9-j symbol

In physics, Wigner's 9-j symbols were introduced by Eugene Paul Wigner in 1937. They are related to recoupling coefficients in quantum mechanics involving four angular momenta

Coupling of two angular momenta ${\displaystyle \mathbf {j} _{1}}$ and ${\displaystyle \mathbf {j} _{2}}$ is the construction of simultaneous eigenfunctions of ${\displaystyle \mathbf {J} ^{2}}$ and ${\displaystyle J_{z}}$, where ${\displaystyle \mathbf {J} =\mathbf {j} _{1}+\mathbf {j} _{2}}$, as explained in the article on Clebsch–Gordan coefficients.

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