Discrete space-time symmetry, polarization eigenmodes and their   degeneracies
  The irreducible representations of the group C4(direct product)V can be used to distinguish polarization eigenmodes, to account for their degeneracies and to associate them with particular magnetic crystal classes. The occurrence of this finite Abelian group points to a possible connection with topology but, irrespective of this, qualitative features of gyrotropy in condensed media can be approached in a way that does not depend on arbitrarily truncated tensor expansions. 
R J Potton
physics.optics