  We consider a single particle which is bound by a central potential and obeys
the Dirac equation. We compare two cases in which the masses are the same but
Va < Vb, where V is the time-component of a vector potential. We prove
generally that for each discrete eigenvalue E whose corresponding (large and
small) radial wave functions have no nodes, it necessarily follows that Ea <
Eb. As an illustration, this general relativistic comparison theorem is applied
to approximate the Dirac spectrum generated by a screened-Coulomb potential.
