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The monodromy of unit-root F-isocrystals with geometric origin

Part of: (Co)homology theory Arithmetic algebraic geometry

Published online by Cambridge University Press:  18 April 2022

Joe Kramer-Miller
Joe Kramer-Miller*
Affiliation:
Department of Mathematics, Lehigh University, Bethlehem, PA18015, USAjjk221@lehigh.edu
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Abstract

Let $C$ be a smooth curve over a finite field of characteristic $p$ and let $M$ be an overconvergent $\mathbf {F}$-isocrystal over $C$. After replacing $C$ with a dense open subset, $M$ obtains a slope filtration. This is a purely $p$-adic phenomenon; there is no counterpart in the theory of lisse $\ell$-adic sheaves. The graded pieces of this slope filtration correspond to lisse $p$-adic sheaves, which we call geometric. Geometric lisse $p$-adic sheaves are mysterious, as there is no $\ell$-adic analogue. In this article, we study the monodromy of geometric lisse $p$-adic sheaves with rank one. More precisely, we prove exponential bounds on their ramification breaks. When the generic slopes of $M$ are integers, we show that the local ramification breaks satisfy a certain type of periodicity. The crux of the proof is the theory of $\mathbf {F}$-isocrystals with log-decay. We prove a monodromy theorem for these $\mathbf {F}$-isocrystals, as well as a theorem relating the slopes of $M$ to the rate of log-decay of the slope filtration. As a consequence of these methods, we provide a new proof of the Drinfeld–Kedlaya theorem for irreducible $\mathbf {F}$-isocrystals on curves.

Keywords

unit-root F-isocrystalsp-adic cohomologyramification theory

MSC classification

Primary: 14F30: $p$-adic cohomology, crystalline cohomology
Secondary: 11G20: Curves over finite and local fields
Type
Research Article
Information
Compositio Mathematica , Volume 158 , Issue 2 , February 2022 , pp. 334 - 365
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Copyright
© 2022 The Author(s). The publishing rights in this article are licensed to Foundation Compositio Mathematica under an exclusive licence

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