1. Bender, C.M., Gianfreda, M., Özdemir, Ş.K., Peng, B., Yang, L.: Phys. Rev. A 88, 062111 (2013)
2. Peng, B.,K. Özdemir, Ş, Lei, F., Monifi, F., Gianfreda, M., Long, G.L., Fan, S., Nori, F., Bender, C.M., Yang, L.: Nonreciprocal light transmission in parity-time-symmetric whispering-gallery microcavities, arXiv:physics.optics/ 1308.4564
3. Cotton, F.A.: Chemical Applications of Group Theory, 3rd edn. Wiley, New York (1990)
4. Tinkham, M.: Group Theory and Quantum Mechanics. McGraw-Hill Book Company, New York (1964)
5. Klainman, S., Cederbaum, L.S.: Phys. Rev. A 78, 062113 (2008)