scholarly journals Upper Estimates of the angle best approximations of generalized Liouville-Weyl derivatives

2021 ◽  
Vol 103 (3) ◽  
pp. 54-67
Author(s):  
A.E. Jetpisbayeva ◽  
◽  
A.A. Jumabayeva ◽  
Keyword(s):  
Best Approximation ◽  
Three Dimensional ◽  
Continuous Functions ◽  
Trigonometric Polynomials ◽  
Dimensional Case ◽  
Best Approximations ◽  
Approximation Of Functions ◽  
The Best Approximation

In this article we consider continuous functions f with period 2π and their approximation by trigonometric polynomials. This article is devoted to the study of estimates of the best angular approximations of generalized Liouville-Weyl derivatives by angular approximation of functions in the three-dimensional case. We consider generalized Liouville-Weyl derivatives instead of the classical mixed Weyl derivative. In choosing the issues to be considered, we followed the general approach that emerged after the work of the second author of this article. Our main goal is to prove analogs of the results of in the three-dimensional case. The concept of general monotonic sequences plays a key role in our study. Several well-known inequalities are indicated for the norms, best approximations of the r-th derivative with respect to the best approximations of the function f. The issues considered in this paper are related to the range of issues studied in the works of Bernstein. Later Stechkin and Konyushkov obtained an inequality for the best approximation f^(r). Also, in the works of Potapov, using the angle approximation, some classes of functions are considered. In subsection 1 we give the necessary notation and useful lemmas. Estimates for the norms and best approximations of the generalized Liouville-Weyl derivative in the three-dimensional case are obtained.

scholarly journals EESTIMATES OF BEST APPROXIMATIONS OF FUNCTIONS WITH LOGARITHMIC SMOOTHNESS IN THE LORENTZ SPACE WITH ANISOTROPIC NORM

2020 ◽  
Vol 6 (1) ◽  
pp. 16
Author(s):  
Gabdolla Akishev
Keyword(s):  
Lorentz Space ◽  
Best Approximation ◽  
Trigonometric Polynomials ◽  
Exact Order ◽  
Sufficient Condition ◽  
Periodic Functions ◽  
Best Approximations ◽  
Approximation Of Functions ◽  
The Best Approximation ◽  
Anisotropic Norm

In this paper, we consider the anisotropic Lorentz space \(L_{\bar{p}, \bar\theta}^{*}(\mathbb{I}^{m})\) of periodic functions of \(m\) variables. The Besov space \(B_{\bar{p}, \bar\theta}^{(0, \alpha, \tau)}\) of functions with logarithmic smoothness is defined. The aim of the paper is to find an exact order of the best approximation of functions from the class \(B_{\bar{p}, \bar\theta}^{(0, \alpha, \tau)}\) by trigonometric polynomials under different relations between the parameters \(\bar{p}, \bar\theta,\) and \(\tau\).The paper consists of an introduction and two sections. In the first section, we establish a sufficient condition for a function \(f\in L_{\bar{p}, \bar\theta^{(1)}}^{*}(\mathbb{I}^{m})\) to belong to the space \(L_{\bar{p}, \theta^{(2)}}^{*}(\mathbb{I}^{m})\) in the case \(1{<\theta^{2}<\theta_{j}^{(1)}},$ ${j=1,\ldots,m},\) in terms of the best approximation and prove its unimprovability on the class \(E_{\bar{p},\bar{\theta}}^{\lambda}=\{f\in L_{\bar{p},\bar{\theta}}^{*}(\mathbb{I}^{m})\colon{E_{n}(f)_{\bar{p},\bar{\theta}}\leq\lambda_{n},}\) \({n=0,1,\ldots\},}\) where \(E_{n}(f)_{\bar{p},\bar{\theta}}\) is the best approximation of the function \(f \in L_{\bar{p},\bar{\theta}}^{*}(\mathbb{I}^{m})\) by trigonometric polynomials of order \(n\) in each variable \(x_{j},\) \(j=1,\ldots,m,\) and \(\lambda=\{\lambda_{n}\}\) is a sequence of positive numbers \(\lambda_{n}\downarrow0\) as \(n\to+\infty\). In the second section, we establish order-exact estimates for the best approximation of functions from the class \(B_{\bar{p}, \bar\theta^{(1)}}^{(0, \alpha, \tau)}\) in the space \(L_{\bar{p}, \theta^{(2)}}^{*}(\mathbb{I}^{m})\).

