1. P. A. M. Dirac:Proc. Roy. Soc.,209 A, 292 (1951);212 A, 330 (1952);223 A, 438 (1954);Nature,168, 906 (1951);169, 146, 702 (1952). See also,E. Schködinger:Nature,169, 538 (1952).
2. J. A. Cronin:Phys. Rev.,161, 1483 (1967).
3. Y. Nambu:Prog. Theor. Phys. Suppl., extra number, 190 (1968). See also the comment byY. Nambu: inProceedings of the 1967 International Conference in Particles and Fields, p. 304.
4. J. S. R. Chisholm:Nucl. Phys.,26, 469 (1961).
5. S. Weinberg:Phys. Rev. D,2, 674, 3085 (1970);L. S. Brown:Phys. Rev. D,2, 3083 (1970).