Lemma 66.7.1. Let $S$ be a scheme. Let $X$ be an algebraic space over $S$. Let $\mathcal{P}$ be a property of schemes which is local in the étale topology, see Descent, Definition 35.15.1. The following are equivalent
for some scheme $U$ and surjective étale morphism $U \to X$ the scheme $U$ has property $\mathcal{P}$, and
for every scheme $U$ and every étale morphism $U \to X$ the scheme $U$ has property $\mathcal{P}$.
If $X$ is representable this is equivalent to $\mathcal{P}(X)$.
Proof. The implication (2) $\Rightarrow $ (1) is immediate. For the converse, choose a surjective étale morphism $U \to X$ with $U$ a scheme that has $\mathcal{P}$ and let $V$ be an étale $X$-scheme. Then $U \times _ X V \rightarrow V$ is an étale surjection of schemes, so $V$ inherits $\mathcal{P}$ from $U \times _ X V$, which in turn inherits $\mathcal{P}$ from $U$ (see discussion following Descent, Definition 35.15.1). The last claim is clear from (1) and Descent, Definition 35.15.1. $\square$
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