[IV Theorem 11.3.1, EGA]

Theorem 37.15.1. Let $S$ be a scheme. Let $f : X \to S$ be a morphism which is locally of finite presentation. Let $\mathcal{F}$ be a quasi-coherent $\mathcal{O}_ X$-module which is locally of finite presentation. Then

$U = \{ x \in X \mid \mathcal{F}\text{ is flat over }S\text{ at }x\}$

is open in $X$.

Proof. We may test for openness locally on $X$ hence we may assume that $f$ is a morphism of affine schemes. In this case the theorem is exactly Algebra, Theorem 10.129.4. $\square$

Comment #2691 by on

A reference is EGA IV_3, Theorem 11.3.1

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