  We study the expansion of a Bose-Einstein condensate trapped in a combined
optical-lattice and axially-symmetric harmonic potentials using the numerical
solution of the mean-field Gross-Pitaevskii equation. First, we consider the
expansion of such a condensate under the action of the optical-lattice
potential alone. In this case the result of numerical simulation for the axial
and radial sizes during expansion is in agreement with two experiments by
Morsch et al.[2002 Phys. Rev. A 66 021601(R) and 2003 Laser Phys. 13 594].
Finally, we consider the expansion under the action of the harmonic potential
alone. In this case the oscillation and the disappearance and revival of the
resultant interference pattern is in agreement with the experiment by M\"uller
et al. [2003 J. Opt. B 5 538].
