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