1. H. Lehmann, K. Symanzik, W. Zimmermann: Nuovo Cimento 1 (1955) 1425; J.D. Bjorken, S.D. Drell: Relativistic quantum fields, p. 136, New York: McGraw-Hill 1965
2. R. Haag: unpublished Copenhagen lectures (1953);
3. G. Barton: Introduction to advanced field theory, Interscience 1963
4. J.D. Bjorken, S.D. Drell: loc. cit. p. 308.
5. G. Källén: Helv. Phys. Acta 25 (1952) 417; J.D. Bjorken, S.D. Drell: loc. cit. Relativistic quantum fields, p. 141, New York: McGraw-Hill 1965