Our approximation of a galaxy represented on Fig. 2 can be represented as the sum of three cubes: a cube (a) with uniform “negative mass” density of total mass −2m, a cube (b) with only the galaxy of mass +m at the center, a cube (c) with a sphere occupying half the volume of the cube and of homogeneous positive density of matter, equal and opposite to that of the first cube, that together with (a) will create the empty depletion zone.