scholarly journals The numerical range of an element of a normed algebra

1969 ◽  
Vol 10 (1) ◽  
pp. 68-72 ◽  
Author(s):  
F. F. Bonsall
Keyword(s):  
Linear Space ◽  
Unit Sphere ◽  
Dual Space ◽  
Normed Linear Space ◽  
Numerical Range ◽  
Normed Algebra ◽  
Bounded Linear ◽  
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Given a normed linear space X, let S(X), X′, B(X) denote respectively the unit sphere {x: ∥x∥ = 1} of X, the dual space of X, and the algebra of all bounded linear mappings of X into X. For each x ∊ S(X) and T ∊ B(X), let Dx(x) = {f e X′:∥f∥ = f(x)= 1}, and V(T; x) = {f(Tx):f∊Dx(x)}. The numerical range V(T) is then defined by

Spatial numerical range of an operator

1974 ◽  
Vol 76 (3) ◽  
pp. 515-520
Author(s):  
K. Tillekeratne
Keyword(s):  
Unit Ball ◽  
Unit Sphere ◽  
Normed Space ◽  
Numerical Range ◽  
Cartesian Product ◽  
Linear Operators ◽  
Normed Algebra ◽  
Bounded Linear Operators ◽  
Bounded Linear ◽  
Image Position

0. Introduction. Let X be a normed space and let T be an operator on X. Let S(X) denote its unit sphere, {x ∈ X: ∥x∥ = 1}, B(X) = {x ∈ X: ∥x∥ ≤ 1} its unit ball, X′ its dual and ℬ(X) the normed algebra of bounded linear operators on X. Let II be the subset of the Cartesian product X × X′ defined by

scholarly journals Operators with a Single Spectrum

1966 ◽  
Vol 15 (1) ◽  
pp. 11-18 ◽  
Author(s):  
T. T. West
Keyword(s):  
Linear Space ◽  
Normed Linear Space ◽  
Linear Operators ◽  
Complex Field ◽  
Unique Extension ◽  
Single Spectrum ◽  
Bounded Linear ◽  
Infinite Dimensional ◽  
Resolvent Set ◽  
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Let X be an infinite dimensional normed linear space over the complex field Z. X will not be complete, in general, and its completion will be denoted by . If ℬ(X) is the algebra of all bounded linear operators in X then T ∈ ℬ(X) has a unique extension and . The resolvent set of T ∈ ℬ(X) is defined to beand the spectrum of T is the complement of ρ(T) in Z.

Complex strict and uniform convexity and hyponormal operators

1984 ◽  
Vol 96 (3) ◽  
pp. 483-493 ◽  
Author(s):  
Kirsti Mattila
Keyword(s):  
Banach Space ◽  
Dual Space ◽  
Numerical Range ◽  
Complex Banach Space ◽  
Linear Operators ◽  
Uniform Convexity ◽  
Bounded Linear Operators ◽  
Bounded Linear ◽  
Hyponormal Operators ◽  
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Let X be a complex Banach space. We denote by X* the dual space of X and by B(X) the space of all bounded linear operators on X. The (spatial) numerical range of an operator TεB(X) is defined as the setIf V(T) ⊂ ℝ, then T is called hermitian. More about numerical ranges may be found in [8] and [9].

The power inequality on Banach spaces

1971 ◽  
Vol 69 (3) ◽  
pp. 411-415 ◽  
Author(s):  
Béla Bollobás
Keyword(s):  
Linear Operator ◽  
Banach Spaces ◽  
Normed Space ◽  
Bounded Linear Operator ◽  
Dual Space ◽  
Numerical Range ◽  
Numerical Radius ◽  
Bounded Linear ◽  
Power Inequality ◽  
Image Position

Let X be a complex normed space with dual space X′ and let T be a bounded linear operator on X. The numerical range of T is defined asand the numerical radius is v(T) = sup {|ν: νε V(T)}. Most known results and problems concerning numerical range can be found in the notes by Bonsall and Duncan (5).

scholarly journals Suns are convex in tangent directions

2019 ◽  
Vol 484 (2) ◽  
pp. 131-133
Author(s):  
A. R. Alimov ◽  
E. V. Shchepin
Keyword(s):  
Linear Space ◽  
Unit Sphere ◽  
Normed Linear Space ◽  
Tangent Line ◽  
Chebyshev Set ◽  
Tangent Direction

