1. A sample of recent developments, with abundant references to previous work, is contained in:Scale and Conformal Symmetry in Hadron Physics, edited byR. Gatto (New York, N. Y., 1973); see alsoS. Ferrara, R. Gatto andA. F. Grillo:Conformal Algebra in Space-Time (Berlin, 1973). Earlier references can also be found in the beautiful paper byG. Mack andA. Salam:Ann. of Phys.,53, 174 (1969).
2. S. Fubini:Nuovo Cimento,34 A, 521 (1976).
3. For one of the first examples, see,e.g. A. Bastai, L. Bertocchi, S. Fubini, G. Furlan andM. Tonin:Nuovo Cimento,30, 1512 (1963).
4. See, for instance,B. G. Wybourne:Classical Groups for Physicists (New York, N. Y., 1974).
5. Ch. P. Boyer andK. B. Wolf:Communicaciones Tecnicas, Vol.6, Serie B, No. 104 (1975).