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. $\square$

In your comment you can use Markdown and LaTeX style mathematics (enclose it like $\pi$). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).