Lemma 20.41.7. Let $X$ be a ringed space. Let $E$ be a well ordered set and let

$X = \bigcup \nolimits _{\alpha \in E} W_\alpha$

be an open covering with $W_\alpha \subset W_{\alpha + 1}$ and $W_\alpha = \bigcup _{\beta < \alpha } W_\beta$ if $\alpha$ is not a successor. Let $K_\alpha$ be an object of $D(\mathcal{O}_{W_\alpha })$ with $\mathop{\mathrm{Ext}}\nolimits ^ i(K_\alpha , K_\alpha ) = 0$ for $i < 0$. Assume given isomorphisms $\rho _\beta ^\alpha : K_\alpha |_{W_\beta } \to K_\beta$ in $D(\mathcal{O}_{W_\beta })$ for all $\beta < \alpha$ with $\rho _\gamma ^\alpha = \rho _\gamma ^\beta \circ \rho ^\alpha _\beta |_{W_\gamma }$ for $\gamma < \beta < \alpha$. Then there exists an object $K$ in $D(\mathcal{O}_ X)$ and isomorphisms $K|_{W_\alpha } \to K_\alpha$ for $\alpha \in E$ compatible with the isomorphisms $\rho _\beta ^\alpha$.

Proof. In this proof $\alpha , \beta , \gamma , \ldots$ represent elements of $E$. Choose a K-injective complex $I_\alpha ^\bullet$ on $W_\alpha$ representing $K_\alpha$. For $\beta < \alpha$ denote $j_{\beta , \alpha } : W_\beta \to W_\alpha$ the inclusion morphism. By transfinite induction, we will construct for all $\beta < \alpha$ a map of complexes

$\tau _{\beta , \alpha } : (j_{\beta , \alpha })_!I_\beta ^\bullet \longrightarrow I_\alpha ^\bullet$

representing the adjoint to the inverse of the isomorphism $\rho ^\alpha _\beta : K_\alpha |_{W_\beta } \to K_\beta$. Moreover, we will do this in such that for $\gamma < \beta < \alpha$ we have

$\tau _{\gamma , \alpha } = \tau _{\beta , \alpha } \circ (j_{\beta , \alpha })_!\tau _{\gamma , \beta }$

as maps of complexes. Namely, suppose already given $\tau _{\gamma , \beta }$ composing correctly for all $\gamma < \beta < \alpha$. If $\alpha = \alpha ' + 1$ is a successor, then we choose any map of complexes

$(j_{\alpha ', \alpha })_!I_{\alpha '}^\bullet \to I_\alpha ^\bullet$

which is adjoint to the inverse of the isomorphism $\rho ^\alpha _{\alpha '} : K_\alpha |_{W_{\alpha '}} \to K_{\alpha '}$ (possible because $I_\alpha ^\bullet$ is K-injective) and for any $\beta < \alpha '$ we set

$\tau _{\beta , \alpha } = \tau _{\alpha ', \alpha } \circ (j_{\alpha ', \alpha })_!\tau _{\beta , \alpha '}$

If $\alpha$ is not a successor, then we can consider the complex on $W_\alpha$ given by

$C^\bullet = \mathop{\mathrm{colim}}\nolimits _{\beta < \alpha } (j_{\beta , \alpha })_!I_\beta ^\bullet$

(termwise colimit) where the transition maps of the sequence are given by the maps $\tau _{\beta ', \beta }$ for $\beta ' < \beta < \alpha$. We claim that $C^\bullet$ represents $K_\alpha$. Namely, for $\beta < \alpha$ the restriction of the coprojection $(j_{\beta , \alpha })_!I_\beta ^\bullet \to C^\bullet$ gives a map

$\sigma _\beta : I_\beta ^\bullet \longrightarrow C^\bullet |_{W_\beta }$

which is a quasi-isomorphism: if $x \in W_\beta$ then looking at stalks we get

$(C^\bullet )_ x = \mathop{\mathrm{colim}}\nolimits _{\beta ' < \alpha } \left((j_{\beta ', \alpha })_!I_{\beta '}^\bullet \right)_ x = \mathop{\mathrm{colim}}\nolimits _{\beta \leq \beta ' < \alpha } (I_{\beta '}^\bullet )_ x \longleftarrow (I_\beta ^\bullet )_ x$

which is a quasi-isomorphism. Here we used that taking stalks commutes with colimits, that filtered colimits are exact, and that the maps $(I_\beta ^\bullet )_ x \to (I_{\beta '}^\bullet )_ x$ are quasi-isomorphisms for $\beta \leq \beta ' < \alpha$. Hence $(C^\bullet , \sigma _\beta ^{-1})$ is a solution to the system $(\{ K_\beta \} _{\beta < \alpha }, \{ \rho ^\beta _{\beta '}\} _{\beta ' < \beta < \alpha })$. Since $(K_\alpha , \rho ^\alpha _\beta )$ is another solution we obtain a unique isomorphism $\sigma : K_\alpha \to C^\bullet$ in $D(\mathcal{O}_{W_\alpha })$ compatible with all our maps, see Lemma 20.41.6 (this is where we use the vanishing of negative ext groups). Choose a morphism $\tau : C^\bullet \to I_\alpha ^\bullet$ of complexes representing $\sigma$. Then we set

$\tau _{\beta , \alpha } = \tau |_{W_\beta } \circ \sigma _\beta$

to get the desired maps. Finally, we take $K$ to be the object of the derived category represented by the complex

$K^\bullet = \mathop{\mathrm{colim}}\nolimits _{\alpha \in E} (W_\alpha \to X)_!I_\alpha ^\bullet$

where the transition maps are given by our carefully constructed maps $\tau _{\beta , \alpha }$ for $\beta < \alpha$. Arguing exactly as above we see that for all $\alpha$ the restriction of the coprojection determines an isomorphism

$K|_{W_\alpha } \longrightarrow K_\alpha$

compatible with the given maps $\rho ^\alpha _\beta$. $\square$

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