Lemma 20.25.1. Let (X, \mathcal{O}_ X) be a ringed space. Let \mathcal{U} : X = \bigcup _{i \in I} U_ i be an open covering. For a bounded below complex \mathcal{F}^\bullet of \mathcal{O}_ X-modules there is a canonical map
\text{Tot}(\check{\mathcal{C}}^\bullet (\mathcal{U}, \mathcal{F}^\bullet )) \longrightarrow R\Gamma (X, \mathcal{F}^\bullet )
functorial in \mathcal{F}^\bullet and compatible with (20.25.0.1) and (20.25.0.2). There is a spectral sequence (E_ r, d_ r)_{r \geq 0} with
E_2^{p, q} = H^ p(\text{Tot}(\check{\mathcal{C}}^\bullet (\mathcal{U}, \underline{H}^ q(\mathcal{F}^\bullet )))
converging to H^{p + q}(X, \mathcal{F}^\bullet ).
Proof.
Let {\mathcal I}^\bullet be a bounded below complex of injectives. The map (20.25.0.1) for \mathcal{I}^\bullet is a map \Gamma (X, {\mathcal I}^\bullet ) \to \text{Tot}(\check{\mathcal{C}}^\bullet ({\mathcal U}, {\mathcal I}^\bullet )). This is a quasi-isomorphism of complexes of abelian groups as follows from Homology, Lemma 12.25.4 applied to the double complex \check{\mathcal{C}}^\bullet ({\mathcal U}, {\mathcal I}^\bullet ) using Lemma 20.11.1. Suppose {\mathcal F}^\bullet \to {\mathcal I}^\bullet is a quasi-isomorphism of {\mathcal F}^\bullet into a bounded below complex of injectives. Since R\Gamma (X, {\mathcal F}^\bullet ) is represented by the complex \Gamma (X, {\mathcal I}^\bullet ) we obtain the map of the lemma using
\text{Tot}(\check{\mathcal{C}}^\bullet ({\mathcal U}, {\mathcal F}^\bullet )) \longrightarrow \text{Tot}(\check{\mathcal{C}}^\bullet ({\mathcal U}, {\mathcal I}^\bullet )).
We omit the verification of functoriality and compatibilities. To construct the spectral sequence of the lemma, choose a Cartan-Eilenberg resolution \mathcal{F}^\bullet \to \mathcal{I}^{\bullet , \bullet }, see Derived Categories, Lemma 13.21.2. In this case \mathcal{F}^\bullet \to \text{Tot}(\mathcal{I}^{\bullet , \bullet }) is an injective resolution and hence
\text{Tot}(\check{\mathcal{C}}^\bullet ({\mathcal U}, \text{Tot}({\mathcal I}^{\bullet , \bullet })))
computes R\Gamma (X, \mathcal{F}^\bullet ) as we've seen above. By Homology, Remark 12.18.4 we can view this as the total complex associated to the triple complex \check{\mathcal{C}}^\bullet ({\mathcal U}, {\mathcal I}^{\bullet , \bullet }) hence, using the same remark we can view it as the total complex associate to the double complex A^{\bullet , \bullet } with terms
A^{n, m} = \bigoplus \nolimits _{p + q = n} \check{\mathcal{C}}^ p({\mathcal U}, \mathcal{I}^{q, m})
Since \mathcal{I}^{q, \bullet } is an injective resolution of \mathcal{F}^ q we can apply the first spectral sequence associated to A^{\bullet , \bullet } (Homology, Lemma 12.25.1) to get a spectral sequence with
E_1^{n, m} = \bigoplus \nolimits _{p + q = n} \check{\mathcal{C}}^ p(\mathcal{U}, \underline{H}^ m(\mathcal{F}^ q))
which is the nth term of the complex \text{Tot}(\check{\mathcal{C}}^\bullet (\mathcal{U}, \underline{H}^ m(\mathcal{F}^\bullet )). Hence we obtain E_2 terms as described in the lemma. Convergence by Homology, Lemma 12.25.3.
\square
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