  Low-momentum nucleon-nucleon interactions are derived within the framework of
a unitary-transformation theory, starting with realistic nucleon-nucleon
interactions. A cutoff momentum Lambda is introduced to specify a border
between the low- and high-momentum spaces. By the Faddeev-Yakubovsky
calculations the low-momentum interactions are investigated with respect to the
dependence of ground-state energies of ^3H and ^4He on the parameter Lambda. It
is found that we need the momentum cutoff parameter Lambda >= 5 fm^-1 in order
to reproduce satisfactorily the exact values of the binding energies for ^3H
and ^4He. The calculation with Lambda =2 fm^-1 recommended by Bogner et al.
leads to considerable overbinding at least for the few-nucleon systems.
