Dihedral Group and Repetitive Achromats with Mirror Symmetric or Mirror   Antisymmetric Basic Cell
  Using the group-theoretical point of view we study in this paper second and third order repetitive achromats with a mirror symmetric or mirror antisymmetric basic cell and compare these achromats with repetitive achromats designed without internal cell symmetries taken into account. 
V. Balandin, R. Brinkmann, W. Decking, N. Golubeva
physics.acc-ph