Computing on the Banach space C [ 0 , 1 ]

Computability ◽  
2021 ◽  
pp. 1-14
Author(s):  
Tyler A. Brown
Keyword(s):  
Banach Space ◽  
Unit Interval ◽  
Computable Presentation

We demonstrate that, within any computable presentation of the Banach space C [ 0 , 1 ], computing 1 is no harder than computing the halting set. Additionally, we prove that the modulus operator | · | is Ø ″ -computable and use this to show that C [ 0 , 1 ] is Δ 3 0 -categorical when we restrict ourselves to the presentations in which at least one homeomorphism of the unit interval onto itself is computable.

scholarly journals Best Simultaneous Approximation in Orlicz Spaces

2007 ◽  
Vol 2007 ◽  
pp. 1-7 ◽  
Author(s):  
M. Khandaqji ◽  
Sh. Al-Sharif
Keyword(s):  
Banach Space ◽  
Simultaneous Approximation ◽  
Closed Subspace ◽  
Orlicz Spaces ◽  
Unit Interval ◽  
Luxemburg Norm ◽  
Integrable Functions ◽  
Best Simultaneous Approximation ◽  
Distance Formula

LetXbe a Banach space and letLΦ(I,X)denote the space of OrliczX-valued integrable functions on the unit intervalIequipped with the Luxemburg norm. In this paper, we present a distance formula dist(f1,f2,LΦ(I,G))Φ, whereGis a closed subspace ofX, andf1,f2∈LΦ(I,X). Moreover, some related results concerning best simultaneous approximation inLΦ(I,X)are presented.

scholarly journals Best Simultaneous approximation of quasi-continuous functions by monotone functions

1991 ◽  
Vol 50 (3) ◽  
pp. 391-408 ◽  
Author(s):  
Salem M. A. Sahab
Keyword(s):  
Banach Space ◽  
Convex Cone ◽  
Simultaneous Approximation ◽  
Continuous Functions ◽  
Unit Interval ◽  
Closed Convex Cone ◽  
Monotone Functions ◽  
Measurable Functions ◽  
Best Simultaneous Approximation ◽  
Nondecreasing Functions

AbstractLet Q denote the Banach space (under the sup norm) of quasi-continuous functions on the unit interval [0, 1]. Let ℳ denote the closed convex cone comprised of monotone nondecreasing functions on [0, 1]. For f and g in Q and 1 < p < ∞, let hp denote the best Lp-simultaneous approximant of f and g by elements of ℳ. It is shown that hp converges uniformly as p → ∞ to a best L∞-simultaneous approximant of f and g by elements of ℳ. However, this convergence is not true in general for any pair of bounded Lebesgue measurable functions. If f and g are continuous, then each hp is continuous; so is limp→∞ hp = h∞.

scholarly journals Biconcave-function characterisations of UMD and Hilbert spaces

1993 ◽  
Vol 47 (2) ◽  
pp. 297-306 ◽  
Author(s):  
Jinsik Mok Lee
Keyword(s):  
Banach Space ◽  
Hilbert Space ◽  
Hilbert Spaces ◽  
Complex Banach Space ◽  
Unit Interval ◽  
Martingale Differences ◽  
Almost Everywhere ◽  
Integrable Functions ◽  
Image Position ◽  
Umd Banach Spaces

Suppose that X is a real or complex Banach space with norm |·|. Then X is a Hilbert space if and only iffor all x in X and all X-valued Bochner integrable functions Y on the Lebesgue unit interval satisfying EY = 0 and |x − Y| ≤ 2 almost everywhere. This leads to the following biconcave-function characterisation: A Banach space X is a Hilbert space if and only if there is a biconcave function η: {(x, y) ∈ X × X: |x − y| ≤ 2} → R such that η(0, 0) = 2 andIf the condition η(0, 0) = 2 is eliminated, then the existence of such a function η characterises the class UMD (Banach spaces with the unconditionally property for martingale differences).

An example regarding Kalton's paper ‘isomorphisms between spaces of vector-valued continuous functions’

2021 ◽  
pp. 1-5
Author(s):  
Félix Cabello Sánchez
Keyword(s):  
Banach Space ◽  
Banach Spaces ◽  
Closed Subspace ◽  
Continuous Functions ◽  
Unit Interval ◽  
Finite Covering ◽  
Covering Dimension ◽  
Striking Result ◽  
Vector Valued ◽  
Locally Convex

Abstract The paper alluded to in the title contains the following striking result: Let $I$ be the unit interval and $\Delta$ the Cantor set. If $X$ is a quasi Banach space containing no copy of $c_{0}$ which is isomorphic to a closed subspace of a space with a basis and $C(I,\,X)$ is linearly homeomorphic to $C(\Delta ,\, X)$ , then $X$ is locally convex, i.e., a Banach space. We will show that Kalton result is sharp by exhibiting non-locally convex quasi Banach spaces $X$ with a basis for which $C(I,\,X)$ and $C(\Delta ,\, X)$ are isomorphic. Our examples are rather specific and actually, in all cases, $X$ is isomorphic to $C(K,\,X)$ if $K$ is a metric compactum of finite covering dimension.

scholarly journals Proximunalty in Orlicz-bochner Function Spaces

2003 ◽  
Vol 34 (1) ◽  
pp. 71-76 ◽  
Author(s):  
M. Khandaqji ◽  
R. Khalil ◽  
D. Hussein
Keyword(s):  
Banach Space ◽  
Closed Subspace ◽  
Function Spaces ◽  
Unit Interval

A (closed) subspace $ Y$ of a Banach space $ X$ is called proximinal if for every $ x\in X$ there exists some $ y\in Y$ such that $ \|x-y\|\le\|x-z\|$ for $ z\in Y$. It is the object of this paper is to study the proximinality of $ L^\Phi(I,Y)$ in $ L^\Phi(I,X)$ for some class of Young's functions $ \Phi$, where $ I$ is the unit interval. We prove (among other results) that if $ Y$ is a separable proximinal subspace of $ X$, then $ L^\Phi(I,Y)$ is proximinal in $ L^\Phi(I,X)$.

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