  We study the effect of the cubic torus topology of the Universe on scalar
cosmological perturbations which define the gravitational potential. We obtain
three alternative forms of the solution for both the gravitational potential
produced by point-like masses, and the corresponding force. The first solution
includes the expansion of delta-functions into Fourier series, exploiting
periodic boundary conditions. The second one is composed of summed solutions of
the Helmholtz equation for the original mass and its images. Each of these
summed solutions is the Yukawa potential. In the third formula, we express the
Yukawa potentials via Ewald sums. We show that for the present Universe, both
the bare summation of Yukawa potentials and the Yukawa-Ewald sums require
smaller numbers of terms to yield the numerical values of the potential and the
force up to desired accuracy. Nevertheless, the Yukawa formula is yet
preferable owing to its much simpler structure.
