Hidden state-variables and a non-Markoffian formulation of reactor noise

A non-Markoffian Langevin equation df(t)/dt=-∫ sub(t0) sup(t) B(t-t')f(t')dt'+Z(t-t0)g(t0)+F(t) for the fluctuations of known state-variables, f(t), is proposed. The non-Markoffian effect and the new stochastic term Z(t-t0)g(t0) are due to ''hidden'' state-variables g(t). The random force F(t) meets the requirement of the causality condition, but the random force given by R(t)=Z(t-t0)g(t0)+F(t) does not. A new relation between the fluctuation R(t) and the dissipation B(t) is found. The effect of external noise sources is formulated to demonstrate that the present formalism is useful for finding out hidden state-variables. (auth.)