Centered four-point cumulant decomposition ($G_4=C_4+\sum C_2{C}_2$)

For a normalized Minkowski source functional $Z=e^{iW}$ with vanishing one-point function, $C_n=i^{1-n}{\delta }^{n}W/\delta J^n$ andDifferentiating $e^{iW}$ assigns the four labels to connected blocks. Blocks of size one vanish, leaving one block of four or one of the three pair partitions. Thus $G_4=i{W}_4-\sum W_2{W}_2$ in this source convention.

// Target: perturbative-quantum-field-theory.bigb