Mathematical Research Letters

Volume 12, Issue 3, May 2005 pp. 303-320.

Frobenius modules and de Jong's theorem

Authors: Kiran S. Kedlaya
Author institution: Massachusetts Institute of Technology

Summary: Let $k$ be an algebraically closed field of characteristic $p>0$. A theorem of de~Jong shows that morphisms of modules over $W(k) \llbracket t \rrbracket$ with Frobenius and connection structure descend from the completion of $W(k)((t))$. A careful reading of de~Jong's proof suggests the possibility that an analogous theorem holds for modules with only a Frobenius structure. We show that this analogue holds in one natural formulation, but fails in a stronger formulation in which $W(k) \llbracket t \rrbracket$ is replaced by $W(k \llbracket t \rrbracket)$.

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