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A recently found additional hyperfine interaction in the Dirac equation extends the validity of this equation to comparable masses of its two spin- 1/2 particles. For equal masses as in positronium, it ensures CP conservation. The new operator simplifies the hyperfine mixing of states with different j. Anomalous magnetic moments are incorporated, and an application to helium-like Rydberg states is mentioned. CPT invariance is demonstrated and discussed, including the extension to n-electron atoms.
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H. Pilkuhn (1995) studied this question.
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