The relevance of chaos for the linear response theory

We discuss the van Kampen objection, about the range of validity of the linear response theory; we show that, in chaotic systems, the instability of the dynamics has a constructive role. For any system with good chaotic properties, a fluctuation response relation holds at macroscopic times, because the relevant perturbative parameter is tc·⋎f⋎, where tc is a correlation time and ⋎f⋎ the strength of the external force. Nevertheless, the van Kampen criticism has relevant consequences on the feasibility of numerical experiments.