Lemma 20.31.3. Let $(X, \mathcal{O}_ X)$ be a ringed space. Let $X = U \cup V$ be the union of two open subspaces of $X$. For objects $E$, $F$ of $D(\mathcal{O}_ X)$ we have a Mayer-Vietoris sequence

$\xymatrix{ & \ldots \ar[r] & \mathop{\mathrm{Ext}}\nolimits ^{-1}(E_{U \cap V}, F_{U \cap V}) \ar[lld] \\ \mathop{\mathrm{Hom}}\nolimits (E, F) \ar[r] & \mathop{\mathrm{Hom}}\nolimits (E_ U, F_ U) \oplus \mathop{\mathrm{Hom}}\nolimits (E_ V, F_ V) \ar[r] & \mathop{\mathrm{Hom}}\nolimits (E_{U \cap V}, F_{U \cap V}) }$

where the subscripts denote restrictions to the relevant opens and the $\mathop{\mathrm{Hom}}\nolimits$'s and $\mathop{\mathrm{Ext}}\nolimits$'s are taken in the relevant derived categories.

Proof. Use the distinguished triangle of Lemma 20.31.1 to obtain a long exact sequence of $\mathop{\mathrm{Hom}}\nolimits$'s (from Derived Categories, Lemma 13.4.2) and use that

$\mathop{\mathrm{Hom}}\nolimits _{D(\mathcal{O}_ X)}(j_{U!}E|_ U, F) = \mathop{\mathrm{Hom}}\nolimits _{D(\mathcal{O}_ U)}(E|_ U, F|_ U)$

by Lemma 20.30.8. $\square$

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