Kohn's theorem and correlation functions for a Fermi liquid

In this paper we derive Kohn`s theorem and calculate the spin and current response functions for a Fermi liquid. A magnetic weight is introduced as the limit {omega}/{ital q}{r_arrow}0 of the spin response function in analogy to the superfluid weight recently introduced by Scalapino and collaborators. It is conjectured that in this limit both the magnetic-ordered and the Fermi-liquid spin response functions have the same value.