The Stacks project

Lemma 69.11.1. Let $S$ be a scheme. Let $f : X \to Y$ be an affine morphism of algebraic spaces over $S$. If $Y$ quasi-compact and quasi-separated, then $X$ is a directed limit $X = \mathop{\mathrm{lim}}\nolimits X_ i$ with each $X_ i$ affine and of finite presentation over $Y$.

Proof. Consider the quasi-coherent $\mathcal{O}_ Y$-module $\mathcal{A} = f_*\mathcal{O}_ X$. By Lemma 69.9.4 we can write $\mathcal{A} = \mathop{\mathrm{colim}}\nolimits \mathcal{A}_ i$ as a directed colimit of finitely presented $\mathcal{O}_ Y$-algebras $\mathcal{A}_ i$. Set $X_ i = \underline{\mathop{\mathrm{Spec}}}_ Y(\mathcal{A}_ i)$, see Morphisms of Spaces, Definition 66.20.8. By construction $X_ i \to Y$ is affine and of finite presentation and $X = \mathop{\mathrm{lim}}\nolimits X_ i$. $\square$

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