The two-body problem of ultra-cold atoms in a harmonic trap
  We consider two bosonic atoms interacting with a short-range potential and trapped in a spherically symmetric harmonic oscillator. The problem is exactly solvable and is relevant for the study of ultra-cold atoms. We show that the energy spectrum is universal, irrespective of the shape of the interaction potential, provided its range is much smaller than the oscillator length. 
Patrick Shea, Brandon P. van Zyl, Rajat K. Bhaduri