The Coulomb gauge is chosen in such a way that ∇ ∇ ⋅ ⋅ A ′ = 0 , which corresponds to the case of magnetostatics. In terms of λ, this means that it must satisfy the equation ∇ ∇ 2 λ λ = − − ∇ ∇ ⋅ ⋅ A .