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We present a new method to investigate the exciton spectrum in the case of degenerate bands. Using symmetry considerations and second-order perturbation theory, we obtain a simple analytical expression for the binding energy as a function of the band parameters. Direct excitons in group IV elements, III-V compounds, and II-VI compounds can be investigated by this method. Results are given for Ge, GaAs, InSb, ZnSe, and CdTe.
Baldereschi et al. (Mon,) studied this question.