Symmetrized correlation function classical limit

To get a classical limit, the symmetrized correlation function is represented by ... 

Before entering the discussion of the classical limit, we will propose another formulation of the FDT as given by Eq. (37) or Eq. (38) in the time-domain. It provides the expression of the dissipative part of the linear response function in terms of the derivative of the symmetrized correlation function, an expression which becomes particularly simple in the classical limit [34]. 

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