Past exam of the mathematics course of the University of Cambridge / 2019 / iii / Paper 304 / 2 / c / Solution

Applying four functional derivatives to the functional equation gives the homogeneous equation for the renormalized four-point one-particle-irreducible correlation function:If the opposite convention for ${\gamma }_{\varphi }$ is used, the last sign changes. The factor four counts the four external renormalized fields.