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Let $p$ be a prime and let $\{A_n\}_{n > 0}$ be an inverse limit of abelian groups such that $A_n$ is $p^n$-torsion with $A_n/p^{n - 1} \cong A_{n - 1}$ (these isomorphisms are part of the data). Let $A = \varprojlim A_n$ be the inverse limit, which surjects onto each $A_n$. Is it necessarily true that $A/p^n \cong A_n$?

This should be elementary but I can neither prove it nor find a likely to exist counterexample. It is equivalent to ask whether $lim^1$ vanishes for the system $\{ A_n[p^m]\}_{n > 0}$ for each fixed $m$.

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See Tag 09B8. Here are some more characters.

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  • $\begingroup$ Thank you. Meanwhile I found that my question is a special case of Lemme 1.2.6 in Berthelot, Breen, Messing "Theorie de Dieudonne Cristalline II" whose proof is left to the reader. The Stacks Project reference is also useful to justify that lemma. $\endgroup$
    – Lisa S.
    Commented Aug 17, 2018 at 11:17

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