Lemma 30.21.4. (For a more general version see More on Morphisms, Lemma 37.50.3.) Let f : X \to Y be a proper morphism of schemes with Y Noetherian. Let \mathcal{L} be an invertible \mathcal{O}_ X-module. Let y \in Y be a point such that \mathcal{L}_ y is ample on X_ y. Then there is an open neighbourhood V \subset Y of y such that \mathcal{L}|_{f^{-1}(V)} is ample on f^{-1}(V)/V.
Proof. Pick d_0 as in Lemma 30.21.3 for \mathcal{F} = \mathcal{O}_ X. Pick d \geq d_0 so that we can find r \geq 0 and sections s_{y, 0}, \ldots , s_{y, r} \in H^0(X_ y, \mathcal{L}_ y^{\otimes d}) which define a closed immersion
This is possible by Morphisms, Lemma 29.39.4 but we also use Morphisms, Lemma 29.41.7 to see that \varphi _ y is a closed immersion and Constructions, Section 27.13 for the description of morphisms into projective space in terms of invertible sheaves and sections. By our choice of d_0, after replacing Y by an open neighbourhood of y, we can choose s_0, \ldots , s_ r \in H^0(X, \mathcal{L}^{\otimes d}) mapping to s_{y, 0}, \ldots , s_{y, r}. Let X_{s_ i} \subset X be the open subset where s_ i is a generator of \mathcal{L}^{\otimes d}. Since the s_{y, i} generate \mathcal{L}_ y^{\otimes d} we see that X_ y \subset U = \bigcup X_{s_ i}. Since X \to Y is closed, we see that there is an open neighbourhood y \in V \subset Y such that f^{-1}(V) \subset U. After replacing Y by V we may assume that the s_ i generate \mathcal{L}^{\otimes d}. Thus we obtain a morphism
with \mathcal{L}^{\otimes d} \cong \varphi ^*\mathcal{O}_{\mathbf{P}^ r_ Y}(1) whose base change to y gives \varphi _ y.
We will finish the proof by a sleight of hand; the “correct” proof proceeds by directly showing that \varphi is a closed immersion after base changing to an open neighbourhood of y. Namely, by Lemma 30.21.2 we see that \varphi is a finite over an open neighbourhood of the fibre \mathbf{P}^ r_{\kappa (y)} of \mathbf{P}^ r_ Y \to Y above y. Using that \mathbf{P}^ r_ Y \to Y is closed, after shrinking Y we may assume that \varphi is finite. Then \mathcal{L}^{\otimes d} \cong \varphi ^*\mathcal{O}_{\mathbf{P}^ r_ Y}(1) is ample by the very general Morphisms, Lemma 29.37.7. \square
Comments (0)