Theorem 76.22.1. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of algebraic spaces over $S$. Let $\mathcal{F}$ be a quasi-coherent sheaf on $X$. Assume $f$ is locally of finite presentation and that $\mathcal{F}$ is an $\mathcal{O}_ X$-module which is locally of finite presentation. Then
\[ \{ x \in |X| : \mathcal{F}\text{ is flat over }Y\text{ at }x\} \]
is open in $|X|$.
Proof.
Choose a commutative diagram
\[ \xymatrix{ U \ar[d]_ p \ar[r]_\alpha & V \ar[d]^ q \\ X \ar[r]^ a & Y } \]
with $U$, $V$ schemes and $p$, $q$ surjective and étale as in Spaces, Lemma 65.11.6. By More on Morphisms, Theorem 37.15.1 the set $U' = \{ u \in |U| : p^*\mathcal{F}\text{ is flat over }V\text{ at }u\} $ is open in $U$. By Morphisms of Spaces, Definition 67.31.2 the image of $U'$ in $|X|$ is the set of the theorem. Hence we are done because the map $|U| \to |X|$ is open, see Properties of Spaces, Lemma 66.4.6.
$\square$
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