Fluctuation-Response Relation in Non-Equilibrium Systems and Active Matter

We use dynamic equations to derive a relation between correlation functions and response or relaxation functions in many-body systems. The relation is very general and holds both in equilibrium, when the usual fluctuation-dissipation theorem is valid, and to linear order in an expansion about arbitrary non-equilibrium states, when it is not. We illustrate our results by discussing fluids, both in equilibrium and non-equilibrium states, as well as several active-matter systems.