1. Castell, L.: Integral transformations in momentum space and conformal invariance. M. P. I. Munich, preprint, October 1969, to be published in J. Math. Phys.
2. Gross, D. J., Wess, J.: CERN preprint, TH 1076-CERN (1969).
3. Coon, D. D.: J. Math. Phys.10, 1314 (1969).
4. Bargmann, V.: Ann. Math.48, 568 (1947).
5. Castell, L.: ICTP, Trieste, preprint IC/67/71, to be published in J. Math. Phys.