what does 6j mean? it appears in the quantum mechanics of angular momentum.
when three angular momenta are coupled, there are two natural ways to combine them:
first couple J₁ and J₂ to get J₃, or first couple j₁ and j₂ to get j₃.
the 6j symbol is the overlap (recoupling coefficient) between these two coupling schemes.
it depends on six angular momentum quantum numbers (three big J's and three small j's)
and is closely related to the racah W-coefficient. it appears in perturbation theory,
in the calculation of matrix elements of tensor operators, and in the theory of
spectroscopic line strengths.