Galois scaffolds and Galois module structure in extensions of characteristic p local fields of degree p2

Abstract

A Galois scaffold, in a Galois extension of local fields with perfect residue fields, is an adaptation of the normal basis to the valuation of the extension field, and thus can be applied to answer questions of Galois module structure. Here we give a sufficient condition for a Galois scaffold to exist in fully ramified Galois extensions of degree p2 of characteristic p local fields. This condition becomes necessary when we restrict to p = 3. For extensions L/K of degree p2 that satisfy this condition, we determine the Galois module structure of the ring of integers by finding necessary and sufficient conditions for the ring of integers of L to be free over its associated order in K[Gal(L/K)].