A direction d is called a tangent direction to the unit sphere S of a normed linear space s  S and lin(s + d) is a tangent line to the sphere S at s imply that lin(s + d) is a one-sided tangent to the sphere S, i. e., it is the limit of secant lines at s. A set M is called convex with respect to a direction d if [x, y]  M whenever x, y in M, (y - x) || d. We show that in a normed linear space an arbitrary sun (in particular, a boundedly compact Chebyshev set) is convex with respect to any tangent direction of the unit sphere.

scholarly journals Some convexity theorems for matrices

1971 ◽  
Vol 12 (2) ◽  
pp. 110-117 ◽  
Author(s):  
P. A. Fillmore ◽  
J. P. Williams
Keyword(s):  
Linear Operator ◽  
Operator Theory ◽  
Bounded Linear Operator ◽  
Numerical Range ◽  
Two Dimensional ◽  
Bounded Linear ◽  
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The numerical range of a bounded linear operator A on a complex Hilbertspace H is the set W(A) = {(Af, f): ║f║ = 1}. Because it is convex andits closure contains the spectrum of A, the numerical range is often a useful toolin operator theory. However, even when H is two-dimensional, the numerical range of an operator can be large relative to its spectrum, so that knowledge of W(A) generally permits only crude information about A. P. R. Halmos [2] has suggested a refinement of the notion of numerical range by introducing the k-numerical rangesfor k = 1, 2, 3, …. It is clear that W1(A) = W(A). C. A. Berger [2] has shown that Wk(A) is convex.

scholarly journals Pythagorean Orthogonality in a Normed Linear Space

1958 ◽  
Vol 9 (4) ◽  
pp. 168-169
Author(s):  
Hazel Perfect
Keyword(s):  
Euclidean Space ◽  
Linear Space ◽  
Normed Linear Space ◽  
Image Position ◽  
Pythagorean Orthogonality

This note presents a proof of the following proposition:Theorem. If Pythagorean orthogonality is homogeneous in a normed linear space T then T is an abstract Euclidean space.The theorem was originally stated and proved by R. C. James ([1], Theorem 5. 2) who systematically discusses various characterisations of a Euclidean space in terms of concepts of orthogonality. I came across the result independently and the proof which I constructed is a simplified version of that of James. The hypothesis of the theorem may be stated in the form:Since a normed linear space is known to be Euclidean if the parallelogram law:is valid throughout the space (see [2]), it is evidently sufficient to show that (l) implies (2).

scholarly journals On Exposed and Farthest Points in Normed Linear Spaces

1971 ◽  
Vol 12 (3) ◽  
pp. 301-308 ◽  
Author(s):  
M. Edelstein ◽  
J. E. Lewis
Keyword(s):  
Linear Space ◽  
Normed Linear Space ◽  
Nonempty Subset ◽  
Linear Spaces ◽  
Farthest Points ◽  
Normed Linear Spaces ◽  
Farthest Point ◽  
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Let S be a nonempty subset of a normed linear space E. A point s0 of S is called a farthest point if for some x ∈ E, . The set of all farthest points of S will be denoted far (S). If S is compact, the continuity of distance from a point x of E implies that far (S) is nonempty.

scholarly journals Some inequalities in Pseudo-Hilbert spaces

2011 ◽  
Vol 42 (4) ◽  
pp. 483-492
Author(s):  
Loredana Ciurdariu
Keyword(s):  
Linear Space ◽  
Number Field ◽  
Hilbert Spaces ◽  
Complex Number ◽  
Normed Linear Space ◽  
Normed Algebra ◽  
The Real ◽  
Complex Number Field

The aim of this paper is to obtain new versions of the reverse of the generalized triangle inequalities given in \cite{SSDNA}, %[4],and \cite{SSDPR} %[5] if the pair $(a_i,x_i),\;i\in\{1,\ldots,n\}$ from Theorem 1 of \cite{SSDNA} %[4] belongs to ${\mathbb C}\times\mathcal H $, where $\mathcal H$ is a Loynes $Z$-space instead of ${\mathbb K}\times X$, $X$ being a normed linear space and ${\mathbb K}$ is the field of scalars. By comparison, in \cite{SSDNA} %[4] the pair $(a_i,x_i),\;i\in\{1,\ldots,n\}$ belongs to $A^2$, where $A$ is a normed algebra over the real or complex number field ${\mathbb K}.$ The results will be given in Theorem 1, Theorem 3, Remark 2 and Corollary 3 which represent other interesting variants of Theorem 2.1, Remark 2.2, Theorem 3.2 and Theorem 3.4., see \cite{SSDNA}. %[4].

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