Extended Best Polynomial Approximation Operator in Orlicz Spaces

In this article we consider the best polynomial approximation operator, defined in an Orlicz space L ^Φ(B), and its extension to L ^?(B) where ? is the derivative function of Φ. A characterization of these operators and several properties are obtained.     

复赋范空间中的同时太阳集的最佳逼近

研究了复赋范空间中的同时太阳集对无穷序列的最佳逼近问题，得到了特征及唯一性定理．     

Characterization of Periodic L1-Unicity Subspaces

We are interested in characterizing the finite-dimensional subspaces of periodic, real-valued, and continuous functions which admit uniqueness of weighted best L¹-approximations. Considering the cases of odd and even dimension separately, we are able to give intrinsic characterizations of periodic L¹-unicity subspaces.     

Best L2 Local Approximation on Two Small Intervals

In this article, we introduce the τ condition, which is weaker than the L² differentiability. If a function satisfies the τ condition on two points of ?, we prove the existence and characterization of the best local polynomial approximation on these points.     

赋Orlicz范数的Orlicz空间中最佳逼近算子

在赋Ｏｒｌｉｃｚ范数的Ｏｒｌｉｃｚ空间中,给出最佳逼近算子单调性的一个充分条件和最佳逼近元存在定理．     

Parsimonious Least Norm Approximation

A theoretically justifiable fast finite successive linear approximation algorithm is proposed for obtaining a parsimonious solutionto a corrupted linear system Ax=b+p, where the corruption p is due to noise or error in measurement. The proposedlinear-programming-based algorithm finds a solution x by parametrically minimizing the number of nonzeroelements in x and the error Ax-b-p₁.Numerical tests on a signal-processing-based exampleindicate that the proposed method is comparable to a method that parametrically minimizesthe 1-norm of the solution x and the error Ax-b-p₁, and that both methods are superior, byorders of magnitude, to solutions obtained by least squares as well by combinatorially choosing an optimal solution with a specific number of nonzero elements.     

Banach空间中太阳集对无穷序列的最佳同时逼近

本文研究太阳集对一列无穷序列的最佳同时逼近问题，建立了特征及唯一性结果．     

Weighted Best Local Approximation

In this survey the notion of a balanced best multipoint local approximation is fully exposed since they were treated in the Lp spaces and recent results in Orlicz spaces.The notion of balanced point,introduced by Chui et al.in 1984 are extensively used.     

Best Simultaneous L p -Approximation on Small Regions

In this article, we study the behavior of best simultaneous L ^p -approximation by algebraic polynomials on a union of intervals when the measure of them tend to zero. We also get an interpolation result.     

Cp Condition and the Best Local Approximation

In this paper, we introduce a condition weaker than the Lpdifferentiability,which we call Cpcondition. We prove that if a function satisfies this condition at a point, then there exists the best local approximation at that point. We also give a necessary and sufficient condition for that a function be Lpdifferentiable. In addition, we study the convexity of the set of cluster points of the net of best appoximations of f,{Pe( f)} as e → 0.     

Best Approximation in Numerical Radius

Let X be a reflexive Banach space. In this article, we give a necessary and sufficient condition for an operator T ∈ 𝒦(X) to have the best approximation in numerical radius from the convex subset 𝒰 ? 𝒦(X), where 𝒦(X) denotes the set of all linear, compact operators from X into X. We also present an application to minimal extensions with respect to the numerical radius. In particular, some results on best approximation in norm are generalized to the case of the numerical radius.     

Best Local Approximation in Orlicz Spaces

We get results in Orlicz spaces L ^φ about best local approximation on non-balanced neighborhoods when φ satisfies a certain asymptotic condition. This fact generalizes known previous results in L ^p spaces.     

