Searcharxiv⌕ Search

arXiv subjects

V. V. Peller

Publications and source records attributed to V. V. Peller.

At least 19 recordsLinked to original sources
arXiv2026

Real spectral shift functions for pairs of contractions and pairs of dissipative operators

Recently the authors solved a long-standing problem and showed that for an arbitrary pair of contractions on Hilbert space with trace class difference has an integrable spectral shift function on the unit circle ${\Bbb T}$ and an analogue of the Lifshits--Krein trace formula holds. It is also known that it may happen that there is no real-values integrable spectral shift function. In this paper we find conditions under which a pair of contractions with trace class difference has {\it a real-valued integrable} spectral shift function. We also consider a similar problem for pairs of dissipative operators. Finally, we find an application of the results in question to dissipative Schrödinger operators.

math.FA↗
arXiv2024

Functions of compact operators under trace class perturbations

The paper studies the problem, for which continuous functions $f$ on the real line ${\Bbb R}$, the difference of the functions $f(B)-f(A)$ of self-adjoint operators $A$ and $B$ with trace class difference must also be of trace class. The main result of the paper shows that this happens if and only if the function $f$ is operator Lipschitz on a neighbourhood of zero.

math.FA↗
arXiv2024

Besov spaces in operator theory

The survey is devoted to diverse applications of Besov classes in operator theory. It is illustrated how Besov classes are used to describe Hankel operators of Schatten--von Neumann classes; various applications of this description are considered. Next, we discuss the role of Besov classes in norm estimates of polynomials of power bounded operators on Hilbert space and related estimates of Hankel matrices in tensor products of the spaces $\ell^1$ and $\ell^\infty$. An essential part of the survey is devoted to the role of Besov spaces in various problems of perturbation theory when studying the behavior of functions of a single operator or of a collection of operators under their perturbation.

math.FA↗
arXiv2024

Triangular projection on $\boldsymbol{S}_p,~0<p<1$, as $\boldsymbol{p}$ approaches 1

This is a continuation of our recent paper. We continue studying properties of the triangular projection ${\mathscr P}_n$ on the space of $n\times n$ matrices. We establish sharp estimates for the $p$-norms of ${\mathscr P}_n$ as an operator on the Schatten--von Neumann class $\boldsymbol{S}_p$ for $0<p<1$. Our estimates are uniform in $n$ and $p$ as soon as $p$ is separated away from 0. The main result of the paper shows that for $p\in(0,1)$, the $p$-norms of ${\mathscr P}_n$ on $\boldsymbol{S}_p$ behave as $n\to\infty$ and $p\to1$ as $n^{1/p-1}\min\big\{(1-p)^{-1},\log n\big\}$.

math.FA↗
arXiv2019

On a trace formula for functions of noncommuting operators

The main result of the paper is that the Lifshits--Krein trace formula cannot be generalized to the case of functions of noncommuting self-adjoint operators. To prove this, we show that for pairs $(A_1,B_1)$ and $(A_2,B_2)$ of bounded self-adjoint operators with trace class differences $A_2-A_1$ and $B_2-B_1$, it is impossible to estimate the modulus of the trace of the difference $f(A_2,B_2)-f(A_1,B_1)$ in terms of the norm of $f$ in the Lipschitz class.

math.FA↗
arXiv2017

Analytic operator Lipschitz functions in the disk and a trace formula for functions of contractions

In this paper we prove that for an arbitrary pair $\{T_1,T_0\}$ of contractions on Hilbert space with trace class difference, there exists a function $\boldsymbolξ$ in $L^1({\Bbb T})$ (called a spectral shift function for the pair $\{T_1,T_0\}$ ) such that the trace formula $\operatorname{trace}(f(T_1)-f(T_0))=\int_{\Bbb T} f'(ζ)\boldsymbolξ(ζ)\,dζ$) holds for an arbitrary operator Lipschitz function $f$ analytic in the unit disk.

math.FA↗
arXiv2017

Functions of triples of noncommuting self-adjoint operators under perturbations of class $\boldsymbol S_p$

In this paper we study properties of functions of triples of not necessarily commuting self-adjoint operators. The main result of the paper shows that unlike in the case of functions of pairs of self-adjoint operators there is no Lipschitz type estimates in any Schatten--von Neumann norm $\boldsymbol S_p$, $1\le p\le\infty$, for arbitrary functions in the Besov class $B_{\infty,1}^1({\Bbb R}^3)$. In other words, we prove that for $p\in[1,\infty]$, there is no constant $K>0$ such that the inequality \begin{align*} \|f(A_1,B_1,C_1)&-f(A_2,B_2,C_2)\|_{\boldsymbol S_p}\\[.1cm] &\le K\|f\|_{B_{\infty,1}^1} \max\big\{\|A_1-A_2\|_{\boldsymbol S_p},\|B_1-B_2\|_{\boldsymbol S_p},\|C_1-C_2\|_{\boldsymbol S_p}\big\} \end{align*} holds for an arbitrary function $f$ in $B_{\infty,1}^1({\Bbb R}^3)$ and for arbitrary finite rank self-adjoint operators $A_1,\,B_1,\,C_1,\,A_2,\,B_2$ and $C_2$.

