\[\sum_{j_{12}\,j_{34}}(2j_{12}+1)(2j_{34}+1)(2j_{13}+1)(2j_{24}+1)\begin{%
Bmatrix}j_{1}&j_{2}&j_{12}\\
j_{3}&j_{4}&j_{34}\\
j_{13}&j_{24}&j\end{Bmatrix}\begin{Bmatrix}j_{1}&j_{2}&j_{12}\\
j_{3}&j_{4}&j_{34}\\
j^{\prime}_{13}&j^{\prime}_{24}&j\end{Bmatrix}=\delta_{j_{13},j^{\prime}_{13}}%
\delta_{j_{24},j^{\prime}_{24}}.\]