Onsager's regression hypothesis adjusted to quantum systems

Some 93 years ago, Onsager postulated his so-called regression hypothesis, which relates the time-dependent relaxation of a close-to-equilibrium system to its temporal correlations at thermal equilibrium. As far as quantum systems are concerned, it is long-known that this hypothesis cannot be quantitatively correct. Here, the authors analytically derive a suitably adjusted version of Onsager's original formula, provide a wide variety of numerical illustrations, and compare it with some pertinent previous works.