  We study the structure of nucleon pairs within a simple model consisting of a
square well in three dimensions and a delta-function residual interaction
between two weakly-bound particles at the Fermi surface. We include the
continuum by enclosing the entire system in a large spherical box. To a good
approximation, the continuum can be replaced by a small set of
optimally-determined resonance states, suggesting that in many nuclei far from
stability it may be possible to incorporate continuum effects within
traditional shell-model based approximations.
