Optimal linear response for Anosov diffeomorphisms

Abstract: It is well known that an Anosov diffeomorphism $T$ enjoys linear response of its SRB measure with respect to infinitesimal perturbations $T_2$. For a fixed observation function $c$, we develop a theory to optimise the response of the SRB-expectation of $c$. Our approach is based on the response of the transfer operator on the anisotropic Banach spaces of Gou\"ezel-Liverani. For any non-constant $c$ we prove that the optimising perturbation $T_2$ exists and is unique, and we provide explicit expressions for the Fourier coefficients of $T_2$. We develop an efficient Fourier-based numerical scheme to approximate the optimal vector field $T_2$, along with a proof of convergence. The utility of our approach is illustrated in two numerical examples, by localising SRB measures with small, optimally selected, perturbations.