Remark 7.15.3. There are many sites that give rise to the topos $\mathop{\mathit{Sh}}\nolimits (pt)$. A useful example is the following. Suppose that $S$ is a set (of sets) which contains at least one nonempty element. Let $\mathcal{S}$ be the category whose objects are elements of $S$ and whose morphisms are arbitrary set maps. Assume that $\mathcal{S}$ has fibre products. For example this will be the case if $S = \mathcal{P}(\text{infinite set})$ is the power set of any infinite set (exercise in set theory). Make $\mathcal{S}$ into a site by declaring surjective families of maps to be coverings (and choose a suitable sufficiently large set of covering families as in Sets, Section 3.11). We claim that $\mathop{\mathit{Sh}}\nolimits (\mathcal{S})$ is equivalent to the category of sets.

We first prove this in case $S$ contains $e \in S$ which is a singleton. In this case, there is an equivalence of topoi $i : \mathop{\mathit{Sh}}\nolimits (pt) \to \mathop{\mathit{Sh}}\nolimits (\mathcal{S})$ given by the functors

7.15.3.1
$$\label{sites-equation-sheaves-pt-sets} i^{-1}\mathcal{F} = \mathcal{F}(e), \quad i_*E = (U \mapsto \mathop{\mathrm{Mor}}\nolimits _{\textit{Sets}}(U, E))$$

Namely, suppose that $\mathcal{F}$ is a sheaf on $\mathcal{S}$. For any $U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{S}) = S$ we can find a covering $\{ \varphi _ u : e \to U\} _{u \in U}$, where $\varphi _ u$ maps the unique element of $e$ to $u \in U$. The sheaf condition implies in this case that $\mathcal{F}(U) = \prod _{u \in U} \mathcal{F}(e)$. In other words $\mathcal{F}(U) = \mathop{\mathrm{Mor}}\nolimits _{\textit{Sets}}(U, \mathcal{F}(e))$. Moreover, this rule is compatible with restriction mappings. Hence the functor

$i_* : \textit{Sets} = \mathop{\mathit{Sh}}\nolimits (pt) \longrightarrow \mathop{\mathit{Sh}}\nolimits (\mathcal{S}), \quad E \longmapsto (U \mapsto \mathop{\mathrm{Mor}}\nolimits _{\textit{Sets}}(U, E))$

is an equivalence of categories, and its inverse is the functor $i^{-1}$ given above.

If $\mathcal{S}$ does not contain a singleton, then the functor $i_*$ as defined above still makes sense. To show that it is still an equivalence in this case, choose any nonempty $\tilde e \in S$ and a map $\varphi : \tilde e \to \tilde e$ whose image is a singleton. For any sheaf $\mathcal{F}$ set

$\mathcal{F}(e) := \mathop{\mathrm{Im}}( \mathcal{F}(\varphi ) : \mathcal{F}(\tilde e) \longrightarrow \mathcal{F}(\tilde e) )$

and show that this is a quasi-inverse to $i_*$. Details omitted.

There are also:

• 5 comment(s) on Section 7.15: Topoi

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).