SearcharxivSearch

arXiv subjects

Emmanuel Chetcuti

Publications and source records attributed to Emmanuel Chetcuti.

9 recordsLinked to original sources

Extending surjective maps preserving the norm of symmetric kubo-ando means

Recently, the question of whether surjective maps preserving the norm of a symmetric Kubo-Ando mean can be extended to Jordan $\ast$-isomorphisms has been tackled. The question was affirmatively answered for surjective maps between $C^{*}$-algebras for certain specific classes of symmetric Kubo-Ando means. Here, we give a comprehensive answer to this question for surjective maps between $AW^{*}$-algebras preserving the norm of any symmetric Kubo-Ando mean.

math.OA

Every symmetric Kubo-Ando connection has the order-determining property on $\mathcal B(H)$

In \cite{molnar} L.~Molnar studied the question of whether the L\"owner partial order on the positive cone of an operator algebra is determined by the norm of any arbitrary Kubo-Ando mean. He affirmatively answered the question for certain classes of Kubo-Ando means and left as an open problem the general case. We here give an answer to this question, by showing that the norm of every symmetric Kubo-Ando mean $\sigma$ on $\mathcal B(H)$ is order-determining, i.e. if $A, B\in \mathcal B(H)^{{\sss{++}}}$ satisfy $\Vert A\sigma X\Vert \le \Vert B\sigma X\Vert$ for every $X\in \mathcal B(H)^{\sss{++}}$, then $A\le B$.

math.FA

Lattice uniformities inducing unbounded convergence

A net $(x_\gamma)_{\gamma\in\Gamma}$ in a locally solid Riesz space $(X,\tau)$ is said to be unbounded $\tau$-convergent to $x$ if $|x_\gamma-x|\wedge u\mathop{\overset{\tau}{\longrightarrow}} 0$ for all $u\in X_+$. We recall that there is a locally solid linear topology $\mathfrak{u}\tau$ on $X$ such that unbounded $\tau$-convergence coincides with $\mathfrak{u}\tau$-convergence, and moreover, $\mathfrak{u}\tau$ is characterised as the weakest locally solid linear topology which coincides with $\tau$ on all order bounded subsets. It is with this motivation that we introduce, for a uniform lattice $(L,u)$, the weakest lattice uniformity $u^\ast$ on $L$ that coincides with $u$ on all the order bounded subsets of $L$. It is shown that if $u$ is the uniformity induced by the topology of a locally solid Riesz space $(X,\tau)$, then the $u^*$-topology coincides with $\mathfrak{u}\tau$. This allows comparing the results of this paper with earlier results on unbounded $\tau$-convergence. It will be seen that despite the fact that in the setup of uniform lattices most of the machinery used in the techniques of [M. A. Taylor 2019: Unbounded topologies and uo-convergence in locally solid vector spaces, J. Math. Anal. Appl. \bf{472} no.1, 981--1000] is lacking, the concept of `unbounded convergence' well fittingly generalizes to uniform lattices. We shall also answer Questions 2.13, 3.3, 5.10 of [M. A. Taylor 2019: Unbounded topologies and uo-convergence in locally solid vector spaces, J. Math. Anal. Appl. \bf{472} no.1, 981--1000] and Question 18.51 of [M. A. Taylor 2018: Unbounded convergence in vector lattices, Thesis University of Alberta].

math.FA

On the commutant of $B(H)$ in its ultrapower

Let $B(H)$ be the algebra of bounded linear operators on a separable infinite-dimensional Hilbert space $H$. We study the commutant of $B(H)$ in its ultrapower. We characterize the class of non-principal ultrafilters for which this commutant is non-trivial. Additionally, we extend the class of ultrafilters for which the commutant is trivial.

