The Stacks project

Lemma 98.11.2. Let $S$ be a scheme. Let $p : \mathcal{X} \to \mathcal{Y}$ and $q : \mathcal{Z} \to \mathcal{Y}$ be $1$-morphisms of categories fibred in groupoids over $(\mathit{Sch}/S)_{fppf}$.

  1. If $\mathcal{X} \to (\mathit{Sch}/S)_{fppf}$ and $\mathcal{Z} \to (\mathit{Sch}/S)_{fppf}$ are limit preserving on objects and $\mathcal{Y}$ is limit preserving, then $\mathcal{X} \times _\mathcal {Y} \mathcal{Z} \to (\mathit{Sch}/S)_{fppf}$ is limit preserving on objects.

  2. If $\mathcal{X}$, $\mathcal{Y}$, and $\mathcal{Z}$ are limit preserving, then so is $\mathcal{X} \times _\mathcal {Y} \mathcal{Z}$.

Proof. This is formal. Proof of (1). Let $T = \mathop{\mathrm{lim}}\nolimits _{i \in I} T_ i$ be the directed limit of affine schemes $T_ i$ over $S$. We will prove that the functor $\mathop{\mathrm{colim}}\nolimits \mathcal{X}_{T_ i} \to \mathcal{X}_ T$ is essentially surjective. Recall that an object of the fibre product over $T$ is a quadruple $(T, x, z, \alpha )$ where $x$ is an object of $\mathcal{X}$ lying over $T$, $z$ is an object of $\mathcal{Z}$ lying over $T$, and $\alpha : p(x) \to q(z)$ is a morphism in the fibre category of $\mathcal{Y}$ over $T$. By assumption on $\mathcal{X}$ and $\mathcal{Z}$ we can find an $i$ and objects $x_ i$ and $z_ i$ over $T_ i$ such that $x_ i|_ T \cong T$ and $z_ i|_ T \cong z$. Then $\alpha $ corresponds to an isomorphism $p(x_ i)|_ T \to q(z_ i)|_ T$ which comes from an isomorphism $\alpha _{i'} : p(x_ i)|_{T_{i'}} \to q(z_ i)|_{T_{i'}}$ by our assumption on $\mathcal{Y}$. After replacing $i$ by $i'$, $x_ i$ by $x_ i|_{T_{i'}}$, and $z_ i$ by $z_ i|_{T_{i'}}$ we see that $(T_ i, x_ i, z_ i, \alpha _ i)$ is an object of the fibre product over $T_ i$ which restricts to an object isomorphic to $(T, x, z, \alpha )$ over $T$ as desired.

We omit the arguments showing that $\mathop{\mathrm{colim}}\nolimits \mathcal{X}_{T_ i} \to \mathcal{X}_ T$ is fully faithful in (2). $\square$


Comments (0)

There are also:

  • 2 comment(s) on Section 98.11: Limit preserving

Post a comment

Your email address will not be published. Required fields are marked.

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).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.




In order to prevent bots from posting comments, we would like you to prove that you are human. You can do this by filling in the name of the current tag in the following input field. As a reminder, this is tag 07XM. Beware of the difference between the letter 'O' and the digit '0'.