ABSTRACT
Weshallnowsetupaformalismbasedontheuseofthe"Green's functions"ofthesystem,describingthepropagationofoneormore specifiedparticles.Thistechniqueisfrequentlyusedinfieldtheory;itis particularlyfruitful,asitallowsforacompactandcoherenttreatmentof alargenumberofproperties.Itsapplicationtothemany-bodyproblem, althoughrelativelyrecent,hasbeenthesubjectofalargenumberof articles;thereaderwillfindashortBibliographyattheendofthebook. ThepresentationwhichweshalladoptissimilartothatofGalitskiiand Migdal.
