Published online by Cambridge University Press: 20 November 2018
Throughout this note all groups are abelian, written additively. We refer to Kurosh (8; 9) for notation, terminology and theorems used without reference. We recall the notion of a serving subgroup (or pure subgroup) of a group
. This is a subgroup
in which for every natural number n every equation nx = s, s ∊
can be solved provided that it can be solved in
. If
is torsion-free, “linearly closed” subgroups coincide with serving subgroups and
is a serving subgroup if and only if
/
is torsionfree.