“…Green functions) can be extracted from S, and that renormalizability is trivially preserved in almost any approximation [2,3], turns these ideas into a powerful framework for considering both perturbative and non-perturbative approximations (see for example refs. [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26]). …”