The paper proves a generalized Barsotti-Weil formula, vanishing of Ext^2 for abelian schemes over general bases, and a complete computation of Ext^1(Ga,Gm) over Q, correcting earlier claims.
Formal GAGA for gerbes
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abstract
Fix an $I$-adically complete Noetherian ring $A$ and suppose $X$ is a proper $A$-scheme. This article concerns the relationship between the Brauer group of $X$ and that of the various $X_n$ where $X_n$ is the fiber over $A/I^{n+1}$. In particular, we answer a question of Grothendieck by showing that, in positive and mixed characteristic, there are examples of $X$ with nontrivial Brauer classes that restrict to zero on all the $X_n$. We characterize such behavior, prove this cannot happen in characteristic zero, and deduce a formal GAGA statement for Brauer classes.
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2025 1verdicts
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Extensions of Abelian Schemes and the Additive Group
The paper proves a generalized Barsotti-Weil formula, vanishing of Ext^2 for abelian schemes over general bases, and a complete computation of Ext^1(Ga,Gm) over Q, correcting earlier claims.