1. E. R. Caianiello:Coll. sur les Probl. Math. de la Théorie des Champs (Lille, 1957);Max Planck Festschr. (1958);
2. Nuovo Cimento,13, 637 (1959) and14, 185 (1959). (The latter two papers will be referred to in the text as I and II).
3. J. Hadamard:Le problème de Cauchy et les equations aux dérivées partielles linéares hyperboliques (Paris, 1932).
4. Cfr. for instanceH. J. Bremermann: O.N.R. Techn. Rep. no 8, Dept. of Math. Berkeley.
5. N. N. Bogoliubov andD. V. Shirkov:Introduction to the Theory of Quantum Fields (London, 1959).