title: Racah seniority number
text: # Racah seniority number
  
  The Racah seniority number (seniority quantum number) $\nu$ was introduced by Giulio Racah for the classification of electrons in an atomic configuration. The "seniority number", in a loosing statement, is quantum number additional to the total angular momentum $L$ and total spin $S$, which gives the degree of unpaired particles.
  
  A spin-independent interaction ${\hat {V}}$ is assumed with the property
  
      $\langle l^{2};LM_{L}|{\hat {V}}|l^{2};LM_{L}\rangle =g(2l+1)\delta _{L0}$,
  
  where $L$ is the combined angular momentum, $M_{L}$ magnetic quantum number, $l$ is electrons' orbital angular momenta, and $g$ is the dimensionless magnetic moment. The equation above shows there is no interaction unless the two electrons' orbital angular momenta are coupled to $L=0$. The eigenvalue is the "seniority number" $\nu$.
  
  