1. An excellent summary of the data on charm production is given by S. Wojcicki: In Proceedings of the XXth International Conference on High Energy Physics, AIP Conf. Proc. 68, p. 1430, eds. L. Durand, L. Pondrom (1981)
2. M. Basile et al.: CERN Report EP/81-38 (1981)
3. R.J.N. Phillips: Ref. 1, An excellent summary of the data on charm production is given by S. Wojcicki: In Proceedings of the XXth International Conference on High Energy Physics, AIP Conf. Proc. 68, p. 1470
4. S. Brodsky, P. Hoyer, C. Peterson, N. Sakai: Phys. Lett.93B, 451 (1980); S. Brodsky, C. Peterson, N. Sakai: SLAC-PUB-2660
5. V. Barger, F. Halzen, W.Y. Keung: Phys. Rev. (to be published);