Spectral flow inside essential spectrum V: on absorbing points of coupling resonances

Abstract

Let $H_0$ and $V$ be self-adjoint operators, such that $V$ admits a factorisation $V = F^*JF$ with bounded self-adjoint $J$ and $|H_0|^{1/2}$-compact $F.$ Coupling resonance functions, $r_j(z),$ of the pair $H_0$ and $V$ can be defined as $r_j(z) = - \sigma_j(z) ^{-1},$ where $\sigma_j(z)$ are eigenvalues of the compact-operator valued holomorphic function $F(H_0-z)^{-1} F^*J.$ Taken together, the functions $r_j(z)$ form an infinite-valued holomorphic function on the resolvent set of~$H_0.$ These functions contain a lot of information about the pair $H_0, V$ (this is well-known in the case of rank one $V$). A point $z_0$ of the resolvent set we call \emph{absorbing} if some $r_j(z)$ goes to $\infty$ as $z \to z_0$ along some half-interval. In this note I present some partial results concerning absorbing points of coupling resonances.