Asymptotic formulas for spectral gaps and deviations of Hill and 1D Dirac operators

Abstract

Let $L$ be the Hill operator or the one dimensional Dirac operator on the interval $[0,\pi].$ If $L$ is considered with Dirichlet, periodic or antiperiodic boundary conditions, then the corresponding spectra are discrete and for large enough $|n|$ close to $n^2 $ in the Hill case, or close to $n, \; n\in \mathbb{Z}$ in the Dirac case, there are one Dirichlet eigenvalue $\mu_n$ and two periodic (if $n$ is even) or antiperiodic (if $n$ is odd) eigenvalues $\lambda_n^-, \, \lambda_n^+ $ (counted with multiplicity). We give estimates for the asymptotics of the spectral gaps $\gamma_n = \lambda_n^+ - \lambda_n^-$ and deviations $ \delta_n =\mu_n - \lambda_n^+$ in terms of the Fourier coefficients of the potentials. Moreover, for special potentials that are trigonometric polynomials we provide precise asymptotics of $\gamma_n$ and $\delta_n.$