Properties and Use of the Green’s Functions
The Green’s functions defined in Chap.8 have similar but not identical analytical properties as the Green’s functions defined in Chap.2 corresponding to second-order (in time) differential equations. They can all be expressed in terms of a generalized DOS and the Fermi or Bose thermal equilibrium distributions. From the Green’s functions (or the generalized DOS) one can easily obtain all thermodynamic quantities and linear response functions, like the conductivity. The poles of an appropriate analytic continuation of G in the complex E-plane can be interpreted as the energy (the real part of the pole) and the inverse life time (the imaginary part of the pole) of quasi-particles. The latter are entities which allow us to map an interacting system to a noninteracting one.
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