On diffusion approximation of a slow component for solution of   stochastic differential equation of Ito
  For the concrete model of Brownian particles dynamics in non-uniform environment, the time interval estimation is constructed, on which phenomenological Fick laws for diffusion phenomenon description can be used. The knowledge of these estimations gives the possibility to judge about adequacy of the initial assumption about dynamics of the real processes. Noted, that such an assessment has not been introduced yet, up to date. 
V.A Doobko
math.PR