But this equation for \rho has no longer the form of a temporal derivative plus the gradient of a current (at least, I can't see how to put it in this form).
But what happens in the gauge A = -1/2 \vec{r} \times \vec{B} , with \vec{B} = B_z \hat{e}_z ? Here you still have the problem that the circuitation outside the cylinder vanishes, but the function defining A is no longer multivalued. Of course, there is a discontinuity, but this happens also...
It's gauge invariant as long as you perform gauge transformations compatible with the Coulomb gauge constraint (which does not fix completely the gauge, as is well known). But what about an equation which does not require this constraint?
The Schrodinger equation with the minimal coupling to the Electromagnetic field, in the Coulomb gauge \nabla \cdot A , has a continuity equation \partial_t \rho = \nabla \cdot j where j \propto Re[p^* D p] (D is the covariant gradient D= \nabla + iA .
My question is: is there any...