$S_3$ Symmetry and Tri-bimaximal Mixing
  The near maximal value for the atmospheric neutrino mixing angle together with the fact that the solar mixing angle satisfies the relation $\sin^2\theta_\odot\simeq {1/3}$ is the basis for the so called tri-bimaximal mixing (tbm) when $\theta_{13}=0$. In this note, we explore the possibility that tri-bimaximal mixing is an indication of a softly broken higher leptonic symmetry $S_3$, the permutation of three lepton families that embeds the $\mu-\tau$ exchange symmetry of leptons. 
R.N. Mohapatra, S. Nasri and Hai-Bo Yu