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Qihui Li

Publications and source records attributed to Qihui Li.

15 recordsLinked to original sources
arXiv2024

Homological theory of representations having pure acyclic injective resolutions

Let $Q$ be a quiver and $R$ an associative ring. A representation by $R$-modules of $Q$ is called strongly fp-injective if it admits a pure acyclic injective resolution in the category of representations. It is shown that such representations possess many nice properties. We characterize strongly fp-injective representations under some mild assumptions, which is closely related to strongly fp-injective $R$-modules. Subsequently, we use such representations to define relative Gorenstein injective representations, called Gorenstein strongly fp-injective representations, and give an explicit characterization of the Gorenstein strongly fp-injective representations of right rooted quivers. As an application, a model structure in the category of representations is given.

math.KT↗
arXiv2021

Generalized Wave Operators in von Neumann Algebras

Let $\mathcal{M}\subseteq\mathcal{B}\left( \mathcal{H}\right) $ be a countable decomposable properly infinite von Neumann algebra with a faithful normal semifinite tracial weight $τ$ where $\mathcal{B}\left( \mathcal{H}\right) $ is the set of all bounded linear operators on Hilbert space $\mathcal{H}.$ The main purpose of this article is to introduce generalized weak wave operators $\widetilde{W}_{\pm}$, generalized weak abelian wave operators $\widetilde{\mathfrak{U}}_{\pm}$ and generalized stationary wave operators $\mathcal{U}_{\pm}$ in $\mathcal{M}$ and then to explore the relation among $\widetilde{W}_{\pm},$ $\widetilde{\mathfrak{U}% }_{\pm}$, $\mathcal{U}_{\pm}$ and generalized wave operators $W_{\pm}.$

math.OA↗
arXiv2021

A stationary approach for the Kato-Rosenblum theorem in von Neumann algebras

Let $\mathcal{M}$ be a countable decomposable properly infinite semifinite von Neumann algebra acting on a Hilbert space $\mathcal{H}.$ An analogue of the Kato-Rosenblum theorem in $\mathcal{M}$ has been proved in [9] by showing the existence of generalized wave operators. It is well-known that there are two typical approaches to show the existence of wave operators in the scattering theory. One is called time-dependent approach and another is called stationary approach. The main purpose of this article is to introduce a stationary approach in $\mathcal{M}$ and then to obtain the Kato-Rosenblum theorem in $\mathcal{M}$ by a stationary approach instead of a time-dependent approach in [9].

math.OA↗
arXiv2020

Approximate Equivalence in von Neumann Algebras

Suppose $\mathcal{A}$ is a separable unital ASH C*-algebra, $\mathcal{R}$ is a sigma-finite II$_{\infty}$ factor von Neumann algebra, and $π,ρ:\mathcal{A}\rightarrow\mathcal{R}$ are unital $\ast$-homomorphisms such that, for every $a\in\mathcal{A}$, the range projections of $π\left( a\right) $ and $ρ\left( a\right) $ are Murray von Neuman equivalent in $\mathcal{R}% $. We prove that $π$ and $ρ$ are approximately unitarily equivalent modulo $\mathcal{K}_{\mathcal{R}}$, where $\mathcal{K}_{\mathcal{R}}$ is the norm closed ideal generated by the finite projections in $\mathcal{R}$. We also prove a very general result concerning approximate equivalence in arbitrary finite von Neumann algebras.

math.OA↗
arXiv2019

Some Results on Inner Quasidiagonal $C^*$-algebras

In the current article, we prove the cross product $C^*$-algebra by a Rokhlin action of finite group on a strongly quasidiagonal $C^*$-algbra is strongly quasidiagonal again. We also show that a just-infinite $C^*$-algebra is quasidiagonal if and only if it is inner quasidiagonal. Finally, we compute the topological free entropy dimension in just-infinite $C^*$-algebras.

math.OA↗
arXiv2017

A generalization of the Voiculescu theorem for normal operators in semifinite von Neumann algebras

In this paper, we provide a generalized version of the Voiculescu theorem for normal operators by showing that, in a von Neumann algebra with separable pre-dual and a faithful normal semifinite tracial weight $τ$, a normal operator is an arbitrarily small $(\max\{\|\cdot\|, \Vert\cdot\Vert_{2}\})$-norm perturbation of a diagonal operator. Furthermore, in a countably decomposable, properly infinite von Neumann algebra with a faithful normal semifinite tracial weight, we prove that each self-adjoint operator can be diagonalized modulo norm ideals satisfying a natural condition.

math.OA↗
arXiv2017

Perturbations of self-adjoint operators in semifinite von Neumann algebras: Kato-Rosenblum theorem

