Araki, H., Hepp, K., Ruelle, D.: Asymptotic behaviour of Wightman functions. Helv. Phys. Acta35, 164 (1962)
Ruelle, D.: On the asymptotic condition in quantum field theory. Helv. Phys. Acta35, 147 (1962)
Hagg, R., Swieca, J. A.: When does a quantum field theory describe particles? Commun. Math. Phys.1, 308 (1965)
Nevanlinna, R.: Analytic functions. Berlin, Heidelberg, New York: Springer 1970