2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/129200Minimal conductivity of a single undoped graphene layer is known to be of the order of the conductance quantum, independent of the electron velocity. We show that this universality does not survive electron-electron interaction which results in the non-trivial frequency dependence. We begin with analyzing the perturbation theory in the interaction parameter 'g' for the electron self-energy and observe the failure of the random-phase approximation. The optical conductivity is then derived from the quantum kinetic equation and the exact result is obtained in the limit when g << 1 << g lnω.4 pages, 3 figures; final versionMesoscale and Nanoscale Physicstext