In [2] B.M. Schein raised the problem of characterizing semigroups
which can become subtraction semigroups, when a subtraction operator
is defined on them in terms of the group composition. In [3] Bohdan Zelinka
solved this problem partially by showing atomic subtraction semigroups can
be characterized as cancellative semigroups containing ‘0’. In this note, we
remove the restriction of atomicity and show that a multiplicative group A0
with ‘0’ can be made into a subtraction group if and only if a suitable self-map
of A0 exists.