `sites.tex` and is located in lines 8439–8448 (see updates for more information).

```
\begin{proposition}
\label{proposition-criterion-points}
Let $\mathcal{C}$ be a site. Assume that
\begin{enumerate}
\item finite limits exist in $\mathcal{C}$, and
\item every covering $\{U_i \to U\}_{i \in I}$
has a refinement by a finite covering of $\mathcal{C}$.
\end{enumerate}
Then $\mathcal{C}$ has enough points.
\end{proposition}
\begin{proof}
We have to show that given any sheaf
$\mathcal{F}$ on $\mathcal{C}$, any $U \in \Ob(\mathcal{C})$,
and any distinct sections $s, s' \in \mathcal{F}(U)$, there exists
a point $p$ such that $s, s'$ have distinct image in
$\mathcal{F}_p$. See Lemma \ref{lemma-enough}.
Consider the system $(J, \geq, V_j, g_{jj'})$
with $J = \{1\}$, $V_1 = U$, $g_{11} = \text{id}_U$.
Apply Lemma \ref{lemma-refine-all-at-once}.
By the result of that lemma we get a system
$(I, \geq, U_i, f_{ii'})$ refining our system such
that $s_p \not = s'_p$ and such that moreover for every
finite covering $\{W_k \to W\}$ of the site $\mathcal{C}$ the map
$\coprod_k u(W_k) \to u(W)$ is surjective.
Since every covering of $\mathcal{C}$ can be refined by
a finite covering we conclude that
$\coprod_k u(W_k) \to u(W)$ is surjective for {\it any}
covering $\{W_k \to W\}$ of the site $\mathcal{C}$.
This implies that $u = p$ is a point, see
Proposition \ref{proposition-point-limits} (and the discussion
at the beginning of this section which guarantees that $u$
commutes with finite limits).
\end{proof}
```

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