2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/147500A microscopic theory of zero wavelength (q=0) interaction in finite--size systems is proposed. Its exact solution interpolates between the Coulomb blockade and the perturbative Altshuler--Aronov theory, in the strong and weak interaction limits respectively. The tunneling density of states and the quasiparticle life--time are calculated. The physical nature of the q=0 component of the interaction is discussed.10 pages, 2 figures, REVTEX 3.0. Figures are tarred compressed, no changes in the text.Condensed Mattertext