1. O. Akimoto andE. Hanamura:Journ. Phys. Soc. Japan,33, 1537 (1972);W.-T. Huang:Phys. Stat. Sol.,60 (b), 309 (1973);W. F. Brinkman, T. M. Rice andB. Bell:Phys. Rev. B,8, 1570 (1973). See also references therein to previous work.
2. This is also called the «interband» interaction. SeeE. Hanamura:Journ. Phys. Soc. Japan,39, 1506 (1975). The «Coulomb» part is recognized in this reference.
3. The exchange interaction of excitons is discussed in the following two useful papers.G. E. Pikus andG. L. Bir:Sov. Phys. JETP,33, 108 (1971)—here in eq. (47) the sign ofΛ 1 should be changed—andM. Suffczynski, L. Swierkowski andW. Wardzynski:J. Phys. C,8, L52 (1975).
4. The dependence of ε onr 12 is neglected in this paper. SeeH. Haken: inPolarons and Excitons, edited byC. G. Kuper andG. D. Whitfield (Edinburg, 1963), p. 295.
5. E. J. Elliott: inPolarons and Excitons, edited byC. G. Kuper andG. D. Whitfield (Edinburgh, 1963), p. 269, especially eq. (6.7).