Gauge coupling flux thresholds, exotic matter and the unification scale in F-SU(5) GUT G. K. Leontaris and N. D. Tracas Eur. Phys. J. C, 67 3-4 (2010) 489-498 Published online: 31 March 2010 DOI: 10.1140/epjc/s10052-010-1298-2