The exposition of linear response theory, in the context of quantum statistics, can be found in a paper by Ryogo Kubo. This defines particularly the Kubo formula, which considers the general case that the "force" h (t) is a perturbation of the basic operator of the system, the Hamiltonian, H ^ ^ 0 → → H ^ ^ 0 − − h ( t ′ ) B ^ ^ ( t ′ ) where B ^ ^ corresponds to a measurable quantity as input, while the output x (t) is the perturbation of the thermal expectation of another measurable quantity A ^ ^ ( t ) . The Kubo formula then defines the quantum-statistical calculation of the susceptibility χ χ ( t − − t ′ ) by a general formula involving only the mentioned operators.