Poisson's equation may be solved using a Green's function : φ φ ( r ) = − − ∭ ∭ f ( r ′ ) 4 π π | r − − r ′ | d 3 r ′ , where the integral is over all of space. A general exposition of the Green's function for Poisson's equation is given in the article on the screened Poisson equation. There are various methods for numerical solution, such as the relaxation method, an iterative algorithm.