The Stacks project

Lemma 91.12.1. In the situation above.

  1. There exists an $\mathcal{O}'$-module $\mathcal{F}'$ flat over $\mathcal{O}_{\mathcal{B}'}$ with $i^*\mathcal{F}' \cong \mathcal{F}$, if and only if the class $o(\mathcal{F}, f^*\mathcal{J} \otimes _\mathcal {O} \mathcal{F}, 1) \in \mathop{\mathrm{Ext}}\nolimits ^2_\mathcal {O}( \mathcal{F}, f^*\mathcal{J} \otimes _\mathcal {O} \mathcal{F})$ of Lemma 91.10.4 is zero.

  2. If such a module exists, then the set of isomorphism classes of lifts is principal homogeneous under $\mathop{\mathrm{Ext}}\nolimits ^1_\mathcal {O}( \mathcal{F}, f^*\mathcal{J} \otimes _\mathcal {O} \mathcal{F})$.

  3. Given a lift $\mathcal{F}'$, the set of automorphisms of $\mathcal{F}'$ which pull back to $\text{id}_\mathcal {F}$ is canonically isomorphic to $\mathop{\mathrm{Ext}}\nolimits ^0_\mathcal {O}( \mathcal{F}, f^*\mathcal{J} \otimes _\mathcal {O} \mathcal{F})$.

Proof. Part (1) follows from Lemma 91.11.8 as we have seen above that $\mathcal{I} = f^*\mathcal{J}$. Part (2) follows from Lemma 91.11.7. Part (3) follows from Lemma 91.11.4. $\square$

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