Lemma 20.26.4. Let $(X, \mathcal{O}_ X)$ be a ringed space. Let $\mathcal{K}^\bullet $ be a complex of $\mathcal{O}_ X$-modules. Then $\mathcal{K}^\bullet $ is K-flat if and only if for all $x \in X$ the complex $\mathcal{K}_ x^\bullet $ of $\mathcal{O}_{X, x}$-modules is K-flat (More on Algebra, Definition 15.58.3).

**Proof.**
If $\mathcal{K}_ x^\bullet $ is K-flat for all $x \in X$ then we see that $\mathcal{K}^\bullet $ is K-flat because $\otimes $ and direct sums commute with taking stalks and because we can check exactness at stalks, see Modules, Lemma 17.3.1. Conversely, assume $\mathcal{K}^\bullet $ is K-flat. Pick $x \in X$ $M^\bullet $ be an acyclic complex of $\mathcal{O}_{X, x}$-modules. Then $i_{x, *}M^\bullet $ is an acyclic complex of $\mathcal{O}_ X$-modules. Thus $\text{Tot}(i_{x, *}M^\bullet \otimes _{\mathcal{O}_ X} \mathcal{K}^\bullet )$ is acyclic. Taking stalks at $x$ shows that $\text{Tot}(M^\bullet \otimes _{\mathcal{O}_{X, x}} \mathcal{K}_ x^\bullet )$ is acyclic.
$\square$

## Post a comment

Your email address will not be published. Required fields are marked.

In your comment you can use Markdown and LaTeX style mathematics (enclose it like `$\pi$`

). A preview option is available if you wish to see how it works out (just click on the eye in the toolbar).

Unfortunately JavaScript is disabled in your browser, so the comment preview function will not work.

All contributions are licensed under the GNU Free Documentation License.

## Comments (2)

Comment #2030 by FĂ©lix BB on

Comment #2069 by Johan on