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The Stacks project

Lemma 37.41.6. Let f : X \to S be a morphism of schemes. Let s \in S. Assume that

  1. f is locally of finite type,

  2. f is separated, and

  3. X_ s has at most finitely many isolated points.

Then there exists an elementary étale neighbourhood (U, u) \to (S, s) and a decomposition

U \times _ S X = W \amalg V

into open and closed subschemes such that the morphism V \to U is finite, and the fibre W_ u of the morphism W \to U contains no isolated points. In particular, if f^{-1}(s) is a finite set, then W_ u = \emptyset .

Proof. This is clear from Lemma 37.41.4 by choosing x_1, \ldots , x_ n the complete set of isolated points of X_ s and setting V = \bigcup V_ i. \square


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