Sources of symmetric potentials
Laboratory experiments on gravitation are usually performed with objects of constant density, so that the analysis of the forces concerns only the geometry of their shape. In an ideal experiment, the shapes of the constituent parts will be optimised to meet certain mathematical criteria, which ensure that the experiment has maximal sensitivity. 
Using this idea, the author suggested an experiment to determine the departure of the gravitational force from Newton's force law [1]. The geometrical problem which has to be solved is to find two shapes which differ significantly, but have the same Newtonian potential. Essentially, the experiment determines whether the two objects are distinguishable by their gravitational force. Here, we consider the case when one of them is a round ball. The result, Theorem 1, establishes a fact which appeared in numerical simulations, that the second object has to have a hole in it.