arXiv subjects

Sen Zhu

Publications and source records attributed to Sen Zhu.

9 recordsLinked to original sources

arXiv2023

The Takai duality of $L^p$-operator algebras with incompressibility

Let $G$ be a countable discrete Abelian group, $A$ be a separable unital $L^p$-operator algebra which has unique $L^p$-operator matrix norms for $p\in [1,\infty)$, and $\alpha$ be an isometric action of $G$ on $A$. We prove in this paper that if $A$ is $p$-isometrically incompressible, then $F^{p}(\hat{G},F^p(G,A,\alpha),\hat{\alpha})$, the iterated $L^p$-operator crossed product, is isometrically isomorphic to $\overline{M}_{G}^{p}\otimes_{p}A$ if and only if $p=2$. Furthermore, we prove that if $A=M_n^p$, then $F^{p}(\hat{G},F^p(G,A,\alpha),\hat{\alpha})$ is isomorphic to $\overline{M}_{G}^{p}\otimes_{p}A$ if and only if either $p=2$ or $G$ is finite. This shows that the Takai duality for $C^*$-algebras can not be generalized to $L^p$-operator algebras with $p\neq 2$, and solves a problem raised by N. C. Phillips in the negative.

math.FA↗

arXiv2023

$p$-nuclearity of $L^p$-operator crossed products

Let $(X,\mathcal{B},\mu)$ be a measure space and $A$ be a norm closed subalgebra of $\mathcal{B}(L^p(X,\mu))$, where $p\in [1,\infty)$. Let $(G,A,\alpha)$ be an $L^p$-operator algebra dynamical system, where $G$ is a countable discrete amenable group. We prove that the full $L^p$-operator crossed product $F^p(G,A,\alpha)$ is $p$-nuclear if and only if $A$ is $p$-nuclear {provided the action} $\alpha$ of $G$ on $A$ is $p$-completely isometric. As applications, we prove that $L^p$-Cuntz algebras and rotation $L^p$-operator algebras are $p$-nuclear. Our results solve { a problem raised by N. C. Phillips concerning {$p$-nuclearity} for $L^p$-Cuntz algebras.}

math.FA↗

arXiv2022

On the Takai duality for $L^{p}$ operator crossed products

The aim of this paper is to study a problem raised by N. C. Phillips concerning the existence of Takai duality for $L^p$ operator crossed products $F^{p}(G,A,\alpha)$, where $G$ is a locally compact Abelian group, $A$ is an $L^{p}$ operator algebra and $\alpha$ is an isometric action of $G$ on $A$. Inspired by D. Williams' proof for the Takai duality theorem for crossed products of $C^*$-algebras, we construct a homomorphism $\Phi$ from $F^{p}(\hat{G},F^p(G,A,\alpha),\hat{\alpha})$ to $\mathcal{K}(l^{p}(G))\otimes_{p}A$ which is a natural $L^p$-analog of D. Williams' map. For countable discrete Abelian groups $G$ and separable unital $L^p$ operator algebras $A$ which have unique $L^p$ operator matrix norms, we show that $\Phi$ is an isomorphism if and only if either $G$ is finite or $p=2$; in particular, $\Phi$ is an isometric isomorphism in the case that $p=2$. Moreover, it is proved that $\Phi$ is equivariant for the double dual action $\hat{\hat{\alpha}}$ of $G$ on $F^p(\hat{G},F^p(G,A,\alpha),\hat{\alpha})$ and the action $\mathrm{Ad}\rho\otimes\alpha$ of $G$ on $\mathcal{K}(l^p(G))\otimes_p A$.

math.OA↗

arXiv2020

The orthogonal Lie algebra of operators: ideals and derivations

We study in this paper the infinite-dimensional orthogonal Lie algebra $\mathcal{O}_C$ which consists of all bounded linear operators $T$ on a separable, infinite-dimensional, complex Hilbert space $\mathcal{H}$ satisfying $CTC=-T^*$, where $C$ is a conjugation on $\mathcal{H}$. By employing results from the theory of complex symmetric operators and skew-symmetric operators, we determine the Lie ideals of $\mathcal{O}_C$ and their dual spaces. We study derivations of $\mathcal{O}_C$ and determine their spectra. These results complete some results of P. de la Harpe and provide interesting contrasts between $\mathcal{O}_C$ and the algebra $\mathcal{B(H)}$ of all bounded linear operators on $\mathcal{H}$.

