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

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 $\mathbb {Z} G$ generate a subgroup of finite index in its unit group $\mathcal {U}(\mathbb {Z} G)$. Our proof provides algorithms to represent some units that contribute to only one simple component of $\mathbb {Q} G$ and generate a subgroup of finite index in $\mathcal {U}(\mathbb {Z} G)$ as product of Bass units. We also obtain a basis $B$ formed by Bass units of a free abelian subgroup of finite index in $\mathcal {U}(\mathbb {Z} G)$ 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 $B$.