Finite modules over non-semisimple group rings

Abstract

Let $G$ be an abelian group of order $n$ and let $R$ be a commutative ring which admits a homomorphism ${\Bbb Z}[ζ_{n}]\ra R$, where $ζ_{n}$ is a (complex) primitive $n$-th root of unity. Given a finite $R[G\e]$-module $M$, we derive a formula relating the order of $M$ to the product of the orders of the various isotypic components $M^{\eχ}$ of $M$, where $χ$ ranges over the group of $R$-valued characters of $G$. We then give conditions under which the order of $M$ is exactly equal to the product of the orders of the $M^χ$. To derive these conditions, we build on work of E.Aljadeff and obtain, as a by-product of our considerations, a new criterion for cohomological triviality which improves the well-known criterion of T.Nakayama. We also give applications to abelian varieties and to class groups of abelian fields, obtaining in particular some new class number formulas. Our results also have applications to "non-semisimple" Iwasawa theory, but we do not develop these here. In general, the results of this paper can be used to strengthen a variety of known results involving finite $R[G\e]$-modules whose hypotheses include (an equivalent form of) the following assumption: ``the order of $G$ is invertible in $R$".}