1. C. Bernardini, G.F. Corazza, G.Ghigo and B.Touschek, Nuovo Cim. 18 (1960) 1293.
2. J. Le Francois, rapporteur talk,Cornell Conf. 1971.
3. V. Sidorov, rapporteur talk, Cornell Conf. 1971.
4. Pioneering work: G.K. O’Neill,Bulletin A. Phys. Soc. Ser. 23 (1958) 158.
5. B.H. Wiik and G. Wolf, DESY 78/23.