Perturbative Methods in Many-Body Problems
The purpose of this chapter is to show how that the perturbative methods, already worked out for calculating the scattering operator using Feynman diagrams, can also be applied to many-body problems. We limit ourselves here to the principal ideas and several applications. The appropriate quantities on which the perturbative calculations should be performed are the Green functions of the many-body problem. They generalize, in a natural manner, the concept of a causal or time-ordering operator, introduced in Sects. 9.3.1 and 9.3.4.
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