David Ruelle
General linear response formula in statistical mechanics, 
and the fluctuation-dissipation theorem.
(17K, Plain TeX)

ABSTRACT.  Given a nonequilibrium steady state $\rho$ we derive
formally the linear response formula $\delta_t\rho(\Phi)=\int_{-\infty}^t
d\tau\rho(\delta_\tau F\cdot\nabla(\Phi\circ f^{t-\tau}))$ for the
variation of the expectation value of $\Phi$ at time $t$ under an
infinitesimal perturbation $\delta_\tau F$ of the acting forces.  This leads to
a form of the fluctuation-dissipation theorem valid far from equilibrium: 
the complex singularities of the susceptibility are in part those of the 
spectral density, and in part of a different nature to be discussed.