On twisted group rings and Galois-stable ideals

Abstract

Let A be a Dedekind domain with perfect field of fractions K, and let B be the integral closure of A in a finite Galois extension L of K, with Galois group G := Gal(L|K). We describe the twisted group ring B~G by means of a Wedderburn-embedding. We give a description of the image of B~G in A^(n×n) via congruences of matrix entries for an extension of the form Q(√d)|Q with d being a nonzero squarefree integer, in case of a cyclotomic field Q(ζ_p)|Q with p ∈ Z_>0 prime, and for the extensions Q(ζ_9)|Q and Q(2^{1/3}, ζ_3)|Q. By means of this description we show in examples that there are non-zero ideals in B~G that are not of the form b(B~G) for some Galois-stable ideal b ⊆ B. In case of A being a finite extension of Z, we obtain an explicit formula for the index of the image of B~G in A^(n×n) in terms of the discriminant.