Theorem 41.10.1. Let Y be a locally Noetherian scheme. Let f : X \to Y be a morphism which is locally of finite type. Let \mathcal{F} be a coherent \mathcal{O}_ X-module. The set of points in X where \mathcal{F} is flat over Y is an open set. In particular the set of points where f is flat is open in X.
Proof. See More on Morphisms, Theorem 37.15.1. \square
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