A response function is a measure of how a property of a system changes in
the presence of one or more perturbations. With our notation (see e.g.
Ref. [28]), 
,
, and
denote linear, quadratic and cubic response functions , respectively, which
provide the first, second, and third-order corrections to the
expectation-value of A, due to perturbations B, C, and D, each of
which is associated with a frequency 
, 
, and
. Often the perturbations are considered to be
external monochromatic fields, or static (e.g. relativistic) perturbations,
in which case the frequency is zero. In general, the perturbations B,
C, and D represent Fourier components of an arbitrary time-dependent
perturbation.