Elementary energy bands in crystalline solids

Elementary energy bands in solids are defined generalizing the notion of si mple and composite energy bands. All energy branches of elementary energy b ands are connected. Following the connectivity of the energy branches in el ementary energy bands, a new notion of unavoidable degeneracy is establishe d, for which the crossings can be moved but not removed. This degeneracy do es not depend on the particular shape of the potential and is present in al l crystals of some given space group symmetry.