The Stacks project

Lemma 33.30.2. Let $A \to B$ be a faithfully flat ring map. Let $X$ be a scheme over $A$ such that

  1. $X$ is quasi-compact and quasi-separated, and

  2. $R = H^0(X, \mathcal{O}_ X)$ is a semi-local ring.

Then the pullback map $\mathop{\mathrm{Pic}}\nolimits (X) \to \mathop{\mathrm{Pic}}\nolimits (X_ B)$ is injective.

Proof. Let $\mathcal{L}$ be an invertible $\mathcal{O}_ X$-module whose pullback $\mathcal{L}'$ to $X_ B$ is trivial. Set $M = H^0(X, \mathcal{L})$ and $N = H^0(X, \mathcal{L}^{\otimes - 1})$. By Lemma 33.30.1 the $R$-modules $M$ and $N$ are invertible. Since $R$ is semi-local $M \cong R$ and $N \cong R$, see Algebra, Lemma 10.78.7. Choose generators $s \in M$ and $t \in N$. Then $st \in R = H^0(X, \mathcal{O}_ X)$ is a unit by the last part of Lemma 33.30.1. We conclude that $s$ and $t$ define trivializations of $\mathcal{L}$ and $\mathcal{L}^{\otimes -1}$ over $X$. $\square$

Comments (0)

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Your email address will not be published.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 0CDY. Beware of the difference between the letter 'O' and the digit '0'.