Special case of [Theorem 3.2.4, BBD] without boundedness assumption.

Theorem 20.41.8 (BBD gluing lemma). In Situation 20.41.3 assume

1. $X = \bigcup _{U \in \mathcal{B}} U$,

2. for $U, V \in \mathcal{B}$ we have $U \cap V = \bigcup _{W \in \mathcal{B}, W \subset U \cap V} W$,

3. for any $U \in \mathcal{B}$ we have $\mathop{\mathrm{Ext}}\nolimits ^ i(K_ U, K_ U) = 0$ for $i < 0$.

Then there exists an object $K$ of $D(\mathcal{O}_ X)$ and isomorphisms $\rho _ U : K|_ U \to K_ U$ in $D(\mathcal{O}_ U)$ for $U \in \mathcal{B}$ such that $\rho ^ U_ V \circ \rho _ U|_ V = \rho _ V$ for all $V \subset U$ with $U, V \in \mathcal{B}$. The pair $(K, \rho _ U)$ is unique up to unique isomorphism.

Proof. A pair $(K, \rho _ U)$ is called a solution in the text above. The uniqueness follows from Lemma 20.41.4. If $X$ has a finite covering by elements of $\mathcal{B}$ (for example if $X$ is quasi-compact), then the theorem is a consequence of Lemma 20.41.6. In the general case we argue in exactly the same manner, using transfinite induction and Lemma 20.41.7.

First we use transfinite induction to choose opens $W_\alpha \subset X$ for any ordinal $\alpha$. Namely, we set $W_0 = \emptyset$. If $\alpha = \beta + 1$ is a sucessor, then either $W_\beta = X$ and we set $W_\alpha = X$ or $W_\beta \not= X$ and we set $W_\alpha = W_\beta \cup U_\alpha$ where $U_\alpha \in \mathcal{B}$ is not contained in $W_\beta$. If $\alpha$ is a limit ordinal we set $W_\alpha = \bigcup _{\beta < \alpha } W_\beta$. Then for large enough $\alpha$ we have $W_\alpha = X$. Observe that for every $\alpha$ the open $W_\alpha$ is a union of elements of $\mathcal{B}$. Hence if $\mathcal{B}_\alpha = \{ U \in \mathcal{B}, U \subset W_\alpha \}$, then

$S_\alpha = (\{ K_ U\} _{U \in \mathcal{B}_\alpha }, \{ \rho _ V^ U\} _{V \subset U\text{ with }U, V \in \mathcal{B}_\alpha })$

is a system as in Lemma 20.41.4 on the ringed space $W_\alpha$.

We will show by transfinite induction that for every $\alpha$ the system $S_\alpha$ has a solution. This will prove the theorem as this system is the system given in the theorem for large $\alpha$.

The case where $\alpha = \beta + 1$ is a successor ordinal. (This case was already treated in the proof of the lemma above but for clarity we repeat the argument.) Recall that $W_\alpha = W_\beta \cup U_\alpha$ for some $U_\alpha \in \mathcal{B}$ in this case. By induction hypothesis we have a solution $(K_{W_\beta }, \{ \rho ^{W_\beta }_ U\} _{U \in \mathcal{B}_\beta })$ for the system $S_\beta$. Then we can consider the collection $\mathcal{B}_\alpha ^* = \mathcal{B}_\alpha \cup \{ W_\beta \}$ of opens of $W_\alpha$ and we see that we obtain a system $(\{ K_ U\} _{U \in \mathcal{B}_\alpha ^*}, \{ \rho _ V^ U\} _{V \subset U\text{ with }U, V \in \mathcal{B}_\alpha ^*})$. Note that this new system also satisfies condition (3) by Lemma 20.41.4 applied to the solution $K_{W_\beta }$. For this system we have $W_\alpha = W_\beta \cup U_\alpha$. This reduces us to the case handled in Lemma 20.41.6.

The case where $\alpha$ is a limit ordinal. Recall that $W_\alpha = \bigcup _{\beta < \alpha } W_\beta$ in this case. For $\beta < \alpha$ let $(K_{W_\beta }, \{ \rho ^{W_\beta }_ U\} _{U \in \mathcal{B}_\beta })$ be the solution for $S_\beta$. For $\gamma < \beta < \alpha$ the restriction $K_{W_\beta }|_{W_\gamma }$ endowed with the maps $\rho ^{W_\beta }_ U$, $U \in \mathcal{B}_\gamma$ is a solution for $S_\gamma$. By uniqueness we get unique isomorphisms $\rho _{W_\gamma }^{W_\beta } : K_{W_\beta }|_{W_\gamma } \to K_{W_\gamma }$ compatible with the maps $\rho ^{W_\beta }_ U$ and $\rho ^{W_\gamma }_ U$ for $U \in \mathcal{B}_\gamma$. These maps compose in the correct manner, i.e., $\rho _{W_\delta }^{W_\gamma } \circ \rho _{W_\gamma }^{W_\beta }|_{W_\delta } = \rho ^{W_\delta }_{W_\beta }$ for $\delta < \gamma < \beta < \alpha$. Thus we may apply Lemma 20.41.7 (note that the vanishing of negative exts is true for $K_{W_\beta }$ by Lemma 20.41.4 applied to the solution $K_{W_\beta }$) to obtain $K_{W_\alpha }$ and isomorphisms

$\rho _{W_\beta }^{W_\alpha } : K_{W_\alpha }|_{W_\beta } \longrightarrow K_{W_\beta }$

compatible with the maps $\rho _{W_\gamma }^{W_\beta }$ for $\gamma < \beta < \alpha$.

To show that $K_{W_\alpha }$ is a solution we still need to construct the isomorphisms $\rho _ U^{W_\alpha } : K_{W_\alpha }|_ U \to K_ U$ for $U \in \mathcal{B}_\alpha$ satisfying certain compatibilities. We choose $\rho _ U^{W_\alpha }$ to be the unique map such that for any $\beta < \alpha$ and any $V \in \mathcal{B}_\beta$ with $V \subset U$ the diagram

$\xymatrix{ K_{W_\alpha }|_ V \ar[r]_{\rho _ U^{W_\alpha }|_ V} \ar[d]_{\rho _{W_\beta }^{W_\alpha }|_ V} & K_ U|_ V \ar[d]^{\rho _ U^ V} \\ K_{W_\beta } \ar[r]^{\rho _ V^{W_\beta }} & K_ V }$

commutes. This makes sense because

$(\{ K_ V\} _{V \subset U, V \in \mathcal{B}_\beta \text{ for some }\beta < \alpha }, \{ \rho _ V^{V'}\} _{V \subset V'\text{ with }V, V' \subset U \text{ and }V, V' \in \mathcal{B}_\beta \text{ for some }\beta < \alpha })$

is a system as in Lemma 20.41.4 on the ringed space $U$ and because $(K_ U, \rho ^ U_ V)$ and $(K_{W_\alpha }|_ U, \rho _ V^{W_\beta }\circ \rho _{W_\beta }^{W_\alpha }|_ V)$ are both solutions for this system. This gives existence and uniqueness. We omit the proof that these maps satisfy the desired compatibilities (it is just bookkeeping). $\square$

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