renormalization group (RG)

#basement
  1. Motivation
    Renormalization: self-interactions should alwalys be taken into account; besides, Haag's theorem points out that continuous QFT in an infinite momentum space isn't consistent, where "Fock" spaces of different kinds of particles/free theory/interacting theory are not in the same Hilbert space (can't be related by unitary operators), so we need a cut-off (namely, the interacting picture just approximately stand in perturbative theory if without cut-off).