Convergence of best approximations in a smooth Banach space

Let X be a reflexive, strictly convex Banach space such that both X and X^* have Fréchet differentiable norms, and let {C_n} be a sequence of non-empty closed convex subsets of X. We prove that the sequence of best approximations {p(x ¦ C_n)} of any x ε X converges if and only if lim C_n exists and is not empty. We also discuss measurability of closed convex set valued functions.     

Approximation by fourier sums and best approximations on classes of analytic functions

We establish asymptotic equalities for upper bounds of approximations by Fourier sums and for the best approximations in the metrics of C and L1 on classes of convolutions of periodic functions that can be regularly extended into a fixed strip of the complex plane.     

Besov空间中的一些最佳逼近与最佳逼近元之间的等价关系

在各类Besov空间中建立若干等价关系.文中的定理1是Cohen等人2000年论文中的定理4.2的实质性扩充,定理2将李松1997年论文中的定理7延拓到0相似文献   

Convergence of multiple fourier series for functions of bounded variation

For functions of bounded variation in the sense of Hardy, we consider the pointwise convergence of the partial sums of Fourier series over a given sequence of bounded sets in the space of harmonics. We obtain sufficient conditions for convergence; necessary and sufficient conditions are obtained for the case in which these sets are convex with respect to each coordinate direction. The Pringsheim convergence of Fourier series in this problem was established by Hardy. Translated fromMatematicheskie Zametki, Vol. 61, No. 4, pp. 583–595, April, 1997. Translated by S. A. Telyakovskii and V. N. Temlyakov     

Convergence of fourier series with respect to multiplicative systems and the p-fluctuation continuity modulus

We obtain some sufficient conditions for convergence of series of the Fourier coefficients with respect to multiplicative systems for functions of bounded p-fluctuation. In some cases we establish the unimprovability of these conditions.     

Convergence of fourier series with respect to an orthogonal system of functions

Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 41, No. 5, pp. 641–647, May, 1989.     

Estimates of rate of convergence of best approximations by local functions

Questions are considered on the rate of convergence (in some abstract space of functions) of approximations that are the best in another space. Under specific conditions it is shown that the best approximations by local functions in a weighted Sobolev space W _p,B ^r yield almost-best approximation in W _q,B ^r with q[p,+).Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova AN SSSR, Vol. 58, pp. 22–37, 1976.     

Convergence of double fourier series for functions of bounded generalized variation. II

Siberian Mathematical Journal -     

Rearrangements of series in Lp

In this note we show that the condition $$\sqrt {\sum\nolimits_{n = 1}^\infty {f_n^2 (x)} } \in L_p$$ is sufficient for the set of sums of the rearranged series ∑_σ f_n to be a closed linear set in L_p.     

On the best approximations and rate of convergence of decompositions in the root vectors of an operator

We establish upper bounds of the best approximations of elements of a Banach space B by the root vectors of an operator A that acts in B. The corresponding estimates of the best approximations are expressed in terms of a K-functional associated with the operator A. For the operator of differentiation with periodic boundary conditions, these estimates coincide with the classical Jackson inequalities for the best approximations of functions by trigonometric polynomials. In terms of K-functionals, we also prove the abstract Dini-Lipschitz criterion of convergence of partial sums of the decomposition of f from B in the root vectors of the operator A to f     

Extistence of best nonlinear Lp -approximations

We are concerned with the Tschebyscheff and L_p-approximation problem, when the approximating family is the union of solutions of linear differential equations. We prove a strong compactness property and a global existence theorem. Application to generalized exponential approximation is given.