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The Stacks project

Lemma 21.18.6. Let (\mathcal{C}, \mathcal{O}) be a ringed site. Let \mathcal{K}^\bullet be a complex of \mathcal{O}-modules.

  1. If \mathcal{K}^\bullet is K-flat, then for every point p of the site \mathcal{C} the complex of \mathcal{O}_ p-modules \mathcal{K}_ p^\bullet is K-flat in the sense of More on Algebra, Definition 15.59.1

  2. If \mathcal{C} has enough points, then the converse is true.

Proof. Proof of (2). If \mathcal{C} has enough points and \mathcal{K}_ p^\bullet is K-flat for all points p of \mathcal{C} 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 on Sites, Lemma 18.14.4.

Proof of (1). Assume \mathcal{K}^\bullet is K-flat. Choose a quasi-isomorphism a : \mathcal{L}^\bullet \to \mathcal{K}^\bullet such that \mathcal{L}^\bullet is K-flat with flat terms, see Lemma 21.17.11. Any pullback of \mathcal{L}^\bullet is K-flat, see Lemma 21.18.1. In particular the stalk \mathcal{L}_ p^\bullet is a K-flat complex of \mathcal{O}_ p-modules. Thus the cone C(a) on a is a K-flat (Lemma 21.17.6) acyclic complex of \mathcal{O}-modules and it suffuces to show the stalk of C(a) is K-flat (by More on Algebra, Lemma 15.59.5). Thus we may assume that \mathcal{K}^\bullet is K-flat and acyclic.

Assume \mathcal{K}^\bullet is acyclic and K-flat. Before continuing we replace the site \mathcal{C} by another one as in Sites, Lemma 7.29.5 to insure that \mathcal{C} has all finite limits. This implies the category of neighbourhoods of p is filtered (Sites, Lemma 7.33.2) and the colimit defining the stalk of a sheaf is filtered. Let M be a finitely presented \mathcal{O}_ p-module. It suffices to show that \mathcal{K}^\bullet \otimes _{\mathcal{O}_ p} M is acyclic, see More on Algebra, Lemma 15.59.9. Since \mathcal{O}_ p is the filtered colimit of \mathcal{O}(U) where U runs over the neighbourhoods of p, we can find a neighbourhood (U, x) of p and a finitely presented \mathcal{O}(U)-module M' whose base change to \mathcal{O}_ p is M, see Algebra, Lemma 10.127.6. By Lemma 21.17.4 we may replace \mathcal{C}, \mathcal{O}, \mathcal{K}^\bullet by \mathcal{C}/U, \mathcal{O}_ U, \mathcal{K}^\bullet |_ U. We conclude that we may assume there exists an \mathcal{O}-module \mathcal{F} such that M \cong \mathcal{F}_ p. Since \mathcal{K}^\bullet is K-flat and acyclic, we see that \mathcal{K}^\bullet \otimes _\mathcal {O} \mathcal{F} is acyclic (as it computes the derived tensor product by definition). Taking stalks is an exact functor, hence we get that \mathcal{K}^\bullet \otimes _{\mathcal{O}_ p} M is acyclic as desired. \square


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