= 3-j symbol
{title2=$\begin{pmatrix}j_1&j_2&j_3\\m_1&m_2&m_3\end{pmatrix}$}
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= Wigner 3j symbol
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A symmetrized form of the <Clebsch-Gordan coefficients> used for coupling three angular momenta. The relation is $\langle j_1m_1,j_2m_2|j_3m_3\rangle=(-1)^{j_1-j_2+m_3}\sqrt{2j_3+1}\begin{pmatrix}j_1&j_2&j_3\\m_1&m_2&-m_3\end{pmatrix}$. The symbol vanishes unless the magnetic indices sum to zero and the angular momenta obey the triangle and integrality conditions. Products of two such symbols express the <Gaunt integral>; the zero-magnetic-index symbol enforces even total multipole for integer spherical harmonics.
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