Introduction

In quantum field theory (QFT) courses, one first learns about weakly coupled field theories, such as quantum electrodynamics and perturbative QCD (It is a big field now, but for readers who need concise and compact introduction, I recommend Feynman lectures published recently with 30+ year delay.). They start with harmonic oscillator quantization, define “quanta” of the fields, and their weak interaction, described by Feynman diagrams (Of course, Feynman diagrams have now much wider range of applications than QFTs. Readers interested in the easiest introduction to Feynman diagrams in quantum/statistical mechanics can find it in my textbook “Manybody theory in the nutshell,” Princeton University Press, where they are explained using basic toy models from quantum statistical mechanics).