The Stacks project

Lemma 29.39.6. Let $f : X \to S$ be a morphism of schemes. Let $\mathcal{L}$ be an invertible sheaf on $X$. Assume $f$ is of finite type. The following are equivalent:

  1. $\mathcal{L}$ is $f$-relatively very ample, and

  2. there exist an open covering $S = \bigcup V_ j$, for each $j$ an integer $n_ j$, and immersions

    \[ i_ j : X_ j = f^{-1}(V_ j) = V_ j \times _ S X \longrightarrow \mathbf{P}^{n_ j}_{V_ j} \]

    over $V_ j$ such that $\mathcal{L}|_{X_ j} \cong i_ j^*\mathcal{O}_{\mathbf{P}^{n_ j}_{V_ j}}(1)$.

Proof. We see that (1) implies (2) by taking an affine open covering of $S$ and applying Lemma 29.39.1 to each of the restrictions of $f$ and $\mathcal{L}$. We see that (2) implies (1) by Lemma 29.38.7. $\square$

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