The Stacks project

Lemma 35.25.6. Let $f : X \to S$ be a morphism of schemes. Let $\mathcal{L}$ be an invertible $\mathcal{O}_ X$-module. Let $\{ g_ i : S_ i \to S\} _{i \in I}$ be an fpqc covering. Let $f_ i : X_ i \to S_ i$ be the base change of $f$ and let $\mathcal{L}_ i$ be the pullback of $\mathcal{L}$ to $X_ i$. The following are equivalent

  1. $\mathcal{L}$ is ample on $X/S$, and

  2. $\mathcal{L}_ i$ is ample on $X_ i/S_ i$ for every $i \in I$.

Proof. The implication (1) $\Rightarrow $ (2) follows from Morphisms, Lemma 29.37.9. Assume $\mathcal{L}_ i$ is ample on $X_ i/S_ i$ for every $i \in I$. By Morphisms, Definition 29.37.1 this implies that $X_ i \to S_ i$ is quasi-compact and by Morphisms, Lemma 29.37.3 this implies $X_ i \to S$ is separated. Hence $f$ is quasi-compact and separated by Lemmas 35.23.1 and 35.23.6.

This means that $\mathcal{A} = \bigoplus _{d \geq 0} f_*\mathcal{L}^{\otimes d}$ is a quasi-coherent graded $\mathcal{O}_ S$-algebra (Schemes, Lemma 26.24.1). Moreover, the formation of $\mathcal{A}$ commutes with flat base change by Cohomology of Schemes, Lemma 30.5.2. In particular, if we set $\mathcal{A}_ i = \bigoplus _{d \geq 0} f_{i, *}\mathcal{L}_ i^{\otimes d}$ then we have $\mathcal{A}_ i = g_ i^*\mathcal{A}$. It follows that the natural maps $\psi _ d : f^*\mathcal{A}_ d \to \mathcal{L}^{\otimes d}$ of $\mathcal{O}_ X$ pullback to give the natural maps $\psi _{i, d} : f_ i^*(\mathcal{A}_ i)_ d \to \mathcal{L}_ i^{\otimes d}$ of $\mathcal{O}_{X_ i}$-modules. Since $\mathcal{L}_ i$ is ample on $X_ i/S_ i$ we see that for any point $x_ i \in X_ i$, there exists a $d \geq 1$ such that $f_ i^*(\mathcal{A}_ i)_ d \to \mathcal{L}_ i^{\otimes d}$ is surjective on stalks at $x_ i$. This follows either directly from the definition of a relatively ample module or from Morphisms, Lemma 29.37.4. If $x \in X$, then we can choose an $i$ and an $x_ i \in X_ i$ mapping to $x$. Since $\mathcal{O}_{X, x} \to \mathcal{O}_{X_ i, x_ i}$ is flat hence faithfully flat, we conclude that for every $x \in X$ there exists a $d \geq 1$ such that $f^*\mathcal{A}_ d \to \mathcal{L}^{\otimes d}$ is surjective on stalks at $x$. This implies that the open subset $U(\psi ) \subset X$ of Constructions, Lemma 27.19.1 corresponding to the map $\psi : f^*\mathcal{A} \to \bigoplus _{d \geq 0} \mathcal{L}^{\otimes d}$ of graded $\mathcal{O}_ X$-algebras is equal to $X$. Consider the corresponding morphism

\[ r_{\mathcal{L}, \psi } : X \longrightarrow \underline{\text{Proj}}_ S(\mathcal{A}) \]

It is clear from the above that the base change of $r_{\mathcal{L}, \psi }$ to $S_ i$ is the morphism $r_{\mathcal{L}_ i, \psi _ i}$ which is an open immersion by Morphisms, Lemma 29.37.4. Hence $r_{\mathcal{L}, \psi }$ is an open immersion by Lemma 35.23.16 and we conclude $\mathcal{L}$ is ample on $X/S$ by Morphisms, Lemma 29.37.4. $\square$

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