  A zero-range approach to atom-molecule coupling is developed in analogy to
the Fermi-Huang pseudo-potential treatment of atom-atom interactions. It is
shown by explicit comparison to an exactly-solvable finite-range model that
replacing the molecular bound-state wavefunction with a regularized
delta-function can reproduce the exact scattering amplitude in the
long-wavelength limit. Using this approach we find an analytical solution to
the two-channel Feshbach resonance problem for two atoms in a spherical
harmonic trap.
