Writing units of integral group rings of finite abelian groups as a product of Bass units

TL;DR

This paper provides a constructive proof that Bass units generate a subgroup of finite index in the units of integral group rings of finite abelian groups, along with algorithms for expressing units as products of Bass units.

Contribution

It offers a constructive proof and algorithms for representing units in integral group rings of finite abelian groups as products of Bass units, including a basis with finite index.

Findings

Algorithms to express units as products of Bass units.

Construction of a basis of Bass units with finite index.

Explicit methods to decompose units into basis elements.

Abstract

We give a constructive proof of the theorem of Bass and Milnor saying that if $G$ is a finite abelian group then the Bass units of the integral group ring $\Z G$ generate a subgroup of finite index in its units group $\U(\Z G)$. Our proof provides algorithms to represent some units that contribute to only one simple component of $\Q G$ and generate a subgroup of finite index in $\U(\Z G)$ as product of Bass units. We also obtain a basis $B$ formed by Bass units of a free abelian subgroup of finite index in $\U(\Z G)$ and give, for an arbitrary Bass unit $b$, an algorithm to express $b^{\varphi(|G|)}$ as a product of a trivial unit and powers of at most two units in this basis $B$.