1. G. Gardner, J. M. Greene, M. D. Kruskal and R. M. Minca: Comm. Pure and Appl. Math., 27, 97 (1974).
2. Y. Nogami and C. S. Warke: Phys. Lett., 59 A, 251 (1976).
3. V. Glasser, H. Grosse and A. Martin: CERN preprint, TH, 2432 (1977).
4. D. V. Choodnovsky and G. V. Choodnovsky: Ecole Polytechnique, Seminaire Herman-Choodnovsky, preprint (Novembre 1977).
5. I. M. Gel’fand and L. A. Dikü: Funct. Anal. and its Appl., 11, No. 2 (1976).