The degrees of freedom of interest here is the velocity formula_2 of the particle, formula_3 denotes the particle's mass. The force acting on the particle is written as a sum of a viscous force proportional to the particle's velocity (Stokes' law), and a "noise term" formula_4 (the name given in physical contexts to terms in stochastic differential equations which are stochastic processes) representing the effect of the collisions with the molecules of the fluid. The force formula_4 has a Gaussian probability distribution with correlation function