Functions of normal operators under perturbations

Abstract

In \cite{Pe1}, \cite{Pe2}, \cite{AP1}, \cite{AP2}, and \cite{AP3} sharp estimates for $f(A)-f(B)$ were obtained for self-adjoint operators $A$ and $B$ and for various classes of functions $f$ on the real line $\R$. In this paper we extend those results to the case of functions of normal operators. We show that if a function $f$ belongs to the H\"older class \L_\a(\R^2), 0<\a<1, of functions of two variables, and $N_1$ and $N_2$ are normal operators, then \|f(N_1)-f(N_2)\|\le\const\|f\|_{\L_\a}\|N_1-N_2\|^\a. We obtain a more general result for functions in the space \L_\o(\R^2)=\big\{f:~|f(\z_1)-f(\z_2)|\le\const\o(|\z_1-\z_2|)\big\} for an arbitrary modulus of continuity \o. We prove that if $f$ belongs to the Besov class B_{\be1}^1(\R^2), then it is operator Lipschitz, i.e., \|f(N_1)-f(N_2)\|\le\const\|f\|_{B_{\be1}^1}\|N_1-N_2\|. We also study properties of $f(N_1)-f(N_2)$ in the case when f\in\L_\a(\R^2) and $N_1-N_2$ belongs to the Schatten-von Neuman class \bS_p.Comment: 32 page