Generating spectral gaps by geometry
  Motivated by the analysis of Schr\"odinger operators with periodic potentials we consider the following abstract situation: Let $\Delta_X$ be the Laplacian on a non-compact Riemannian covering manifold $X$ with a discrete isometric group $\Gamma$ acting on it such that the quotient $X/\Gamma$ is a compact manifold. We prove the existence of a finite number of spectral gaps for the operator $\Delta_X$ associated with a suitable class of manifolds $X$ with non-abelian covering transformation groups $\Gamma$. This result is based on the non-abelian Floquet theory as well as the Min-Max-principle. Groups of type I specify a class of examples satisfying the assumptions of the main theorem. 
Fernando Lled\'o and Olaf Post