Zeroes of the spectral density of the Schroedinger operator with the   slowly decaying Wigner-von Neumann potential
  We consider the Schr\"odinger operator $\mathcal L_{\alpha}$ on the half-line with a periodic background potential and a perturbation which consists of two parts: a summable potential and the slowly decaying Wigner--von Neumann potential $\frac{c\sin(2\omega x+\delta)}{x^{\gamma}}$, where $\gamma\in(\frac12,1)$. The continuous spectrum of this operator has the same band-gap structure as the continuous spectrum of the unperturbed periodic operator. In every band there exist two points, called critical, where the eigenfunction equation has square summable solutions. Every critical point $\nu_{cr}$ is an eigenvalue of the operator $\mathcal L_{\alpha}$ for some value of the boundary parameter $\alpha=\alpha_{cr}$, specific to that particular point. We prove that for $\alpha\neq\alpha_{cr}$ the spectral density of the operator $\mathcal L_{\alpha}$ has a zero of the exponential type at $\nu_{cr}$. 
Sergey Simonov
math.SP