Linear-Response Theory, Kubo Formula, Kramers
... (Actually, because of causality, the upper integration limit, ∞, can be replaced by t, and the
lower one, t0 , by −∞, if the perturbation is switched on adiabatically.)
The function XÂ,B̂ (t − t′ ) is (apart from a minus sign) identical with the retarded Green’s
function GÂ,B̂ (t − t′ ), and, whic ...