Diffraction for the Dirac–Coulomb Propagator

The Dirac equation in $$\mathbb {R}^{1,3}$$ with potential $${\textsf{Z}}/r$$ is a relativistic field modeling the hydrogen atom. We analyze singularity structure of propagator for this equation, showing that singularities Schwartz kernel are along an expanding spherical wave away from rays miss at origin, but also may include additional diffracted emanating origin. This wavefront $$1-{\epsilon }$$ derivatives smoother than main singularities, all $${\epsilon }>0,$$ and conormal singularity.