In the paper, we prove an analogue of the Kato-Rosenblum theorem in a semifinite von Neumann algebra. Let $\mathcal{M}$ be a countably decomposable, properly infinite, semifinite von Neumann algebra acting on a Hilbert space $\mathcal{H}$ and let $τ$ be a faithful normal semifinite tracial weight of $\mathcal M$. Suppose that $H$ and $H_1$ are self-adjoint operators affiliated with $\mathcal{M}$. We show that if $H-H_1$ is in $\mathcal{M}\cap L^{1}\left(\mathcal{M},τ\right)$, then the ${norm}$ absolutely continuous parts of $H$ and $H_1$ are unitarily equivalent. This implies that the real part of a non-normal hyponormal operator in $\mathcal M$ is not a perturbation by $\mathcal{M}\cap L^{1}\left(\mathcal{M},τ\right)$ of a diagonal operator. Meanwhile, for $n\ge 2$ and $1\leq p<n$, by modifying Voiculescu's invariant we give examples of commuting $n$-tuples of self-adjoint operators in $\mathcal{M}$ that are not arbitrarily small perturbations of commuting diagonal operators modulo $\mathcal{M}\cap L^{p}\left(\mathcal{M},τ\right)$.

math.OA↗
arXiv2016

Topological Orbit Dimension of MF $C^*$-algebras

This paper is a continuation of our work on D. Voiculescu's topological free entropy dimension in unital C*-algebras. In this paper we first prove the topological free entropy dimension of a MF-nuclear and inner QD algebra is irrelevant to its generating family. Then we give the relation between the topological orbit dimension $K_{top}^2$ and the modified free orbit dimension$ K_2^2$ by using MF-traces. We also introduce a new invariant $K_{top}^3$ which is a modification of the topological orbit dimension $K_{top}^2$ when$ K_{top}^2$ is defined. As the applications of $K_{top}^3$, We prove that$ K_{top}^3(A)=0$ if A has property $ c^*-Γ$ and has no finite-dimensional representations. We also give the definition of property MF-c^*-Γ. We then conclude that, for the unital MF $ C^*$-algebra with no finite-dimensional representations, if A has property MF-c*-Γ, then $K_{top}^3(A)=0$.

math.OA↗
arXiv2015

A Note On Inner Quasidiagonal C*-Algebras

In the paper, we give two new characterizations of separable inner quasidiagonal C*-algebras. Base on these characterizations, we show that a unital full free product of two inner quasidiagonal C*-algebras is inner quasidiagonal again. As an application, we show that a unital full free product of two inner quasidiagoanl C*-algebras with amalgmation over a full matrix algebra is inner quasidiagonal. Meanwhile, we conclude that a unital full free product of two AF algebras with amalgamation over a finite-dimensional C*-algebra is inner quasidiagonal if there are faithful tracial states on each of these two AF algebras such that the restrictions on the common subalgebra agree.

math.OA↗
arXiv2014

A note on unital full amalgamated free products of quasi-diagonal C*-algebras

In the paper, we consider the question whether a unital full amalgamated free product of quasidiagonal C*-algebras is quasidiagonal again. We give a sufficient condition such that a unital full amalgamated free product of quasidiagonal C*-algebras with amalgamation over a finite dimensional C*- algebra is quasidiagonal. Applying this result, we conclude that a unital full free product of two AF algebras with amalgamation over a finite-dimensional C*-algebra is AF if there are faithful tracial states on each of these two AF algebras such that the restrictions on the common subalgebra agree.

math.OA↗
arXiv2010

Unital Full Amalgamated Free Products of MF Algebras

In this paper, we consider the question whether a unital full free product of MF algebras with amalgamation over a finite dimensional C*-algebra is an MF algebra. First, we show that, under a natural condition, a unital full free product of two separable residually finite dimensional (RFD) C*-algebras with amalgamation over a finite dimensional C*-algebra is again a separable RFD C*-algebra. Applying this result on MF C*-algebras, we show that, under a natual condition, a unital full free product of two MF algebras is again an MF algebra. As an application, we show that a unital full free product of two AF algebras with amalgamation over an AF algebra is an MF algebra if there are faithful tracial states on each of these two AF algebras such that the restrictions on the common subalgebra agree.

math.OA↗
arXiv2010

On The MF Propety of Reduced Amalgamated Free Products of UHF Algebras

In this paper, we concentrate on the MF property of reduced free products of unital C*-algebras with amalgamation over finite dimensional C*-algebras. More specifically, we give a necessary and sufficient condition for a reduced free product of two UHF algebras amalgamated over a finite-dimensional C*-algebra with respect to trace preserving conditional expectations to be MF.

math.OA↗
arXiv2008

Topological Free Entropy Dimensions in Nuclear C$^*$-algebras and in Full Free Products of C$^*$-algebras

In the paper, we introduce a new concept of topological orbit dimension of $n$-tuples of elements in a unital C$^*$ algebra. Using this concept, we conclude that the Voiculescu's topological free entropy dimension of any family of self-adjoint generators of a nuclear C$^*$ algebra is less than or equal to 1. We also show that the topological free entropy dimension is additive in the full free products of unital C$^*$ algebras. In the appendix, we show that unital full free product of Blackadar and Kirchberg's unital MF algebras is also MF algebra.

math.OA↗