Interacting Bose Gas in an Optical Lattice

A grand canonical system of hard-core bosons in an optical lattice is considered. The bosons can occupy randomly $N$ equivalent states at each lattice site. The limit $N\to\infty$ is solved exactly in terms of a saddle-point integration, representing a weakly-interacting Bose gas. At T=0 there is only a condensate in the limit $N\to\infty$. Corrections in 1/N increase the total density of bosons but suppress the condensate. This indicates a depletion of the condensate due to increasing interaction at finite values of N.