In particular, let ϕ ϕ 1 denote the electric potential resulting from a total charge density ρ ρ 1 . The electric potential satisfies Poisson's equation, − − ∇ ∇ 2 ϕ ϕ 1 = ρ ρ 1 / ε ε 0 , where ε ε 0 is the vacuum permittivity. Similarly, let ϕ ϕ 2 denote the electric potential resulting from a total charge density ρ ρ 2 , satisfying − − ∇ ∇ 2 ϕ ϕ 2 = ρ ρ 2 / ε ε 0 . In both cases, we assume that the charge distributions are localized, so that the potentials can be chosen to go to zero at infinity. Then, Green's reciprocity theorem states that, for integrals over all space: