Define mean values ξ ξ i ( t ) and Ξ Ξ i ( t ) of fluctuating quantities x i and X i respectively such that they take given values x 1 , x 2 , … … , X 1 , X 2 , … … at t = 0 . Note that ξ ξ ˙ ˙ i ( t ) = − − γ γ i k Ξ Ξ k ( t ) .