1. R. Fedele, G. Miele andG. Ricciardi: University of Pisa and INFN Sezione di Pisa preprint IFUP-TH 11/89, (march 1989), unpublished.
2. M. Sands:Proceedings of the International School of Physics «Enrico Fermi», Course XLVI, edited byB. Toushek (Academic Press, New York, N.Y., 1971), p., 257.
3. K. Steffen: inProceedings of the CERN Accelerator School, Gif-sur-Yvette, Paris, september 3–14 1984, edited byP. Bryant andS. Turner, Vol.1 (Geneva, 1985), p. 25.
4. S. Solimeno, B. Crosignani andP. Di Porto:Guiding, Diffraction and Confinement of Optical Radiation (Academic Press, London, 1986).
5. L. D. Landau andE. M. Lifsits:Elettrodinamica dei mezzi continui (Editori Riuniti, Roma, 1986), p. 424.