Let Ω Ω ⊆ ⊆ R 3 be a bounded open set, and E 0 ( r ) = 1 4 π π ε ε 0 ∫ ∫ Ω Ω ρ ρ ( r ′ ) r − − r ′ ‖ r − − r ′ ‖ 3 d r ′ ≡ ≡ 1 4 π π ε ε 0 ∫ ∫ Ω Ω e ( r , r ′ ) d r ′ be the electric field, with ρ ρ ( r ′ ) a continuous function (density of charge).