The Low-Frequency Behaviour of Response Functions
The matrix N(z) introduced in the last section is also well suited for the discussion of the low-frequency behavior of the response functions. Of course, N(z) is nothing but the analytic continuation of the memory function N(ω) introduced in (4.24) or, in matrix form, in the context of (5.15) in the upper frequency half plane. It may be considered as a frequency-dependent generalization of kinetic coefficients. The traditional kinetic coefficients are given by the z → 0 limit of N(z). Nore precisely, since N(z), like the response functions, may be discontinuous across the real axis, one has to consider N(+i0).
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