1. Dirac P. A. M.,Proc. R. Soc. London, Ser. A,133 (1931) 60;Phys. Rev.,74 (1948) 817.
2. Fierz M.,Helv. Phys. Acta,17 (1944) 27, and alsoBanderet P. P.,Helv. Phys. Acta,19 (1946) 503. See further:Bocchieri P. andLoinger A.,Nuovo Cimento A,59 (1980) 121;Loinger A.,Riv. Nuovo Cimento,10 (1987) 1.
3. Schrödinger E.,Space-Time Structure (The University Press, Cambridge) 1960, p. 24 and following.
4. «… one musn't believe in anything too strongly; one must always be prepared that various beliefs one has had for a long time may be overthrown.» (From theLectures on Quantum Field Theory byP. A. M. Dirac (Academic Press Inc., New York, N.Y.) 1966, p. 8.)