ABSTRACT
In this chapter, we discuss the central concept in many-body physics, namely the single-particle Green function. It is shown that many physical observables, such as current and number density, are related to this object. However, this quantity is more important than that since it provides information of the nature of ‘quasiparticles’. If we choose to characterize the atomic constituents of the model being studied minus the mutual interactions among the constituents-‘particles’, then ‘quasiparticles’ would be the effective description of such particles in the presence of such mutual interaction between the constituents. The single-particle Green function contains information about the energy momentum relation of these quasiparticles and also their lifetime. We consider this concept both for systems in thermodynamic equilibrium and for systems out of equilibrium.
