A Multi-Scale Analysis Scheme on Abelian Groups with an Application to Operators Dual to Hill's Equation

Abstract

We present an abstract multiscale analysis scheme for matrix functions $(H_{\varepsilon}(m,n))_{m,n\in \mathfrak{T}}$, where $\mathfrak{T}$ is an Abelian group equipped with a distance $|\cdot|$. This is an extension of the scheme developed by Damanik and Goldstein for the special case $\mathfrak{T} = \mathbb{Z}^ν$. Our main motivation for working out this extension comes from an application to matrix functions which are dual to certain Hill operators. These operators take the form $H_{\tildeω}=-\frac{d^2}{dx^2} + \varepsilon U(\tildeωx)$, where $U$ is a real smooth function on the torus $\mathbb{T}^ν$, $\tildeω\in \mathbb{R}^ν$ is a vector with rational components, and $\varepsilon$ is a small parameter. The group in this particular case is the quotient $\mathfrak{T} = \mathbb{Z}^ν/\{m\in\mathbb{Z}^ν:m\tildeω=0\}$. We show that the general theory indeed applies to this special case, provided that the rational frequency vector $\tildeω$ obeys a suitable Diophantine condition in a large box of modes. Despite the fact that in this setting the orbits $k + mω$, $k \in \mathbb{R}$, $m\in\mathbb{Z}^ν$ are not dense, the dual eigenfunctions are exponentially localized and the eigenvalues of the operators can be described as $E(k+mω)$ with $E(k)$ being a "nice" monotonic function of the impulse $k \ge 0$. This enables us to derive a description of the Floquet solutions and the band-gap structure of the spectrum, which we will use in a companion paper to develop a complete inverse spectral theory for the Sturm-Liouville equation with small quasi-periodic potential via periodic approximation of the frequency. The analysis of the gaps in the range of the function $E(k)$ plays a crucial role in this approach.