| Fluctuation dissipation theorem is a well stated result in physics, connecting equilibrium fluctuations to the relaxation of an external perturbation. While this theory has been initially developed in the context of equilibrium statistical mechanics, in the last decades it has been extended to a larger class of systems, ranging from chaotic to non-equilibrium dynamics. In this talk I will discuss the main issues of this generalized response theory by focusing on recent theoretical and experimental results on vibrated granular gases. |