this general theorem and the proof of its corollaries are entirely similar to those given in the section 2 for the particular case of electromagnetic expressions in SI units. Furthermore, if we make the particular specifications $$\begin{aligned}
\mathscr{C}=c,\;\cJ=\textbf{J},\; \mathscr{G}=\rho,\; \mA=\textbf{A}/\mu_0, \; \mathscr{P}=\epsilon_0\Phi,\; {\cB}=\textbf{B}/\mu_0,\;
{\cE}=\epsilon_0\textbf{E},\end{aligned}$$ in the general theorem and its corollaries then we obtain the corresponding electromagnetic expressions in SI units. In the Minkowski spacetime the existence theorem is indeed elegant: *Existence Theorem*. Let ${\cal J}^\nu$ a localised four-vector that satisfies the continuity equation $\partial_\nu {\cal J}^\nu =0$ then there exists a four-vector ${\cal A}^\nu$ defined as $$\begin{aligned}
{\cal A}^\nu =\int {\cal G} {\cal J}^\nu d^4x',\end{aligned}$$ that satisfies the field equation $\partial_\nu {\cal A}^\nu =0$, where the Green function is defined by ${\cal G}=\delta\{t'\!-t+R/\mathscr{C}\}/(4\pi R)$ with $\mathscr{C}$ being a constant with units of velocity. *Corollary 1.* The four-vector ${\cal A}^\nu$ satisfies the wave equation $\partial_\mu\partial^\mu {\cal A}^\nu={\cal J}^\nu$, where $\partial_\mu\partial^\mu\!=-\nabla^2\!+\!(1/\mathscr{C}^2)\partial^2/\partial t^2$. *Corollary 2.* There exists the antisymmetric tensor ${\cal F}^{\mu\nu}= \partial^\mu {\cal A}^\nu-\partial^\nu {\cal A}^\mu$ that satisfies the field equations $\partial_\mu {\cal F}^{\mu\nu} ={\cal J}^\nu$ and $\partial_\mu\,\!^*{\cal F}^{\mu\nu}=0$, where $^*\!{\cal F}^{\mu\nu}\!=\!\varepsilon^{\mu\nu\alpha\beta}\partial_{\alpha}{\cal A}_\beta$. The proof of this covariant form of the theorem and the proof of its corollaries are entirely similar 