  In calculations to date [1,2] of multi-layer stacks of dipolar condensates,
created in one-dimensional optical lattices, the condensates have been assumed
to be two-dimensional. In a real experiment, however, the condensates do not
extend to infinity in the oblate direction, but have to be confined by a trap
potential, too. By three-dimensional numerical simulations of this realistic
experimental situation we find a crucial dependence of the phonon instability
boundary on the number of layers. Moreover, near the boundary of the phonon
instability, a variety of structured ground-state wave functions emerges, which
may indicate the onset of a roton instability [3,4].
