the_zeroth

\[ \begin{gathered} \nabla \cdot \mathbf{D}=\rho_{f} \\ \nabla \cdot \mathbf{B}=0 \\ \nabla \times \mathbf{E}=-\frac{\partial \mathbf{B}}{\partial t} \\ \nabla \times \mathbf{H}=\mathbf{J}_{f}+\frac{\partial \mathbf{D}}{\partial t} \end{gathered} \]