Flipped SU(5) GUT phenomenology: proton decay and John Ellis, Jason L. Evans, Natsumi Nagata, Dimitri V. Nanopoulos and Keith A. Olive Eur. Phys. J. C, 81 12 (2021) 1109 Published online: 15 December 2021 DOI: 10.1140/epjc/s10052-021-09896-x