2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/44349We discuss the question why the mixing $U_{e3}$ is small. The natural answer is $U_{e3}=0$ in some symmetric limit, in which two large mixings are realized. It is possible to force $U_{e3}$ and $\cos{2 θ_{23}}$ to be zero by imposing a discrete symmetry. We investigate a special class of symmetries $Z_2$ and of the consequences of their perturbative violation.6 pages, 2 figures, Talk at Rencontres de Moriond 2005(EW Interactions and Unified Theories)High Energy Physics - Phenomenologytext