  We study the collective excitation frequencies of a harmonically trapped 85Rb
Bose-Einstein condensate (BEC) in the vicinity of a Feshbach resonance. To this
end, we solve the underlying Gross-Pitaevskii (GP) equation by using a Gaussian
variational approach and obtain the coupled set of ordinary differential
equations for the widths and the center of mass of the condensate. A
linearization shows that the dipole mode frequency decreases when the bias
magnetic field approaches the Feshbach resonance, so the Kohn theorem is
violated.
