The spectral density of a difference of spectral projections
  Let $H_0$ and $H$ be a pair of self-adjoint operators satisfying some standard assumptions of scattering theory. It is known from previous work that if $\lambda$ belongs to the absolutely continuous spectrum of $H_0$ and $H$, then the difference of spectral projections $$D(\lambda)=1_{(-\infty,0)}(H-\lambda)-1_{(-\infty,0)}(H_0-\lambda)$$ in general is not compact and has non-trivial absolutely continuous spectrum. In this paper we consider the compact approximations $D_\varepsilon(\lambda)$ of $D(\lambda)$, given by $$D_\varepsilon(\lambda)=\psi_\varepsilon(H-\lambda)-\psi_\varepsilon(H_0-\lambda),$$ where $\psi_\varepsilon(x)=\psi(x/\varepsilon)$ and $\psi(x)$ is a smooth real-valued function which tends to $\mp1/2$ as $x\to\pm\infty$. We prove that the eigenvalues of $D_\varepsilon(\lambda)$ concentrate to the absolutely continuous spectrum of $D(\lambda)$ as $\varepsilon\to+0$. We show that the rate of concentration is proportional to $|\log\varepsilon|$ and give an explicit formula for the asymptotic density of these eigenvalues. It turns out that this density is independent of $\psi$. The proof relies on the analysis of Hankel operators. 
Alexander Pushnitski
math.SP