  Using the mean-field time-dependent Gross-Pitaevskii equation we study the
formation of a repulsive Bose-Einstein condensate on a combined optical and
harmonic traps in two and three dimensions and subsequent generation of the
interference pattern upon the removal of the combined traps as in the
experiment by Greiner {\it et al.} [Nature (London) {\bf 415}, 39 (2002)]. For
optical traps of moderate strength, interference pattern of 27 (9) prominent
bright spots is found to be formed in three (two) dimensions on a cubic
(square) lattice in agreement with experiment. Similar interference pattern can
also be formed upon removal of the optical lattice trap only. The pattern so
formed can oscillate for a long time in the harmonic trap which can be observed
experimentally.