On the Best Approximation by Ridge Functions in the Uniform Norm

We consider the best approximation of some function classes by the manifold M _n consisting of sums of n arbitrary ridge functions. It is proved that the deviation of the Sobolev class W _p ^r,d from the manifold M _n in the space L _q for any 2≤ q≤ p≤∈fty behaves asymptotically as n ^-r/(d-1) . In particular, we obtain this asymptotic estimate for the uniform norm p=q=∈fty . January 10, 2000. Date revised: March 1, 2001. Date accepted: March 12, 2001.     

赋Amemiya-Orlicz范数的Musielak-Orlicz-Sobolev空间单调性在最佳逼近中的应用

利用Musielak-Orlicz-Sobolev空间的构成特点,借鉴Orlicz-Sobolev空间的单调性在最佳逼近中的一些应用,以Orlicz空间中Jensen'S不等式的推广为主要工具,讨论了赋Amemiya-Orlicz范数的Musielak-Orlicz-Sobolev空间中的最佳逼近问题,主要是唯一性、存在性、稳定性.     

Best Simultaneous Local Approximation in the Lp Norms

We study the behavior of the best simultaneous approximation to two functions from a convex set in L^p spaces, 2<p<∞, on a finite union of intervals when its measure tends to zero. In particular, we give su?cient conditions over the differentiability of two functions to assure existence of the best simultaneous local approximation from the class of algebraic polynomials of a fixed degree. These conditions are weaker than the ordinary differentiability given in previous works. More precisely, we consider differentiable functions in the sense L^p.     

Greedy Bases Are Best for m-Term Approximation

We study the approximation of a subset K in a Banach space X by choosing first basis B and then using n-term approximation. Into the competition for best bases we enter all unconditional bases for X. We show that if the subset K⊂X is well aligned with respect to a greedy basis B, then, in a certain sense, this basis is the best for this type of approximation. Our result improves the recent result of DeVore, Petrova and Temlyakov.      

Near Best Tree Approximation

Tree approximation is a form of nonlinear wavelet approximation that appears naturally in applications such as image compression and entropy encoding. The distinction between tree approximation and the more familiar n-term wavelet approximation is that the wavelets appearing in the approximant are required to align themselves in a certain connected tree structure. This makes their positions easy to encode. Previous work [4,6] has established upper bounds for the error of tree approximation for certain (Besov) classes of functions. This paper, in contrast, studies tree approximation of individual functions with the aim of characterizing those functions with a prescribed approximation error. We accomplish this in the case that the approximation error is measured in L ₂, or in the case p2, in the Besov spaces B _p ⁰(L _p ), which are close to (but not the same as) L _p . Our characterization of functions with a prescribed approximation order in these cases is given in terms of a certain maximal function applied to the wavelet coefficients.     

赋Luxemburg范数的Musielak-Orlicz-Sobolev空间单调性在最佳逼近中的应用

借鉴Orlicz-Sobolev空间的单调性在最佳逼近中一些应用,利用Musielak-Orlicz-Sobolev空间的构成特点,给出了赋Luxemburg范数的Musielak-Orlicz-Sobolev空间的单调性在最佳逼近中一些应用的充分性定理.     

Best uniform approximation to a class of rational functions

We explicitly determine the best uniform polynomial approximation to a class of rational functions of the form 1/²(x−c)+K(a,b,c,n)/(x−c) on [a,b] represented by their Chebyshev expansion, where a, b, and c are real numbers, n−1 denotes the degree of the best approximating polynomial, and K is a constant determined by a, b, c, and n. Our result is based on the explicit determination of a phase angle η in the representation of the approximation error by a trigonometric function. Moreover, we formulate an ansatz which offers a heuristic strategies to determine the best approximating polynomial to a function represented by its Chebyshev expansion. Combined with the phase angle method, this ansatz can be used to find the best uniform approximation to some more functions.     

广义权函数的最大类逼近及其应用

本文研究了广义权函数的最大类逼近问题,给出了K-最大类和A-最大类的特征刻划．同时也给出了关于广义权函数逼近的唯一性元特征刻划的结果和对某些具体逼近问题的应用,从而推广和改进了他人的结果．