1. N. N. Bogoliubov andD. V. Shirkov:Introduction to the Theory of Quantized Fields (Moscow, 1957), chap. IX.
2. G. R. Screaton:Dispersion Relations (New York, 1960).
3. S. S. Schweber:An Introduction to Relativistic Quantum Field Theory (New York, 1961), chap. 18.
4. C. DeWitt andR. Omnès:Dispersion Relations and Elementary Particles (New York, 1960).
5. G. F. Chew:S-matrix Theory of Strong Interactions (New York, 1961), p. 3. Contributions to the development and application of this idea have been made by:L. D. Landau:Nucl Phys.,13, 181 (1959);