1. N. N. Bogoliubov, B. V. Medvedev andM. K. Polivanov:Fort. d. Phys.,6, 159 (1959). SeeN. N. Bogoliubov andD. V. Shirkov:Introduction to the Theory of Quantized Fields (New York, 1959), chap.IX.
2. H. J. Bremermann, R. Oehme andJ. G. Taylor:Phys. Rev.,109, 2178 (1958).
3. H. Lehmann:Nuovo Cimento,10, 579 (1958).
4. S. Mandelstam:Phys. Rev.,115, 1741 (1959).
5. G. Källén andA. Wightman:Mat. Fys. Skr. Dan. Vid. Selsk.,1, no. 6 (1958).