Lemma 35.22.1. Let $f : X \to Y$ be a flat, quasi-compact, surjective monomorphism. Then f is an isomorphism.

Proof. As $f$ is a flat, quasi-compact, surjective morphism we see $\{ X \to Y\}$ is an fpqc covering of $Y$. The diagonal $\Delta : X \to X \times _ Y X$ is an isomorphism. This implies that the base change of $f$ by $f$ is an isomorphism. Hence we see $f$ is an isomorphism by Lemma 35.20.17. $\square$

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