“…Examples of periodic quantum, acoustic and elastic waveguides with gaps in their essential spectra have been presented in the literature, see [2,3,5,6,10,11,23,24,29], and [7,8,22,28], respectively. However, the approach of these papers, or the general approach of [21], cannot be applied to the piezoelectric waveguides of the present paper, because a direct weak formulation of the piezoelectricity problem does not correspond to a semi-bounded self-adjoint operator; the essential spectrum of such a problem would be non-physical, covering the whole plane C as shown in Sect.…”