Line geometry and electromagnetism III: groups of transformations
  The role of linear and projective groups of transformations in line geometry and electromagnetism is examined in accordance with Klein's Erlanger Programm for geometries. The group of collineations of real projective space is chosen as the most general group, and reductions to some of its various subgroups are then detailed according to their relevance to electromagnetic fields, and especially wave-like ones. 
D. H. Delphenich
gr-qc