An exact expression for the down norm is given in terms of the level function on all rearrangement invariant spaces and a useful approximate expression is given for the down norm on all rearrangement invariant spaces whose upper Boyd index is not one.

Given a nonsymmetric matrix A, we investigate the effect of perturbations on an invariant subspace of A. The result derived in this paper is less restrictive on the norm of the perturbation and provides a potentially tighter bound compared to Stewart’s classical result. Moreover, we provide norm estimates for the remainder terms in well-known perturbation expansions for invariant subspaces, eig...

In this paper, we obtain optimal perturbation bounds of the weighted Moore-Penrose inverse under the weighted unitary invariant norm, the weighted Q-norm and the weighted F -norm, and thereby extend some recent results.

In this paper, we obtain optimal perturbation bounds of the weighted Moore-Penrose inverse under the weighted unitary invariant norm, the weighted Q-norm and the weighted F -norm, and thereby extend some recent results.

This paper explores the concept of reparametrization invariant norm (RPI-norm) for C1-functions that vanish at −∞ and whose derivative has compact support, such as C1 c -functions. An RPI-norm is any norm invariant under composition with orientation-preserving diffeomorphisms. The L∞-norm and the total variation norm are well-known instances of RPI-norms. We prove the existence of an infinite f...

In this article we study the Heinz and Hermite-Hadamard inequalities. We derive whole series of refinements these inequalities involving unitarily invariant norms, which improve some recent results, known from literature. We also prove that if $A , B, X\in M_n(\mathbb{C})$ such $A$ $B$ are positive definite $f$ is an operator monotone function on $(0,\infty)$. Then \begin{equation*} |||f(A)X-...

Simple answers are given to the following and related questions: For what Hubert space operator A is it true that the smallest ideal (alternatively, the smallest norm ideal, the smallest maximal norm ideal) containing A is a norm (an intermediate norm, a principal norm) ideal? Do these ideals support a nontrivial unitary invariant positive linear functional?

We give a lower bound for the error of any unitarily invariant algorithm learning half-spaces against the uniform or related distributions on the unit sphere. The bound is uniform in the choice of the target half-space and has an exponentially decaying deviation probability in the sample. The technique of proof is related to a proof of the Johnson Lindenstrauss Lemma. We argue that, unlike prev...

We first show that every γ-contractive commuting multioperator is unitarily equivalent to the restriction of S⊕W to an invariant subspace, where S is a backwards multi-shift and W a γ-isometry. We then describe γ-isometries in terms of (γ, 1)-isometries, and establish that under an additional assumption on T , W above can be chosen to be a commuting multioperator of isometries. Our methods prov...

We work out a theory of approximate quantum error correction that allows us to derive a general lower bound for the entanglement fidelity of a quantum code. The lower bound is given in terms of Kraus operators of the quantum noise. This result is then used to analyze the average error correcting performance of codes that are randomly drawn from unitarily invariant code ensembles. Our results co...