Meromorphic functions with bounded boundary rotation

In this article, we generalize the class of meromorphic functions with bounded boundary rotation and related classes. Characterizations and some properties of these classes of functions are given.     

Functions of bounded variation and polarization

It is known that, if u is a real valued function on ?^N of bounded variation, then its total variation decreases under polarization. In this paper we identify the difference between the total variation of u and that one of its polar u_Π (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Functions of bounded variation, the derivative of the one dimensional maximal function, and applications to inequalities

We prove that if is of bounded variation, then the uncentered maximal function is absolutely continuous, and its derivative satisfies the sharp inequality . This allows us to obtain, under less regularity, versions of classical inequalities involving derivatives.

     

Projections and Non-Linear Approximation in the Space BV(Rd)

The aim of this paper is to provide an analysis of non-linearapproximation in the L_p-norm p = d / (d – 1) of functionsof bounded variation on R^d with d > 1 by polynomials in theHaar system. The exponent p is the natural exponent as it isthe correct exponent in the Sobolev inequality. The approximationschemes that we discuss in this paper are mostly related toHaar thresholding and m-term approximation. These problems ford = 2 are studied in detail in a paper by Cohen, DeVore, Petrushevand Xu. The main aim of this paper is to extend their resultsto the case d 2. We obtain the optimal order of the m-term Haar approximationand prove the stability of Haar thresholding in the BV-norm.As one of the main tools, we establish the boundedness of certainaveraging projections in BV. 2000 Mathematics Subject Classification41A46, 41A63.     

Sharpness of estimates for the least uniform derivation from the class of functions with bounded second derivative

We study the uniform approximation of continuous functions on an interval by functions with bounded second derivative. We prove the sharpness of estimates for the least uniform deviation of a function in terms of its local deviations on uniform and nonuniform three-point grids.     

The bounded approximation property for spaces of holomorphic mappings on infinite dimensional spaces

We study the bounded approximation property for spaces of holomorphic functions. We show that if U is a balanced open subset of a Fréchet–Schwartz space or (DFM )‐space E , then the space ??(U ) of holomorphic mappings on U , with the compact‐open topology, has the bounded approximation property if and only if E has the bounded approximation property. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

BV ‐extension of rate independent operators

Rate independent operators naturally arise in the mathematical analysis of hysteresis. Among rate independent operators, the locally monotone ones are those better suited for the study of PDE's with hysteresis. We prove that a rate independent operator R: Lip (0, T) → BV (0, T) ∩ C (0, T) which is locally monotone and continuous with respect to the strict topology of BV admits a unique continuous extension R?: BV (0, T) → BV (0, T). This general result applies to several concrete hysteresis operators. For many of these operators the existence of a continuous extension was previously known at most to the space BV (0, T) ∩ C (0, T). (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Greedy algorithms in Banach spaces

We study efficiency of approximation and convergence of two greedy type algorithms in uniformly smooth Banach spaces. The Weak Chebyshev Greedy Algorithm (WCGA) is defined for an arbitrary dictionary D and provides nonlinear m-term approximation with regard to D. This algorithm is defined inductively with the mth step consisting of two basic substeps: (1) selection of an mth element _m ^c from D, and (2) constructing an m-term approximant G _m ^c . We include the name of Chebyshev in the name of this algorithm because at the substep (2) the approximant G _m ^c is chosen as the best approximant from Span( ₁ ^c ,..., _m ^c ). The term Weak Greedy Algorithm indicates that at each substep (1) we choose _m ^c as an element of D that satisfies some condition which is t _m -times weaker than the condition for _m ^c to be optimal (t _m =1). We got error estimates for Banach spaces with modulus of smoothness (u)u ^q , 1<q2. We proved that for any f from the closure of the convex hull of D the error of m-term approximation by WCGA is of order (1+t ₁ ^p ++t _m ^p )^–1/p , 1/p+1/q=1. Similar results are obtained for Weak Relaxed Greedy Algorithm (WRGA) and its modification. In this case an approximant G ^r _m is a convex linear combination of 0,₁ ^r ,..., ^r _m . We also proved some convergence results for WCGA and WRGA.     

Hankel-type operators on the space of bounded harmonic functions

We study Hankel-type operators on the space of bounded harmonic functions on the open unit disk. These operators are related to tight uniform algebras, the Dunford-Pettis property, and Bourgain algebras.

