\[Y_{{l_{1}},{m_{1}}}\left(\theta,\phi\right)Y_{{l_{2}},{m_{2}}}\left(\theta,%
\phi\right)=\sum_{l,m}\left(\frac{(2l_{1}+1)(2l_{2}+1)(2l+1)}{4\pi}\right)^{%
\frac{1}{2}}\begin{pmatrix}l_{1}&l_{2}&l\\
m_{1}&m_{2}&m\end{pmatrix}\overline{Y_{{l},{m}}\left(\theta,\phi\right)}\begin%
{pmatrix}l_{1}&l_{2}&l\\
0&0&0\end{pmatrix},\]