LINEAR-RESPONSE IN STOCHASTIC MEAN-FIELD THEORIES AND THE ONSET OF INSTABILITIES

We study the small amplitude response of stochastic one-body theories, such as the Boltzmann-Langevin approach. Whereas the two-time correla tion function only describes the propagation of fluctuations initially present, the equal-time correlation function is related to the source of stochasticity. For stable systems it yields the Einstein relation, while for unstable systems it determines the growth of the instabilit ies. These features are illustrated for unstable nuclear matter in two dimensions.