Bose-Einstein condensation in an optical lattice: A perturbation   approach
  We derive closed analytical expressions for the order parameter $\Phi (x)$ and for the chemical potential $\mu $ of a Bose-Einstein Condensate loaded into a harmonically confined, one dimensional optical lattice, for sufficiently weak, repulsive or attractive interaction, and not too strong laser intensities. Our results are compared with exact numerical calculations in order to map out the range of validity of the perturbative analytical approach. We identify parameter values where the optical lattice compensates the interaction-induced nonlinearity, such that the condensate ground state coincides with a simple, single particle harmonic oscillator wave function. 
C. Trallero-Giner, V. Lopez-Richard, Ming-Chiang Chung, Andreas   Buchleitner