1. G. V. Efimov: preprint JINR (Dubna), E-1204, (1963);
2. Žurn. Ėksp. Teor. Fiz.,44, 2107 (1963).
3. G. V. Efimov:Phys. Lett.,4, 314 (1963).
4. For example:R. E. Cutkosky:Lectures on Analytic Properties of the S-Matrix in Perturbation Theory (Nordita, 1962).
5. N. N. Bogoliubov andD. V. Shirkov:Introduction to the Theory of Quantized Fields (New York, 1959).