THE ORDER OF THE BEST APPROXIMATION OF FUNCTIONS IN LEBESQUE SPACE

2020 ◽  
pp. 43-51
Author(s):  
A.A. Jumabayeva ◽  
◽  
A.E. Zhetpisbayeva ◽  
Keyword(s):  
Best Approximation ◽  
Upper Bounds ◽  
Trigonometric Polynomials ◽  
Best Approximations ◽  
Approximation Of Functions ◽  
Hyperbolic Cross ◽  
The Best Approximation ◽  
Functions Of Two Variables ◽  
Step Hyperbolic Cross

The article considers the LP(T2) Lebesque space of periodec functions of two variables. The problems of approximation of functions of two variables by trigonometric polynomials with “numbers” of harmonics from step hyperbolic crosses are stydied. Value EQγn(f)p=inft∈(Qγn)⌈f−t⌉p,i≤p≤∞ the best approximation of the function f(x) by trigonometric polynomials with “numbers” of harmonics from a step hyperbolic cross of Qγn The article consists of two sections. The first section contains some well-known statements necessary to prove the main results. In the second section, exact estimates of the best approximations of certain functions are established. These estimates make it possible to estimate the upper bounds of the best approximations for certain classes of functions. As approximation apparatuses, trigonometric polynomials with a spector from a stepwise hyperbolic cross are used. The questions considered in this work belong to the circle of questions studied in the works of K. I. Babenko, S. A.Telyakovsky, I. S.Bugrova, N.S.Nikolsky.

On the Jackson-Type Inequality for the Best Sp-Approximations of Functions by Trigonometric Polynomials

2017 ◽  
Vol 8 ◽  
pp. 27-35
Author(s):  
Alexander N. Shchitov
Keyword(s):  
Best Approximation ◽  
Type Inequality ◽  
Trigonometric Polynomials ◽  
Sharp Constant ◽  
Jackson Type ◽  
Moduli Of Continuity ◽  
Approximation Of Functions ◽  
The Best Approximation

We find the sharp constant in the Jackson-type inequality between the value of the best approximation of functions by trigonometric polynomials and moduli of continuity of m-th order in the spaces Sp, 1 ≤ p < ∞. In the particular case we obtain one result which in a certain sense generalizes the result obtained by L.V. Taykov for m = 1 in the space L2 for the arbitrary moduli of continuity of m-th order (m Є N).

Best Trigonometric Approximation, Fractional Order Derivatives and Lipschitz Classes

1977 ◽  
Vol 29 (4) ◽  
pp. 781-793 ◽  
Author(s):  
P. L. Butzer ◽  
H. Dyckhoff ◽  
E. Görlich ◽  
R. L. Stens
Keyword(s):  
Fractional Order ◽  
Best Approximation ◽  
Continuous Functions ◽  
Trigonometric Approximation ◽  
Trigonometric Polynomials ◽  
Lipschitz Classes ◽  
The Best Approximation ◽  
Image Position ◽  
Best Trigonometric Approximation

Let C2π denote the space of 2π-periodic continuous functions and πn the set of trigonometric polynomials of degree ≦ n, where n ϵ P = {0, 1, … } . Given θ > 0, the well-known theorem of Stečkin and its converse state that the best approximation of an ƒ ϵ C2π with respect to the max-norm satisfies

NOTE ON JACKSON AND BERNSTE N TYPE APPROXIMATION THEOREMS IN THE CASE OF APPROXIMATION BY ALGEBRAIC POLYNOMIALS N THE SPACES L AND C

2000 ◽  
Vol 36 (3-4) ◽  
pp. 353-358 ◽  
Author(s):  
S. Pawelke
Keyword(s):  
Periodic Function ◽  
Best Approximation ◽  
Trigonometric Polynomials ◽  
Algebraic Polynomials ◽  
Cient Condition ◽  
Approximation Theorems ◽  
Type Approximation ◽  
The Best Approximation ◽  
Approximation By Algebraic Polynomials

We con ider the best approximation E (n,f)by algebraic polynomials of degree at most n for function f in L 1 (-1, 1)or C [-1, 1]and give imple necessary and u .cient condition for E (n,f)=O (n-.),n ›.,u ing the well-known results in the ca e of ap- proximation of periodic function by trigonometric polynomials.

Sign in / Sign up

Export Citation Format

Share Document