Gravitational potentials and forces in the Lattice Universe: a slab
  We study the effect of the slab topology $T\times R\times R$ of the Universe
on the form of gravitational potentials and forces created by point-like
masses. We obtain two alternative forms of solutions: one is based on the
Fourier series expansion of the delta function using the periodical property
along the toroidal dimension, and another one is derived by direct summation of
solutions of the Helmholtz equation for the source particle and all its images.
The latter one takes the form of the sum of Yukawa-type potentials. We
demonstrate that for the present Universe the latter solution is preferable for
numerical calculations since it requires less terms of the series to achieve
the necessary precision.
