Lemma 67.31.4 (05VX)—The Stacks project

Lemma 67.31.4. Let $S$ be a scheme. Let $X \to Y \to Z$ be morphisms of algebraic spaces over $S$ . Let $\mathcal{F}$ be a quasi-coherent sheaf on $X$ . Let $x \in |X|$ with image $y \in |Y|$ .

If $\mathcal{F}$ is flat at $x$ over $Y$ and $Y$ is flat at $y$ over $Z$ , then $\mathcal{F}$ is flat at $x$ over $Z$ .
Let $x : \mathop{\mathrm{Spec}}(K) \to X$ be a representative of $x$ . If
1. $\mathcal{F}$ is flat at $x$ over $Y$ ,
2. $x^*\mathcal{F} \not= 0$ , and
3. $\mathcal{F}$ is flat at $x$ over $Z$ ,
then $Y$ is flat at $y$ over $Z$ .
Let $\overline{x}$ be a geometric point of $X$ lying over $x$ with image $\overline{y}$ in $Y$ . If $\mathcal{F}_{\overline{x}}$ is a faithfully flat $\mathcal{O}_{Y, \overline{y}}$ -module and $\mathcal{F}$ is flat at $x$ over $Z$ , then $Y$ is flat at $y$ over $Z$ .

Proof. Pick $\overline{x}$ and $\overline{y}$ as in part (3) and denote $\overline{z}$ the induced geometric point of $Z$ . Via the characterization of flatness in Lemmas 67.31.1 and 67.30.8 the lemma reduces to a purely algebraic question on the local ring map $\mathcal{O}_{Z, \overline{z}} \to \mathcal{O}_{Y, \overline{y}}$ and the module $\mathcal{F}_{\overline{x}}$ . Part (1) follows from Algebra, Lemma 10.39.4. We remark that condition (2)(b) guarantees that $\mathcal{F}_{\overline{x}}/ \mathfrak m_{\overline{y}} \mathcal{F}_{\overline{x}}$ is nonzero. Hence (2)(a) $+$ (2)(b) imply that $\mathcal{F}_{\overline{x}}$ is a faithfully flat $\mathcal{O}_{Y, \overline{y}}$ -module, see Algebra, Lemma 10.39.15. Thus (2) is a special case of (3). Finally, (3) follows from Algebra, Lemma 10.39.10. $\square$

The Stacks project

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