Features of normal wave dispersion in periodic structures

The dispersion dependences of normal waves in periodic structures are multivalued in nature. A physically justified choice of a single dispersion branch for each wave requires additional information. Questions are discussed on reducing the uncertainty in choosing dispersion, in particular, based on a limited transition from an analyzed periodic structure to another one, for which dispersion dependences are known with more certainty. Certain inaccuracies and errors in known publications connected with ambiguity of dispersion are noted and corrected.