  Dynamic Hubbard models describe coupling of a boson degree of freedom to the
on-site electronic double occupancy. In the limit of infinite boson frequency
this coupling gives rise to a correlated hopping term in the effective
Hamiltonian and to superconductivity when the Fermi level is near the top of
the band. Here we study the effect of finite boson frequency through a
generalized Lang-Firsov transformation and a high frequency expansion. It is
found that finite frequency enhances the tendency to superconductivity in this
model.
