Few exact results on gauge symmetry factorizability on intervals
  We study the gauge symmetry factorizability by boundary conditions on intervals of any dimensions. With Dirichlet-Neumann BCs, the Kaluza-Klein decomposition in five-dimension for arbitrary gauge group can always be factorized into that for separate subsets of at most two gauge symmetries, and so is completely solvable. Accordingly, we formulate a limit theorem on gauge symmetry factorizability on intervals to recapitulate this remarkable feature of five-dimension case. In higher-dimensional space-time, an interesting chained mixing of gauge symmetries by Dirichlet-Neumann BCs is explicitly constructed. The systematic decomposition picture obtained in this work constitutes the initial step towards determining the general symmetry breaking scheme by boundary conditions. 
Ngoc-Khanh Tran