When SU(5) is extended by the addition of its Higgs representations [75.sub.H] and [15.sub.H], it achieves two objectives: (i) neutrino mass and mixing generation through type-II seesaw mechanism and (ii) precision gauge coupling unification with experimentally accessible proton lifetime.
Section 3 deals with the problem of gauge coupling unification. In Section 4 we make proton lifetime prediction including possible uncertainties.
But as we will find in the subsequent sections, gauge coupling unification in the present SU(5) framework imposes the lower bound [[mu].sub.[DELTA]] [equivalent] [M.sub.[DELTA]] [greater than or equal to] [10.sup.9.23] GeV.
Blumenhagen, "Gauge coupling unification
in F-theory grand unified theories," Physical Review Letters, vol.
Using the D-Parity breaking mechanism of [55, 56], an interesting model of linear seesaw mechanism in the context of supersymmetric SO(10) with successful gauge coupling unification  has been suggested in the presence of three gauge singlet fermions.
Gauge coupling unification in these SO(10) models requires the [G.sub.2213] symmetry to occur at the intermediate scale in the renormalisable model .
Zhang, "Dark matter and gauge coupling unification in nonsupersymmetric SO(10) grand unified models," Physical Review D, vol.
Weiser, "Gauge coupling unification
in left-right symmetric models," Physics Letters.