Selection Rule for Generation Numbers and Gauge Anomalies for   Unification Groups
  The possibilities of global (non-perturbative) gauge anomalies for a class of gauge groups are investigated. Intimately connected to branching rules and topological aspect of gauge groups, the results are applied to the study of unification gauge groups such as SO(10), SU(5), $E_6$, $E_8$ etc. Especially, we discuss extensively about the selection rule for generation numbers $N_f+N_{mf}=even\ge 4$ in SO(10) and supersymmetric SO(10) unification theories as originally proposed by the author$^1$, where $N_f$ and $N_{mf}$ denote the generation numbers for ordinary fermions and mirror fermions respectively. This is due to the global gauge anomalies from some subgroups of SO(10) in a fundamental spinor representation such that the ill-defined 'large' gauge transformations in the subgroup cannot be unwrapped in SO(10) in the quantum theory as we noted$^1$. A similar result related to left-right symmetric models is also given. 
Huazhong Zhang