A non-perturbative scheme, based on the functional generalization of the Callan-Symanzik equation is developed to treat the Coulomb interaction in an electron gas. The one-particle irreducible vertex functions are shown to satisfy an evolution equation whose initial condition is given by means of the classical action and the final point corresponds to the physical system, This equation is truncated by expanding it in momenta and excitation energies. leaving the electric charge as an arbitrary, not necessarily small parameter, Exact coupled partial differential equations up to first order in the frequencies and excitation energies are derived, The numerical integration of these equations is left to a later stage. Nevertheless, in order to demonstrate the relation with the perturbation expansion the one-loop Lindhard function and screening are reproduced in the independent mode approximation of the evolution equation. (C) 2002 Elsevier Science (USA). [References: 16]
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