Riesz basis property of Hill operators with potentials in weighted spaces

Consider the Hill operator $L(v) = - d^2/dx^2 + v(x)$ on $[0,π]$ with Dirichlet, periodic or antiperiodic boundary conditions; then for large enough $n$ close to $n^2$ there are one Dirichlet eigenvalue $μ_n$ and two periodic (if $n$ is even) or antiperiodic (if $n$ is odd) eigenvalues $λ_n^-, \, λ_n^+$ (counted with multiplicity). We describe classes of complex potentials $v(x)= \sum_{2\mathbb{Z}} V(k) e^{ikx}$ in weighted spaces (defined in terms of the Fourier coefficients of $v$) such that the periodic (or antiperiodic) root function system of $L(v)$ contains a Riesz basis if and only if $V(-2n) \asymp V(2n) \quad \text{as} \;\; n \in 2\mathbb{N}\;\; (\text{or} \; n \in 1+ 2\mathbb{N}), \;\; n \to \infty.$ For such potentials we prove that $λ_n^+ - λ_n^- \sim \pm 2\sqrt{V(-2n)V(2n)}$ and $μ_n - \frac{1}{2}(λ_n^+ + λ_n^-) \sim -\frac{1}{2} (V(-2n) + V(2n)).$

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