Linear response function

Summary

A linear response function describes how an observable of a system changes, to first order, when a weak time-dependent force acts on it. The change at time $t$ is the integral of the force at earlier times against a kernel $\chi$. Causality makes $\chi$ vanish for negative times. Response functions give susceptibilities, conductivities and absorption spectra. In equilibrium statistical mechanics they are computed from correlation functions by the Kubo formula.