Lemma 37.16.6. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of schemes over $S$. Assume

$X$ is locally of finite presentation over $S$,

$X$ is flat over $S$, and

$Y$ is locally of finite type over $S$.

Then the set

is open in $X$ and its formation commutes with arbitrary base change.

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