Perturbation expansion

Abstract

This chapter introduces the perturbation expansion as a means to approximately calculate the moment-generating function for theories that contain a solvable part and a small perturbation. The general concept of an action allows the definition of an interaction potential that introduces corrections, which can be systematically calculated via Taylor expansions and expectation values with regard to the solvable part of the theory. Diagrammatic rules for Gaussian solvable theories are presented to organize perturbation expansions using Feynman diagrams. The general rules are exemplified using a “ϕ3 + ϕ4” theory.