The Quarterly Journal of Mathematics Advance Access published online on October 31, 2007
The Quarterly Journal of Mathematics, doi:10.1093/qmath/ham037
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ON THE INTEGRAL GALOIS MODULE STRUCTURE OF CYCLIC EXTENSIONS OF P-ADIC FIELDS

School of Engineering, Computing and Mathematics, University of Exeter, Exeter EX4 1QF
E-mail: n.p.byott{at}ex.ac.uk
Received 25 August 2006;
revised 30 April 2007
| Abstract |
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We strengthen results of Miyata on the integral Galois module structure of totally ramified cyclic Kummer extensions K of degree p n of a p-adic field k. Let c 1(K/k) be the first ramification number of K/k, and let c(K/k) be the least non-negative residue of c 1(K/k) modulo p n . Suppose that K is of the form k(
) with
and val K (
–1)>0, (val K (
–1), p) = 1. Then the valuation ring of K is free over its associated order
if c(K/k) divides p m –1 for some m with 1
m
n; the converse holds if n = 2; and
is a Hopf order (or a Gorenstein order) if and only if c(K/k) = p n – 1.