 The expression gives the gravitational potential associated to a point mass or the Coulomb potential associated to a point charge.The expansion using Legendre polynomials might be useful, for instance, when integrating this expression over a continuous mass or charge distribution.Legendre polynomials occur in the solution of Laplace's equation of the static potential, , in a charge-free region of space, using the method of separation of variables, where the boundary conditions have axial symmetry (no dependence on an azimuthal angle).