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Galois cohomology of Witt vectors of algebraic integers

Published online by Cambridge University Press:  02 November 2004

LARS HESSELHOLT
Affiliation:
Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A. e-mail: larsh@math.mit.edu

Extract

Let $K$ be a complete discrete valuation field of characteristic zero with residue field $k_K$ of characteristic $p\gt 0$. Let $L/K$ be a finite Galois extension with Galois group $G\,{=}\,G_{L/K}$ and suppose that the induced extension of residue fields $k_L/k_K$ is separable. By the normal basis theorem, $L$ is always a projective $K[G]$-module. But the ring of integers $\mathcal{O}_L$ is a projective$\mathcal{O}_K[G]$-module if and only $L/K$ is tamely ramified. For wildly ramified extensions, the structure of $\mathcal{O}_L$ as an $\mathcal{O}_K[G]$-module is very complicated and quite far from understood. We propose that the ring of Witt vectors $W\s(\mathcal{O}_L)$ is a more well-behaved object. Indeed, we show that for a large class of extensions $L/K$, the pro-abelian group $H^1(G,W\s(\mathcal{O}_L))$ is zero. We conjecture that this is true in general.

Type
Research Article
Copyright
© 2004 Cambridge Philosophical Society

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