Lemma 100.32.1. Let $Y$ be an algebraic space. Let $(U, R, s, t, c)$ be a groupoid in algebraic spaces over $Y$. Assume $U \to Y$ is flat and locally of finite presentation and $R \to U \times _ Y U$ an open immersion. Then $X = [U/R] = U/R$ is an algebraic space and $X \to Y$ is étale.
Proof. The quotient stack $[U/R]$ is an algebraic stacks by Criteria for Representability, Theorem 96.17.2. On the other hand, since $R \to U \times U$ is a monomorphism, it is an algebraic space (by our abuse of language and Algebraic Stacks, Proposition 93.13.3) and of course it is equal to the algebraic space $U/R$ (of Bootstrap, Theorem 79.10.1). Since $U \to X$ is surjective, flat, and locally of finite presenation (Bootstrap, Lemma 79.11.6) we conclude that $X \to Y$ is flat and locally of finite presentation by Morphisms of Spaces, Lemma 66.31.5 and Descent on Spaces, Lemma 73.8.2. Finally, consider the cartesian diagram
\[ \xymatrix{ R \ar[d] \ar[r] & U \times _ Y U \ar[d] \\ X \ar[r] & X \times _ Y X } \]
Since the right vertical arrow is surjective, flat, and locally of finite presentation (small detail omitted), we find that $X \to X \times _ Y X$ is an open immersion as the top horizonal arrow has this property by assumption (use Properties of Stacks, Lemma 99.3.3). Thus $X \to Y$ is unramified by Morphisms of Spaces, Lemma 66.38.9. We conclude by Morphisms of Spaces, Lemma 66.39.12. $\square$
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