1. M. Deutsch andB. Dulit:Phys. Rev.,84, 601 (1951).
2. O. Halpern:Phys. Rev.,94, 904 (1954).
3. A. Bisi, A. Dupasquier andL. Zappa:J. Phys. C,4, L33 (1971).
4. A. Bisi, A. Dupasquier andL. Zappa:J. Phys. C,6, 1125 (1973).
5. A. Dupasquier: inPositron Solid Slate Physics, edited byW. Brandt andA. Dupasquier (North Holland, Amsterdam, New York, N. Y., and Oxford, 1983), p. 510. Equation 5 of this reference is wrong; the correct expression, is given here as eq. 1.