  We derive an exact expression of the response function to an infinitesimal
magnetic field for an Ising-Glauber-like model with arbitrary exchange
couplings. The result is expressed in terms of thermodynamic averages and does
not depend on the initial conditions or on the dimension of the space. The
response function is related to time-derivatives of a complicated correlation
function and so the expression is a generalisation of the equilibrium
fluctuation-dissipation theorem in the special case of this model.
Correspondence with the Ising-Glauber model is discussed. A discrete-time
version of the relation is implemented in Monte Carlo simulations and then used
to study the aging regime of the ferromagnetic two-dimensional Ising-Glauber
model quenched from the paramagnetic phase to the ferromagnetic one. Our
approach has the originality to give direct access to the response function and
the fluctuation-dissipation ratio.
