The Stacks project

Lemma 75.14.8. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of algebraic spaces over $S$. The following are equivalent:

  1. $f$ is unramified and a monomorphism,

  2. $f$ is unramified and universally injective,

  3. $f$ is locally of finite type and a monomorphism,

  4. $f$ is universally injective, locally of finite type, and formally unramified.

Moreover, in this case $f$ is also representable, separated, and locally quasi-finite.

Proof. We have seen in Lemma 75.14.7 that being formally unramified and locally of finite type is the same thing as being unramified. Hence (4) is equivalent to (2). A monomorphism is certainly formally unramified hence (3) implies (4). It is clear that (1) implies (3). Finally, if (2) holds, then $\Delta : X \to X \times _ Y X$ is both an open immersion (Morphisms of Spaces, Lemma 66.38.9) and surjective (Morphisms of Spaces, Lemma 66.19.2) hence an isomorphism, i.e., $f$ is a monomorphism. In this way we see that (2) implies (1). Finally, we see that $f$ is representable, separated, and locally quasi-finite by Morphisms of Spaces, Lemmas 66.27.10 and 66.51.1. $\square$


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