Generalized Aubry-André formula and continuity of the intersection spectrum of the Almost Mathieu operator
We consider the spectrum of the Almost Mathieu operator (AMO) and show that the moments of the restriction of the Lebesgue measure to the intersection spectrum $\text{Leb}|_{Σ_{α,λ}}$ are polynomials in coupling $λ$ with coefficients that are trigonometric polynomials in frequency $α$. The statement can be considered as a generalization of the Aubry-André formula for the measure of the spectrum of AMO. As a corollary, we obtain that the restriction of the Lebesgue measure to the intersection spectrum that we denote by $μ^{-}_{α, λ}$ depends continuously on the parameters (frequency $α$ and coupling $λ$) in weak-* topology. Moreover, we prove that the dependence is not just continuous but analytic in $λ$ and $C^{\infty}$ in $α$ in a sense that an integral of an analytic test function $φ(x)$ with respect to $μ^{-}_{α, λ}$ has the same kind of dependence. In particular, this implies that the Lebesgue measure of the part of the spectrum $Σ_{α,λ}$ that lies between two gaps depends analytically on the coupling constant $λ$ and $C^{\infty}$ on the frequency $α$ in an open domain (away from the critical coupling $λ=1$) where these gaps do not bifurcate.