Cusp-Analogous Condition at the Nucleus

In the Dirac many-electron case, Eqs. (9.151) and (9.152) allow us to determine the ratio of the first coefficients in the series expansion of Pj(r) and Qi r) in the point-like nucleus case  

This ratio is comparable to Kato s nonrelativistic cusp condition [482], which is the ratio of the derivative of the nonrelativistic radial function to itself (note 

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