Hi. Here, somebody apparently derives Maxwell's equations using only symmetry of second derivatives and the Lorenz gauge condition. Unfortunately it's in German, but I think the basic ideas are clear from the maths only. In this derivation, the magnetic field turns out to be divergence-free, even before the application of the gauge condition. Why? Shouldn't such a general derivation allow for magnetic monopoles? Also, it seems a bit suspicious that this derivation only works by fixing a specific gauge. How can this be the same as usual ED which allows different gauges?