I find that some book e.g. Many-body theory of solids by John C. Inkson P145 says that :" By analogy with the classical polarization, we define a polarization propagator through the relationship ... ε=1-vP" , where ε is the dielectronic response function, and v is the bare Coulomb potential. I do not remember such relationship in electrodynamics. Can anybody remind me of it? In fact, on the next page in the book, I find a new relationship i.e. P=δρ/δV， where V is the total potential instead of external potential and ρ is the electron density. But is there any similarity between this polarization function P and the classical polarization?