  We show that the derivative of the (measure) transfer operator with respect
to the parameter of the map is a divergence. Then, for physical measures of
discrete-time hyperbolic chaotic systems, we derive an equivariant divergence
formula for the unstable perturbation of transfer operators along unstable
manifolds. This formula and hence the linear response, the parameter-derivative
of physical measures, can be sampled by recursively computing only $2u$ many
vectors on one orbit, where $u$ is the unstable dimension. The numerical
implementation of this formula in \cite{far} is neither cursed by
dimensionality nor the sensitive dependence on initial conditions.
