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Elementary energy bands in solids are defined generalizing the notion of simple and composite energy bands. All energy branches of elementary energy bands are connected. Following the connectivity of the energy branches in elementary energy bands, a new notion of unavoidable degeneracy is established, for which the crossings can be moved but not removed. This degeneracy does not depend on the particular shape of the potential and is present in all crystals of some given space group symmetry.
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Michel et al. (2000) studied this question.
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