Spectral flow inside essential spectrum III: coupling resonances near essential spectrum

TL;DR

This paper investigates the behavior of coupling resonance functions associated with a pair of operators near the essential spectrum, showing they are well-behaved under certain conditions and relate to spectral flow during operator deformation.

Contribution

It demonstrates that coupling resonance functions are well-behaved near the essential spectrum when the limiting absorption principle holds, clarifying their structure and impact on spectral flow.

Findings

Coupling resonance functions are either single-valued or do not take real values in a specified neighborhood.

Under the limiting absorption principle, resonance functions exhibit controlled behavior near the essential spectrum.

The results connect resonance functions to spectral flow during operator perturbations.

Abstract

Given a self-adjoint operator $H_0$ and a relatively $H_0$-compact self-adjoint operator $V,$ the functions $r_j(z) = - \sigma_j^{-1}(z),$ where $\sigma_j(z)$ are eigenvalues of the compact operator $(H_0-z)^{-1}V,$ bear a lot of important information about the pair $H_0$ and $V.$ We call them coupling resonances. In case of rank one (and positive) perturbation $V,$ there is only one coupling resonance function, which is a Herglotz function. This case has been studied in depth in the literature, and appears in different situations, such as Sturm-Liouville theory, random Schr\"odinger operators, harnomic and spectral analyses, etc. The general case is complicated by the fact that the resonance functions are no longer single valued holomorphic functions, and potentially can have quite an erratic behaviour, typical for infinitely-valued holomorphic functions. Of special interest are those coupling resonance functions $r_z$ which approach a real number $r_{\lambda+i0}$ from the interval $[0,1]$ as the spectral parameter $z=\lambda+iy$ approaches a point $\lambda$ of the essential spectrum, since they are responsible for spectral flow through $\lambda$ inside essential spectrum when $H_0$ gets deformed to $H_1 = H_0+V$ via the path $H_0 + rV, r \in [0,1].$ In this paper it is shown that if the pair $H_0,$ $V$ satisfies the limiting absorption principle, then the coupling resonance functions are well-behaved near the essential spectrum in the following sense. Let $I$ be an open interval inside the essential spectrum of $H_0$ and $\epsilon>0.$ Then there exists a compact subset~$K$ of~$I$ such that $| I \setminus K | < \epsilon,$ and $K$ has a "non-tangential" neighbourhood in the upper complex half-plane, such that any coupling resonance function is either single-valued in the neighbourhood, or does not take a real value in the interval $[0,1].$