E. Jespers et G. Leal, FREE-PRODUCTS OF ABELIAN-GROUPS IN THE UNIT GROUP OF INTEGRAL GROUP-RINGS, Proceedings of the American Mathematical Society, 126(5), 1998, pp. 1257-1265
We classify finite groups G which are such that the unit group of the
integral group ring ZG has a subgroup of finite index which is a non-t
rivial free product of abelian groups.