2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/179303We present a formalism for the description of oscillations in matter of solar, atmospheric and long-baseline neutrinos in the two schemes of four-neutrino mixing that are allowed by the results of all existing neutrino oscillation experiments.31 pages including 3 figures, RevTeXHigh Energy Physics - Phenomenologytext