The fibered rotation number for ergodic symplectic cocycles and its applications: I. Gap Labelling Theorem

Abstract

Let $ (Θ,T,μ) $ be an ergodic topological dynamical system. The fibered rotation number for cocycles in $ Θ\times \mathrm{SL}(2,\mathbb{R}) $, acting on $ Θ\times \mathbb{R}\mathbb{P}^1 $ is well-defined and has wide applications in the study of the spectral theory of Schrödinger operators. In this paper, we will provide its natural generalization for higher dimensional cocycles in $ Θ\times\mathrm{SP}(2m,\mathbb{R}) $ or $ Θ\times \mathrm{HSP}(2m,\mathbb{C}) $, where $ \mathrm{SP}(2m,\mathbb{R}) $ and $ \mathrm{HSP}(2m,\mathbb{C}) $ respectively refer to the $ 2m $-dimensional symplectic or Hermitian-symplectic matrices. As a corollary, we establish the equivalence between the integrated density of states for generalized Schrödinger operators and the fibered rotation number; and the Gap Labelling Theorem via the Schwartzman group, as expected from the one dimensional case [AS1983, JM1982].