\[
\vec{A} \ =\ \left( \frac{\phi}{c} , \ A_x, \ A_y, \ A_z \right) \\
\vec{j} \ =\ \left( \rho c , \ i_x, \ i_y, \ i_z \right) \\
\left( \triangle - \frac{1}{c^2} \frac{\partial^2}{\partial t^2} \right) \vec{A} \ =\ - \mu_0 \,\vec{j} \\
\]
さらに，
\[
\square \ \equiv \ \triangle - \frac{1}{c^2} \frac{\partial^2}{\partial t^2} \\
\square \vec{A} \ =\ - \mu_0 \,\vec{j}
\]
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