1. V. de Alfaro, S. Fubini, G. Furlan and C. Rossetti, Phys. Lett. 21, 576 (1966). The idea that strong interaction sum rules can be obtained from asymptotic limits has been independently developed by L. D. Soloviev, Sov. Journ. of Nucl. Phys. 3, 131 (1966).
2. K. Hepp, Heiv. Phys. Acta, 37, 55 (1964). See, also for general references, the paper by G. C. Fox, “Methods for constructing invariant amplitudes free from kinematical singularities and zeros”. Cavendish Lab. preprint, Nov. 1966.
3. T. L. Trueman, Phys. Rev. Lett. 17, 1198 (1966). See also R. Odorico, University of Trieste preprint.
4. V. de Alfaro, S. Fubini, G. Furlan and C. Rossetti, “Superconvergence and current algebra”, University of Torino preprint, Dec. 1966.
5. M, Gell-Mann, S. Frautschi and F. Zachariasen, Ptiys. Rev. 126, 2204 (1962).