math.FA↗
arXiv2015

Functions of noncommuting self-adjoint operators under perturbation and estimates of triple operator integrals

We define functions of noncommuting self-adjoint operators with the help of double operator integrals. We are studying the problem to find conditions on a function $f$ on ${\Bbb R}^2$, for which the map $(A,B)\mapsto f(A,B)$ is Lipschitz in the operator norm and in Schatten--von Neumann norms $\boldsymbol{S}_p$. It turns out that for functions $f$ in the Besov class $B_{\infty,1}^1({\Bbb R}^2)$, the above map is Lipschitz in the $\boldsymbol{S}_p$ norm for $p\in[1,2]$. However, it is not Lipschitz in the operator norm, nor in the $\boldsymbol{S}_p$ norm for $p>2$. The main tool is triple operator integrals. To obtain the results, we introduce new Haagerup-like tensor products of $L^\infty$ spaces and obtain Schatten--von Neumann norm estimates of triple operator integrals. We also obtain similar results for functions of noncommuting unitary operators.

math.FA↗
arXiv2009

Operator Hölder--Zygmund functions

It is well known that a Lipschitz function on the real line does not have to be operator Lipschitz. We show that the situation changes dramatically if we pass to Hölder classes. Namely, we prove that if $f$ belongs to the Hölder class $Ł_\a(\R)$ with $0<\a<1$, then $\|f(A)-f(B)\|\le\const\|A-B\|^\a$ for arbitrary self-adjoint operators $A$ and $B$. We prove a similar result for functions $f$ in the Zygmund class $Ł_1(\R)$: for arbitrary self-adjoint operators $A$ and $K$ we have $\|f(A-K)-2f(A)+f(A+K)\|\le\const\|K\|$. We also obtain analogs of this result for all Hölder--Zygmund classes $Ł_\a(\R)$, $\a>0$. Then we find a sharp estimate for $\|f(A)-f(B)\|$ for functions $f$ of class $Ł_ø\df\{f: ø_f(\d)\le\constø(\d)\}$ for an arbitrary modulus of continuity $ø$. In particular, we study moduly of continuity, for which $\|f(A)-f(B)\|\le\constø(\|A-B\|)$ for self-adjoint $A$ and $B$, and for an arbitrary function $f$ in $Ł_ø$. We obtain similar estimates for commutators $f(A)Q-Qf(A)$ and quasicommutators $f(A)Q-Qf(B)$. Finally, we estimate the norms of finite differences $\sum\limits_{j=0}^m(-1)^{m-j}(m j)f\big(A+jK\big)$ for $f$ in the class $Ł_{ø,m}$ that is defined in terms of finite differences and a modulus continuity $ø$ of order $m$. We also obtaine similar results for unitary operators and for contractions.

math.FA↗
arXiv2009

Functions of operators under perturbations of class $\bS_p$

This is a continuation of our paper \cite{AP2}. We prove that for functions $f$ in the Hölder class $Ł_\a(\R)$ and $1<p<\be$, the operator $f(A)-f(B)$ belongs to $\bS_{p/\a}$, whenever $A$ and $B$ are self-adjoint operators with $A-B\in\bS_p$. We also obtain sharp estimates for the Schatten--von Neumann norms $\big\|f(A)-f(B)\big\|_{\bS_{p/\a}}$ in terms of $\|A-B\|_{\bS_p}$ and establish similar results for other operator ideals. We also estimate Schatten--von Neumann norms of higher order differences $\sum\limits_{j=0}^m(-1)^{m-j}(m\j)f\big(A+jK\big)$. We prove that analogous results hold for functions on the unit circle and unitary operators and for analytic functions in the unit disk and contractions. Then we find necessary conditions on $f$ for $f(A)-f(B)$ to belong to $\bS_q$ under the assumption that $A-B\in\bS_p$. We also obtain Schatten--von Neumann estimates for quasicommutators $f(A)Q-Qf(B)$, and introduce a spectral shift function and find a trace formula for operators of the form $f(A-K)-2f(A)+f(A+K)$.

