Lemma 35.25.1. 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 (Schemes, Lemma 26.23.2). This implies that the base change of $f$ by $f$ is an isomorphism. Hence we see $f$ is an isomorphism by Lemma 35.23.17.
$\square$

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