# Linear response function

A linear response function describes the input-output relationship of a signal transducer, such as a radio turning electromagnetic waves into music or a neuron turning synaptic input into a response. Because of its many applications in information theory, physics and engineering there exist alternative names for specific linear response functions such as susceptibility, impulse response or impedance; see also transfer function. The concept of a Green's function or fundamental solution of an ordinary differential equation is closely related.

## Mathematical definition

Denote the input of a system by $h(t)$ (e.g. a force), and the response of the system by $x(t)$ (e.g. a position). Generally, the value of $x(t)$ will depend not only on the present value of $h(t)$, but also on past values. Approximately $x(t)$ is a weighted sum of the previous values of $h(t')$, with the weights given by the linear response function $\chi (t-t')$: $x(t)=\int _{-\infty }^{t}dt'\,\chi (t-t')h(t')+\cdots \,.$
The explicit term on the right-hand side is the leading order term of a Volterra expansion for the full nonlinear response. If the system in question is highly non-linear, higher order terms in the expansion, denoted by the dots, become important and the signal transducer cannot adequately be described just by its linear response function.
The complex-valued Fourier transform ${\tilde {\chi }}(\omega )$ of the linear response function is very useful as it describes the output of the system if the input is a sine wave $h(t)=h_{0}\sin(\omega t)$ with frequency $\omega$. The output reads
$x(t)=\left|{\tilde {\chi }}(\omega )\right|h_{0}\sin(\omega t+\arg {\tilde {\chi }}(\omega ))\,,$
with amplitude gain $|{\tilde {\chi }}(\omega )|$ and phase shift $\arg {\tilde {\chi }}(\omega )$.

## Example

Consider a damped harmonic oscillator with input given by an external driving force $h(t)$,
${\ddot {x}}(t)+\gamma {\dot {x}}(t)+\omega _{0}^{2}x(t)=h(t).$
The complex-valued Fourier transform of the linear response function is given by
${\tilde {\chi }}(\omega )={\frac {{\tilde {x}}(\omega )}{{\tilde {h}}(\omega )}}={\frac {1}{\omega _{0}^{2}-\omega ^{2}+i\gamma \omega }}.$
The amplitude gain is given by the magnitude of the complex number ${\tilde {\chi }}(\omega ),$ and the phase shift by the arctan of the imaginary part of the function divided by the real one.
From this representation, we see that for small $\gamma$ the Fourier transform ${\tilde {\chi }}(\omega )$ of the linear response function yields a pronounced maximum ("Resonance") at the frequency $\omega \approx \omega _{0}$. The linear response function for a harmonic oscillator is mathematically identical to that of an RLC circuit. The width of the maximum, $\Delta \omega ,$ typically is much smaller than $\omega _{0},$ so that the Quality factor $Q:=\omega _{0}/\Delta \omega$ can be extremely large.

## Kubo formula

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, ${\hat {H}}_{0}\to {\hat {H}}_{0}-h(t'){\hat {B}}(t')$ where ${\hat {B}}$ corresponds to a measurable quantity as input, while the output x(t) is the perturbation of the thermal expectation of another measurable quantity ${\hat {A}}(t)$. The Kubo formula then defines the quantum-statistical calculation of the susceptibility $\chi (t-t')$ by a general formula involving only the mentioned operators.
As a consequence of the principle of causality the complex-valued function ${\tilde {\chi }}(\omega )$ has poles only in the lower half-plane. This leads to the Kramers–Kronig relations, which relates the real and the imaginary parts of ${\tilde {\chi }}(\omega )$ by integration. The simplest example is once more the damped harmonic oscillator.