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We propose an SO(18) grand-unified model that predicts three light generations, if the horizontal symmetry below the grand-unification scale is chosen to be Sp(4)SU(2)d. A novel feature of this model is the existence of an unbroken SU(2) gauge symmetry, under which the light generations are singlets. This raises some interesting phenomenological possibilities.
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Chang et al. (1985) studied this question.
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