It is instructive to consider the special case of a parabolic confining potential V0(x) = 12mω20x2 in more detail, for which the zeroth-order dispersion relation can be obtained exactly (Section II.F). Since the confinement is symmetric in x, the minigaps in this case occur at the Brillouin zone boundaries k = pπ/a. Other gaps at points where the periodic modulation induces transitions between different 1D subbands are ignored for simplicity. From Eq. (2.59) one then finds that the Fermi energy lies in a minigap when