math.FA

On different modes of order convergence and some applications

Different notions for order convergence have been considered by various authors. Associated to every notion of order convergence corresponds a topology, defined by taking as the closed sets those subsets of the poset satisfying that no net in them order converges to a point that is outside of the set. We shall give a thorough overview of these different notions and provide a systematic comparison of the associated topologies. Then, in the last section we shall give an application of this study by giving a result on von Neumann algebras complementing the study started in \cite{ChHaWe}. We show that for every atomic von Neumann algebra (not necessarily $σ$-finite) the restriction of the order topology to bounded parts of $M$ coincides with the restriction of the $σ$-strong topology $s(M,M_\ast)$. We recall that the methods of \cite{ChHaWe} rest heavily on the assumption of $σ$-finiteness. Further to this, for a semi-finite measure space, we shall give a complete picture of the relations between the topologies on $L^\infty$ associated with the duality $\langle L^1, L^\infty\rangle$ and its order topology.

math.FA

Order topology on orthocomplemented posets of linear subspaces of a pre-Hilbert space

Motivated by the Hilbert-space model for quantum mechanics, we define a pre-Hilbert space logic to be a pair $(S,\el)$, where $S$ is a pre-Hilbert space and $\el$ is an orthocomplemented poset of orthogonally closed linear subspaces of $S$, closed w.r.t. finite dimensional perturbations, (i.e. if $M\in\el$ and $F$ is a finite dimensional linear subspace of $S$, then $M+F\in \el$). We study the order topology $τ_o(\el)$ on $\el$ and show that completeness of $S$ can by characterized by the separation properties of the topological space $(\el,τ_o(\el))$. It will be seen that the remarkable lack of a proper probability-theory on pre-Hilbert space logics -- for an incomplete $S$ -- comes out elementarily from this topological characterization.

math.FA

The order topology on duals of C$^\ast$-algebras and von Neumann algebras

For a von Neumann algebra $\mathcal M$ we study the order topology associated with the hermitian part $\mathcal M_*^s$ and to intervals of the predual $\mathcal M_*$. It is shown that the order topology on $\mathcal M_*^s$ coincides with the topology induced by the norm. In contrast to this, it is proved that the condition of having the order topology associated to the interval $[0,φ]$ equal to that induced by the norm for every $φ\in \mathcal M_*^+$, is necessary and sufficient for the commutativity of $\mathcal M$. It is also proved that if $φ$ is a positive bounded linear functional on a C$^\ast$-algebra $\mathcal A$, then the norm-null sequences in $[0,φ]$ coincide with the null sequences with respect to the order topology on $[0,φ]$ if and only if the von Neumann algebra $π_φ(\mathcal A)'$ is of finite type (where $π_φ$ denotes the corresponding GNS representation). This fact allows us to give a new topological characterization of finite von Neumann algebras. Moreover, we demonstrate that convergence to zero for norm and order topology on order-bounded parts of dual spaces are nonequivalent for all C$^\ast$-algebras that are not of Type $I$.

math.OA

Equilateral weights on the unit ball of $\mathbb R^n$

An equilateral set (or regular simplex) in a metric space $X$, is a set $A$ such that the distance between any pair of distinct members of $A$ is a constant. An equilateral set is standard if the distance between distinct members is equal to $1$. Motivated by the notion of frame-functions, as introduced and characterized by Gleason in \cite{Gl}, we define an equilateral weight on a metric space $X$ to be a function $f:X\to \mathbb R$ such that $\sum_{i\in I}f(x_i)=W$, for every maximal standard equilateral set $\{x_i:i\in I\}$ in $X$, where $W\in\mathbb R$ is the weight of $f$. In this paper we characterize the equilateral weights associated with the unit ball $B^n$ of $\mathbb R^n$ as follows: For $n\ge 2$, every equilateral weight on $B^n$ is constant.

math.FA

The order topology for a von Neumann algebra

The order topology $τ_o(P)$ (resp. the sequential order topology $τ_{os}(P)$) on a poset $P$ is the topology that has as its closed sets those that contain the order limits of all their order convergent nets (resp. sequences). For a von Neumann algebra $M$ we consider the following three posets: the self-adjoint part $M_{sa}$, the self-adjoint part of the unit ball $M_{sa}^1$, and the projection lattice $P(M)$. We study the order topology (and the corresponding sequential variant) on these posets, compare the order topology to the other standard locally convex topologies on $M$, and relate the properties of the order topology to the underlying operator-algebraic structure of $M$.

math.OA