ON GROUPS IN WHICH SUBNORMAL SUBGROUPS OF INFINITE RANK ARE COMMENSURABLE WITH SOME NORMAL SUBGROUP

We study soluble groups G in which each subnormal subgroup H with infinite rank is commensurable with a normal subgroup, i.e. there exists a normal subgroup N such that H ∩ N has finite index in both H and N. We show that if such a G is periodic, then all subnormal subgroups are commensurable with a normal subgroup, provided either the Hirsch-Plotkin radical of G has infinite rank or G is nilpotent-by-abelian (and has infinite rank).