The analytic properties of Feynman amplitudes in the many-fermion problem are investigated by means of techniques recently developed for use in field theory. A prescription is given for calculation of discontinuities across branch cuts. This prescription leads to a reformulation of the theory in terms of quasi-particles; but considerations of unitarity are no longer so simple as they are in field theories dealing with stable particles. These analytic methods turn out to be very useful in electron transport problems.
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J. S. Langer (1961) studied this question.
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