The best way to tackle the many body problems in Physics is the density functional theory (DFT). This is due to the fact that the density in the many body systems plays exactly the same role as position function in few body systems. Once we have the density of a system, we can say a lot about it. To obtain the density of the system we need to first of all obtain the grand potential functional of the system which is a functional of the density function and other system variables. But the point here is that how to know the grand potential functional. The answer to this question is simply that we need to guess the grand potential functional. Our guess is based on general properties of the system. Here is the most probable guess for the grand potential functional of the polymer brushes.