1. Suitable reviews and an extended list of references can be found in the 1976 Les Houches Session XXIX “Weak and Electromagnetic Interactions at High Energy” (ed. R. Balian and C. H. Llewellyn Smith). See also the other lectures in this volume and, in particular, H. D. Politzer, Phys. Rep. 14C, 129 (1974).
2. See, e.g., I. Hinchliffe and C. H. Llewellyn Smith, Nucl. Phys. B128, 93 (1977).
3. G. B. West, Phys. Rep. 18C, 265 (1975).
4. H. Harari, SLAC preprint.
5. Within the covariant parton model, there exists an attempt to calculate σγ using F2(x) as input: see G. B. West, Phys. Rev. Lett. 31, 798 (1973).