Grand Unification from Gauge Theory on $M_4 \times Z_N$
  The SU(5) grand unified theory (GUT) is derived from the geometrical point of view of gauge theory on three-sheeted space-time, i.e., $M_4 \times Z_3$ manifold without recourse to noncommutative geometry. A derivation of SO(10) GUT is also discussed in the same point of view. 
Masahiro Kubo, Ziro Maki, Mikio Nakahara (Kinki University), Takesi   Saito (Kwansei Gakuin University)