Linear response, or else

Abstract

Consider a smooth one-parameter family t -> f_t of dynamical systems f_t, with |t|<epsilon. Assume that for all t (or for many t close to t=0) the map f_t admits a unique SRB invariant probability measure m_t. We say that linear response} holds if t -> m_t is differentiable at t=0 (possibly in the sense of Whitney), and if its derivative can be expressed as a function of f_0, m_0, and d_t f_t|_(t=0). The goal of this note is to present to a general mathematical audience recent results and open problems in the theory of linear response for chaotic dynamical systems, possibly with bifurcations.