Lemma 72.11.7 (0E05)—The Stacks project

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Table of contents
Part 4: Algebraic Spaces
Chapter 72: Algebraic Spaces over Fields
Section 72.11: Geometrically reduced algebraic spaces
Lemma 72.11.7 (cite)

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Lemma 72.11.7. Let $k$ be a field. Let $f : X \to Y$ be a morphism of algebraic spaces over $k$ . Let $x \in |X|$ be a point with image $y \in |Y|$ .

if $f$ is étale at $x$ , then $X$ is geometrically reduced at $x$ $\Leftrightarrow$ $Y$ is geometrically reduced at $y$ ,
if $f$ is surjective étale, then $X$ is geometrically reduced $\Leftrightarrow$ $Y$ is geometrically reduced.

Proof. Part (1) is clear because $\mathcal{O}_{X, \overline{x}} = \mathcal{O}_{Y, \overline{y}}$ if $f$ is étale at $x$ . Part (2) follows immediately from part (1). $\square$

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