1. J.C. Pati and A. Salam, Lepton Number as the Fourth Color, Phys. Rev. D 10 (1974) 275 [Erratum ibid. D 11 (1975) 703] [ INSPIRE ].
2. P. Fileviez Perez and M.B. Wise, Low Scale Quark-Lepton Unification, Phys. Rev. D 88 (2013) 057703 [ arXiv:1307.6213 ] [ INSPIRE ].
3. S.M. Barr, Some Comments on Flipped SU(5) × U(1) and Flipped Unification in General, Phys. Rev. D 40 (1989) 2457 [ INSPIRE ].
4. A. De Rujula, H. Georgi and S.L. Glashow, Flavor goniometry by proton decay, Phys. Rev. Lett. 45 (1980) 413 [ INSPIRE ].
5. H. Georgi, S.L. Glashow and M. Machacek, μ + Polarization in Proton Decay: A Probe of Flavor Mixing in Unified Models, Phys. Rev. D 23 (1981) 783 [ INSPIRE ].