Is<i>S</i><sub>4</sub>the horizontal symmetry of tri-bimaximal lepton mixing?
Is<i>S</i><sub>4</sub>the horizontal symmetry of tri-bimaximal lepton mixing?
We determine the symmetry groups under which the charged-lepton and the Majorana-neutrino mass terms are invariant. We note that those two groups always exist trivially, i.e. independently of the presence of any symmetries in the Lagrangian, and that they always have the same form. Using this insight, we re-evaluate the …