Physical Review A
Relativistic generalizations are derived for the well-known nonrelativistic electric-dipole oscillator-strength sum rules. The relativistic sum rules include both positive- and negative-energy states. The derivations are valid for a Dirac electron in an arbitrary local potential. We also present a number of simple integral properties related to the relativistic virial theorem which are useful in the calculation of matrix elements. © 1982 The American Physical Society.
Goldman, S. P. and Drake, Gordon W. F.. (1982). Relativistic sum rules and integral properties of the Dirac equation. Physical Review A, 25 (6), 2877-2881.