1. J.C. Pati, A. Salam, Phys. Rev. D 10, 275 (1974)
2. R.N. Mohapatra, R. Marshak, Phys. Rev. Lett. 44, 1316 (1980)
3. P. Fileviez, Perez. Phys. Lett. B 654, 189 (2007). arXiv:hep-ph/0702287
4. R. Barbier, C. Berat, M. Besancon, M. Chemtob, A. Deandrea et al., Phys. Rept. 420, 1 (2005). arXiv:hep-ph/0406039
5. R. Mohapatra, N. Okada, H.-B. Yu, Phys. Rev. D 77, 011701 (2008). arXiv:0709.486