1. M. Toller, CERN Report 67/582/5 - Th. 780. This contains an extensive list of past references and summarises the author's own work on the topic
2. J. F. Boyce, R. Delbourgo, A. Salam and J. Strathdee, Trieste preprint IC/67/9.
3. Regge poles and the scattering of unequal mass particles
4. G. Domokos, Phys. Rev., to be published
5. 0(4) Symmetry and Regge-Pole Theory