**Acknowledgements**

The author would like to thank Dr. Lucia Biagini for her helpful advice on various technical issues examined in this paper, and for her help in the graphic realization of the figures that exemplify the concepts expressed in the paragraphs of this chapter. The author also wishes to thank Dr. Livio Galosi, for his usual support activities in the revision of the proofs and in the editing of this and other papers written by the author. We also thank Mrs. Subeide Mari for her technical assistance for all the experimental parts of the numerous papers that have been written by the author over the years on the topics covered in this chapter.
