Effective Drag Between Strongly Inhomogeneous Layers: Exact Results and   Applications
  We generalize Dykhne's calculation of the effective resistance of a 2D two-component medium to the case of frictional drag between the two parallel two-component layers. The resulting exact expression for the effective transresistance, $\rho^D_{eff}$, is analyzed in the limits when the resistances and transresistances of the constituting components are strongly different - situation generic for the vicinity of the {\em classical} (percolative) metal-insulator transition (MIT). On the basis of this analysis we conclude that the evolution of $\rho^D_{eff}$ across the MIT is determined by the type of correlation between the components, constituting the 2D layers. Depending on this correlation, in the case of two electron layers, $\rho^D_{eff}$ changes either monotonically or exhibits a sharp maximum. For electron-hole layers $\rho^D_{eff}$ is negative and $|\rho^D_{eff}|$ exhibits a sharp minimum at the MIT. 
V. M. Apalkov and M. E. Raikh (University of Utah)