where R Ω Ω is the radius of the region, A , θ θ , R Ω Ω ∈ ∈ R 1 . Its solution can be expressed in terms of the double layer potential, which leads to an inhomogeneous nonlinear Hammerstein integral equation for the gravitational potential U ( x ) = λ λ ~ ~ G ^ ^ ( U ) + α α ( θ θ , Λ Λ ) x 2 , G ^ ^ ( U ) ≡ ≡ ∫ ∫ Ω Ω ′ K ( | x − − x ′ | ) exp ⁡ ⁡ ( − − U ( x ′ ) ) d x ′ ,