1. C. G. Bollini andJ. J. Giambiagi:Nuovo Cimento B,12, 20 (1972);G. ’t Hooft andM. Veltman:Nucl. Phys. B,44, 189 (1972).
2. For earlier work, seeM. S. Chanowitz andJ. Ellis:Phys. Lett. B,40, 397 (1972);R. J. Crewther:Phys. Rev. Lett.,28, 1421 (1972).
3. For reviews, seeS. L. Adler: inLectures on Elementary Particles and Quantum Field Theory, edited byS. Deser, M. Grisaru andH. Pendleton (Cambridge, Mass., 1970), p. 1;S. B. Treiman, R. Jackiw andD. J. Gross:Lectures on Current Algebra and Its Applications (Princeton, N. J., 1972), p. 97.
4. S. N. Gupta:Proc. Phys. Soc. London Ser. A,64, 426 (1951). See alsoS. N. Gupta:Quantum Electrodynamics (New York, N. Y., 1977), p. 161.