Zs. Marciniak et Sk. Sehgal, CONSTRUCTING FREE SUBGROUPS OF INTEGRAL GROUP-RING UNITS, Proceedings of the American Mathematical Society, 125(4), 1997, pp. 1005-1009
Let G be an arbitrary group. It is proved that if ZG contains a bicycl
ic unit u not equal 1, then [u, u] is a nonabelian free subgroup of i
nvertible elements.