The E and B fields can be replaced by the magnetic vector potential A and (scalar) electrostatic potential ϕ by E = − − ∇ ∇ ϕ ϕ − − ∂ ∂ A ∂ ∂ t B = ∇ ∇ × × A where ∇ is the gradient, ∇⋅ is the divergence, and ∇× is the curl.