The Stacks project

Remark 37.35.3. In terms of topologies Lemmas 37.35.1 and 37.35.2 mean the following. Let $S$ be any scheme. Let $\{ f_ i : U_ i \to S\} $ be an étale covering of $S$. There exists a Zariski open covering $S = \bigcup V_ j$, for each $j$ a finite locally free, surjective morphism $W_ j \to V_ j$, and for each $j$ a Zariski open covering $\{ W_{j, k} \to W_ j\} $ such that the family $\{ W_{j, k} \to S\} $ refines the given étale covering $\{ f_ i : U_ i \to S\} $. What does this mean in practice? Well, for example, suppose we have a descent problem which we know how to solve for Zariski coverings and for fppf coverings of the form $\{ \pi : T \to S\} $ with $\pi $ finite locally free and surjective. Then this descent problem has an affirmative answer for étale coverings as well. This trick was used by Gabber in his proof that $\text{Br}(X) = \text{Br}'(X)$ for an affine scheme $X$, see [Hoobler].


Comments (1)

Comment #418 by on

It's a question of English language (and I'm by no means an expert), but I think "descend problem" should be "descent problem".


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