math.FA↗

arXiv2019

The Jordan algebra of complex symmetric operators

For a conjugation $C$ on a separable, complex Hilbert space $\mathcal{H}$, the set $\mathcal{S}_C$ of $C$-symmetric operators on $\mathcal{H}$ forms a weakly closed, selfadjoint, Jordan operator algebra. In this paper we study $\mathcal{S}_C$ in comparison with the algebra $\mathcal{B(H)}$ of all bounded linear operators on $\mathcal{H}$, and obtain $\mathcal{S}_C$-analogues of some classical results concerning $\mathcal{B(H)}$. We determine the Jordan ideals of $\mathcal{S}_C$ and their dual spaces. Jordan automorphisms of $\mathcal{S}_C$ are classified. We determine the spectra of Jordan multiplication operators on $\mathcal{S}_C$ and their different parts. It is proved that those Jordan invertible ones constitute a dense, path connected subset of $\mathcal{S}_C$.

math.OA↗

arXiv2018

Reducible and irreducible approximation of complex symmetric operators

This paper aims to study reducible and irreducible approximation in the set $\textsl{CSO}$ of all complex symmetric operators on a separable, complex Hilbert space $\mathcal H$. When ${\rm dim} \mathcal H=\infty$, it is proved that both those reducible ones and those irreducible ones are norm dense in $\textsl{CSO}$. When ${\rm dim} \mathcal H<\infty$, irreducible complex symmetric operators constitute an open, dense subset of $\textsl{CSO}$.

math.FA↗

arXiv2018

Random weighted shifts

In this paper we initiate the study of a fundamental yet untapped random model of non-selfadjoint, bounded linear operators acting on a separable complex Hilbert space. We replace the weights $w_n=1$ in the classical unilateral shift $T$, defined as $Te_n=w_ne_{n+1}$, where $\{e_n\}_{n=1}^\infty$ form an orthonormal basis of a complex Hilbert space, by a sequence of i.i.d. random variables $\{X_n\}_{n=1}^{\infty}$; that is, $w_n=X_n$. This paper answers basic questions concerning such a model. We propose that this model can be studied in comparison with the classical Hardy/Bergman/Dirichlet spaces in function-theoretic operator theory. We calculate the spectra and determine their fine structures (Section 3). We classify the samples up to four equivalence relationships (Section 4). We introduce a family of random Hardy spaces and determine the growth rate of the coefficients of analytic functions in these spaces (Section 5). We compare them with three types of classical operators (Section 6); this is achieved in the form of generalized von Neumann inequalities. The invariant subspaces are shown to admit arbitrarily large indices and their semi-invariant subspaces model arbitrary contractions almost surely. We discuss a Beurling-type theorem (Section 7). We determine various non-selfadjoint algebras generated by $T$ (Section 8). Their dynamical properties are clarified (Section 9). Their iterated Aluthge transforms are shown to converge (Section 10). In summary, they provide a new random model from the viewpoint of probability theory, and they provide a new class of analytic functional Hilbert spaces from the viewpoint of operator theory. The technical novelty in this paper is that the methodology used draws from three (largely separate) sources: probability theory, functional Hilbert spaces, and the approximation theory of bounded operators.

math.FA↗

arXiv2009

Approximation of chaotic operators

As well-known, the concept "hypercyclic" in operator theory is the same as the concept "transitive" in dynamical system. Now the class of hypercyclic operators is well studied. Following the idea of research in hypercyclic operators, we consider classes of operators with some kinds of chaotic properties in this article. First of all, the closures of the sets of all Li-Yorke chaotic operators or distributionally chaotic operators are discussed. We give a spectral description of them and prove that the two closures coincide with each other. Moreover, both the set of all Li-Yorke chaotic operators and the set of all distributionally chaotic operators have nonempty interiors which coincide with each other as well. The article also includes the containing relation between the closure of the set of all hypercyclic operators and the closure of the set of all distributionally chaotic operators. Finally, we get connectedness of the sets considered above.

math.FA↗

arXiv2008

Topologies on Quantum Effects

Quantum effects play an important role in quantum measurement theory. The set of all quantum effects can be organized into an algebraical structure called effect algebra. In this paper, we study various topologies on the Hilbert space effect algebra and the projection lattice effect algebra.

quant-ph↗