The product of two spherical harmonics can be written as $$ Y_{l_1m_1} Y_{l_2m_2}=\sum_{lm}\sqrt{\frac{(2l_1+1)(2l_2+1)(2l+1)}{4\pi}} \left(\begin{array}{ccc} l_1&l_2&l \\ m_1&m_2&m\end{array}\right) $$ $$ \times \left(\begin{array}{ccc} l_1&l_2&l \\ 0 &0 &0\end{array}\right) Y_{l-m}(-1)^m. $$