QED Effective Actions in Space-Dependent Gauge and Electromagnetic Duality

We develop the in-out formalism for one-loop effective actions in electromagnetic fields in the space-dependent gauge. We further advance a method using the inverse scattering matrix to calculate the effective actions in pure magnetic fields, find the effective actions in a constant magnetic field and a localized Sauter-type magnetic field and apply the uniform semiclassical approximation to the effective action in a general magnetic field. In the in-out formalism we show that the one-loop effective actions in constant fields and Sauter-type fields exhibit the electromagnetic duality.