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It is shown that, in the relativistic hydrogenic approximation, expectationvalues of the type $\langle n\ell j|r^k|n\ell j\rangle$ can be expressed interms of Clebsch-Gordan coefficients or $3jm$ symbols. This generalizes theresults obtained by Varshalovich and Karpova [Opt. Spectrosc. 118, 1-5 (2015);Opt. Spektrosk. 118, 3-7] in the non-relativistic hydrogenic case.

Expression of relativistic expectation values of powers of $r$ in terms of Clebsch-Gordan coefficients