Spectral comparison theorem for the Dirac equation

We consider a single particle which is bound by a central potential and obe ys the Dirac equation. We compare two cases, a and b, in which the masses a re the same but V-a < V-b, where V is the time component of a vector potent ial. We prove generally that for each discrete eigenvalue E whose correspon ding (large and small) radial wave functions have no nodes, it necessarily follows that E-a < E-b. As an illustration, this general relativistic compa rison theorem is applied to approximate the Dirac spectrum generated by a s creened-Coulomb potential.