G

G(ji, jj, jk, jl, k, sqrt=np.sqrt)

Wigner-6j part of four-fold reduced matrix element.

\[\begin{split}G(J_i, J_j, J_k, J_l; k) = (2k+1)\begin{Bmatrix} k & k & 0\\ J_i & J_i & J_k \end{Bmatrix} \begin{Bmatrix} 1 & 1 & k\\ J_k & J_i & J_j \end{Bmatrix} \begin{Bmatrix} 1 & 1 & k\\ J_k & J_i & J_l \end{Bmatrix}\end{split}\]