\Box F_{ab} \ \stackrel{\mathrm{def}}{=}\ F_{ab;}{}^d{}_d = \, -2 R_{acbd} F^{cd} + R_{ae}F^e{}_b - R_{be}F^e{}_a + J_{a;b} - J_{b;a} .
Maxwell's source equations can be written in terms of the 4-potential [ref 2, p. 569] as, .