     

Three-space problems for the bounded compact approximation property

In this paper, the notion of the bounded compact approximation property (BCAP) of a pair [Banach space and its subspace] is used to prove that if X is a closed subspace of L∞ with the BCAP, then L∞/X has the BCAP. We also show that X* has the λ-BCAP with conjugate operators if and only if the pair (X, Y) has the λ-BCAP for each finite codimensional subspace Y∈X. Let M be a closed subspace of X such that M⊥ is complemented in X*. If X has the (bounded) approximation property of order p, then M has the (bounded) approximation property of order p.     

Wave fans are special

It is shown that self-similar BV solutions of genuinely nonlinear strictly hyperbolic systems of conservation laws are special functions of bounded variation, with vanishing Cantor part.     

Gaussian BV Functions and Gaussian BV Capacity on Stratified Groups

Let $G$ be a stratified Lie group and let $\{X_1, \cdots, X_{n_1}\}$ be a basis of the first layer of the Lie algebra of $G$. The sub-Laplacian $\Delta_G$ is defined by $$\Delta_G= -\sum^{n_1}_{j=1} X^2_j. $$ The operator defined by $$\Delta_G-\sum^{n_1}_{j=1}\frac{X_jp}{p}X_j$$ is called the Ornstein-Uhlenbeck operator on $G$, where $p$ is a heat kernel at time 1 on $G$. In this paper, we investigate Gaussian BV functions and Gaussian BV capacities associated with the Ornstein-Uhlenbeck operator on the stratified Lie group.     

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     

On Bounded Variation Functions by General MKZD Operators

In this paper, we study the rate of convergence for functions of bounded variation for the recently introduced Bzier variant of the Meyer-Knig-Zeller-Durrmeyer operators.     

Adaptive wavelet methods for elliptic operator equations: Convergence rates

This paper is concerned with the construction and analysis of wavelet-based adaptive algorithms for the numerical solution of elliptic equations. These algorithms approximate the solution of the equation by a linear combination of wavelets. Therefore, a benchmark for their performance is provided by the rate of best approximation to by an arbitrary linear combination of wavelets (so called -term approximation), which would be obtained by keeping the largest wavelet coefficients of the real solution (which of course is unknown). The main result of the paper is the construction of an adaptive scheme which produces an approximation to with error in the energy norm, whenever such a rate is possible by -term approximation. The range of 0$"> for which this holds is only limited by the approximation properties of the wavelets together with their ability to compress the elliptic operator. Moreover, it is shown that the number of arithmetic operations needed to compute the approximate solution stays proportional to . The adaptive algorithm applies to a wide class of elliptic problems and wavelet bases. The analysis in this paper puts forward new techniques for treating elliptic problems as well as the linear systems of equations that arise from the wavelet discretization.

     

Accelerating the convergence of the method of alternating projections

The powerful von Neumann-Halperin method of alternating projections (MAP) is an algorithm for determining the best approximation to any given point in a Hilbert space from the intersection of a finite number of subspaces. It achieves this by reducing the problem to an iterative scheme which involves only computing best approximations from the individual subspaces which make up the intersection. The main practical drawback of this algorithm, at least for some applications, is that the method is slowly convergent. In this paper, we consider a general class of iterative methods which includes the MAP as a special case. For such methods, we study an ``accelerated' version of this algorithm that was considered earlier by Gubin, Polyak, and Raik (1967) and by Gearhart and Koshy (1989). We show that the accelerated algorithm converges faster than the MAP in the case of two subspaces, but is, in general, not faster than the MAP for more than two subspaces! However, for a ``symmetric' version of the MAP, the accelerated algorithm always converges faster for any number of subspaces. Our proof seems to require the use of the Spectral Theorem for selfadjoint mappings.

     

Gap series and functions of bounded variation

Summary We prove that the gap seriesΣf(n _k x) does not behave like an independent random series when f is a function of bounded variation with rational discontinuity.     

Optimal recovery on the classes of functions with bounded mixed derivative

Temlyakov considered the optimal recovery on the classes of functions with bounded mixed derivative in the Lp metrics and gave the upper estimates of the optimal recovery errors. In this paper, we determine the asymptotic orders of the optimal recovery in Sobolev spaces by standard information, i.e., function values, and give the nearly optimal algorithms which attain the asymptotic orders of the optimal recovery.     

An estimate for the least uniform deviation from the class of functions with bounded second derivative

We consider the problem of uniform approximation of a continuous function on an interval by means of the class of functions with bounded second derivative. We prove an estimate for the least uniform deviation of a function in terms of its local deviations on uniform and nonuniform three-point grids.     

Some critical minimization problems for functions of bounded variations

Using a new elementary method, we prove the existence of minimizers for various critical problems in BV(Ω) and also in W1,p(Ω), 1<p<∞.