math.FA↗
arXiv2009

Functions of perturbed operators

We prove that if $0<\a<1$ and $f$ is in the Hölder class $Ł_\a(\R)$, then for arbitrary self-adjoint operators $A$ and $B$ with bounded $A-B$, the operator $f(A)-f(B)$ is bounded and $\|f(A)-f(B)\|\le\const\|A-B\|^\a$. We prove a similar result for functions $f$ of the Zygmund class $Ł_1(\R)$: $\|f(A+K)-2f(A)+f(A-K)\|\le\const\|K\|$, where $A$ and $K$ are self-adjoint operators. Similar results also hold for all Hölder-Zygmund classes $Ł_\a(\R)$, $\a>0$. We also study properties of the operators $f(A)-f(B)$ for $f\inŁ_\a(\R)$ and self-adjoint operators $A$ and $B$ such that $A-B$ belongs to the Schatten--von Neumann class $\bS_p$. We consider the same problem for higher order differences. Similar results also hold for unitary operators and for contractions.

math.FA↗
arXiv2009

The behavior of functions of operators under perturbations

This is a survey article. We consider different problems in connection with the behavior of functions of operators under perturbations of operators. We deal with three classes of operators: unitary operators, self-adjoint operators, and contractions. We study operator Lipschitz and operator differentiable functions. We also study the behavior of functions under perturbations of an operator by an operator of Schatten--von Neumann class $\bS_p$ and apply the results to the Livschits--Krein and Koplienko--Neidhardt trace formulae. We also include in this survey article recent unexpected results obtained in a joint paper with Aleksandrov on operator Hölder--Zygmund functions.

math.FA↗
arXiv2008

Analytic approximation of matrix functions in $L^p$

We consider the problem of approximation of matrix functions of class $L^p$ on the unit circle by matrix functions analytic in the unit disk in the norm of $L^p$, $2\le p<\be$. For an $m\times n$ matrix function $Φ$ in $L^p$, we consider the Hankel operator $H_Φ:H^q(C^n)\to H^2_-(C^m)$, $1/p+1/q=1/2$. It turns out that the space of $m\times n$ matrix functions in $L^p$ splits into two subclasses: the set of respectable matrix functions and the set of weird matrix functions. If $Φ$ is respectable, then its distance to the set of analytic matrix functions is equal to the norm of $H_Φ$. For weird matrix functions, to obtain the distance formula, we consider Hankel operators defined on spaces of matrix functions. We also describe the set of $p$-badly approximable matrix functions in terms of special factorizations and give a parametrization formula for all best analytic approximants in the norm of $L^p$. Finally, we introduce the notion of $p$-superoptimal approximation and prove the uniqueness of a $p$-superoptimal approximant for rational matrix functions.

math.FA↗
arXiv2008

Differentiability of functions of contractions

In this paper we study differentiability properties of the map $T\mapstoϕ(T)$, where $ϕ$ is a given function in the disk-algebra and $T$ ranges over the set of contractions on Hilbert space. We obtain sharp conditions (in terms of Besov spaces) for differentiability and existence of higher derivatives. We also find explicit formulae for directional derivatives (and higher derivatives) in terms of double (and multiple) operator integrals with respect to semi-spectral measures.

math.FA↗
arXiv2008

Analytic approximation of matrix functions and dual extremal functions

We study the question of the existence of a dual extremal function for a bounded matrix function on the unit circle in connection with the problem of approximation by analytic matrix functions. We characterize the class of matrix functions, for which a dual extremal function exists in terms of the existence of a maximizing vector of the corresponding Hankel operator and in terms of certain special factorizations that involve thematic matrix functions.

math.FA↗
arXiv2007

On S. Mazur's problems 8 and 88 from the Scottish Book

The paper discusses Problems 8 and 88 posed by Stanislaw Mazur in the Scottish Book. It turns out that negative solutions to both problems are immediate consequences of the results of Section 5 of my paper "Estimates of functions of power bounded operators on Hilbert spaces", J. Operator Theory 7 (1982), 341-372. We discuss here some quantitative aspects of Problems 8 and 88 and give answers to open problems discussed in a recent paper by Pelczynski and Sukochev.

math.FA↗
arXiv2006

Analytic approximation of rational matrix functions

For a rational matrix function $Φ$ with poles outside the unit circle, we estimate the degree of the unique superoptimal approximation $\AΦ$ by matrix functions analytic in the unit disk. We obtain sharp estimates in the case of $2\times2$ matrix functions. It turns out that ``generically'' $°\AΦ\le\degΦ-2$. We prove that for an arbitrary $2\times2$ rational function $Φ$, $°\AΦ\le2\degΦ-3$ whenever $\degΦ\ge2$. On the other hand, for $k\ge2$, we construct a $2\times2$ matrix function $Φ$, for which $\degΦ=k$, while $°\AΦ=2k-3$. Moreover, we conduct a detailed analysis of the situation when the inequality $°\AΦ\le\degΦ-2$ can violate and obtain best possible results.

math.FA↗