Pointwise limits of analytic functions

This note is an addendum to the above-mentioned paper, whichwas recently published in this journal. Unfortunately, the authorwas unaware that the main results of Section 1 of this paper,Theorems 1.9 and 1.12, had already been proved in papers ofDanielyan and Mergelyan [1,2]. The author is grateful to ProfessorDanielyan for drawing his attention to these references, andapologises for the unintentional duplication of part of theirwork. Footnotes 2000 Mathematics Subject Classification 30E10 (primary), 46J10(secondary). Received May 6, 2008; published online August 22, 2008.     

Pointwise multipliers for Besov spaces of dominating mixed smoothness-Ⅱ

We continue our investigations on pointwise multipliers for Besov spaces of dominating mixed smoothness. This time we study the algebra property of the classes S_(p,q)~rB(R~d) with respect to pointwise multiplication. In addition, if p≤q, we are able to describe the space of all pointwise multipliers for S_(p,q)~rB(R~d).     

Dimensions of the coordinate functions of space-filling curves

The graphs of coordinate functions of space-filling curves such as those described by Peano, Hilbert, Pólya and others, are typical examples of self-affine sets, and their Hausdorff dimensions have been the subject of several articles in the mathematical literature. In the first half of this paper, we describe how the study of dimensions of self-affine sets was motivated, at least in part, by these coordinate functions and their natural generalizations, and review the relevant literature. In the second part, we present new results on the coordinate functions of Pólya's one-parameter family of space-filling curves. We give a lower bound for the Hausdorff dimension of their graphs which is fairly close to the box-counting dimension. Our techniques are largely probabilistic. The fact that the exact dimension remains elusive seems to indicate the need for further work in the area of self-affine sets.     

Pointwise compact sets of measurable functions

If X is a compact Radon measure space, and A is a pointwise compact set of real-valued measurable functions on X, then A is compact for the topology of convergence in measure (Corollary 2H). Consequently, if X_o,..., X_n are Radon measure spaces, then a separately continuous real-valued function on X_o×X₁×...×X_n is jointly measurable (Theorem 3E). If we seek to generalize this work, we encounter some undecidable problems (§4).     

Approximation of differentiable functions by functions of large smoothness

The order of the quantity \(\delta (L) = \mathop {\sup }\limits_{\chi _{_{_{_1 } } } } \mathop {\inf }\limits_{\chi _2 } \parallel \chi _1 - \chi _2 \parallel L_S [0,2 = ]\) as L → ∞ is studied for the classes of periodic functionsx ₁εW _p ⁿ (I), andx ₂ εW _q ^m (L). Necessary and sufficient conditions under which the inequality $$\parallel x^{(n)} \parallel _{L_p } \leqslant C\parallel x\parallel _{L_q }^x \parallel x^{(m)} \parallel _{L_s }^\beta $$ with the constant independent of x holds for all periodic functions x(t) with \(\int_0^{2\pi } \chi (l)dl = 0\) andx ^(m) (t εL _s [0, 2π] are found.     

Pointwise Lipschitz functions on metric spaces

For a metric space X, we study the space D^∞(X) of bounded functions on X whose pointwise Lipschitz constant is uniformly bounded. D^∞(X) is compared with the space LIP^∞(X) of bounded Lipschitz functions on X, in terms of different properties regarding the geometry of X. We also obtain a Banach-Stone theorem in this context. In the case of a metric measure space, we also compare D^∞(X) with the Newtonian-Sobolev space N1,∞(X). In particular, if X supports a doubling measure and satisfies a local Poincaré inequality, we obtain that D^∞(X)=N1,∞(X).     

Differential and smoothness properties of continuous functions

Necessary and sufficient conditions on functions and are found in order for the classes of functions andH ^+k to coincide (k and r are natural numbers).These results are generalizations of [1–4].Translated from Matematicheskie Zametki, Vol. 22, No. 6, pp. 785–794, December, 1977.The authors thank V. V. Salaev for his unflagging interest in their work.     

Estimates of maximal functions measuring local smoothness

Letη be a nondecreasing function on (0, 1] such thatη(t)/t decreases andη(+0)=0. Letf ∈L(I ⁿ ) (I≡[0,1]. Set $${\mathcal{N}}_\eta f(x) = \sup \frac{1}{{\left| Q \right|\eta (\left| Q \right|^{1/n} )}} \smallint _Q \left| {f(t) - f(x)} \right|dt,$$ , where the supremum is taken over all cubes containing the pointx. Forη=t ^α (0<α≤1) this definition was given by A.Calderón. In the paper we prove estimates of the maximal functions ${\mathcal{N}}_\eta f$ , along with some embedding theorems. In particular, we prove the following Sobolev type inequality: if $$1 \leqslant p< q< \infty , \theta \equiv n(1/p - 1/q)< 1, and \eta (t) \leqslant t^\theta \sigma (t),$$ , then $$\parallel {\mathcal{N}}_\sigma {f} {\parallel_{q,p}} \leqslant c \parallel {\mathcal{N}}_\eta {f} {\parallel_p} .$$ . Furthermore, we obtain estimates of ${\mathcal{N}}_\eta f$ in terms of theL ^p -modulus of continuity off. We find sharp conditions for ${\mathcal{N}}_\eta f$ to belong toL ^p (I ⁿ ) and the Orlicz class?(L), too.     

Constructive properties of functions with variable smoothness

Analogs of theorems of Jensen and Weierstrass are proven for functions of variable smoothness, defined on the real line.Translated from Matematicheskie Zametki, Vol. 8, No. 4, pp. 443–449, October, 1970.In conclusion I wish to express my gratitude to S. M. Nikol'skii and S. B. Stechkin for their valuable advice concerning this work.     

Pointwise convergence and radial limits of harmonic functions

This paper characterizes those real-valued functions on a compact setK in ℝ ⁿ that can be expressed as the pointwise limit of a sequence (h _m ), where each functionh _m is harmonic on some neighbourhood ofK. It also characterizes those functions on the unit sphere that can arise as the radial limit function at infinity of an entire harmonic function. Both results rely on important recent work of Lukeš et al. concerning approximation of affine Baire-one functions.     

Identifying local smoothness for spatially inhomogeneous functions

We consider a problem of estimating local smoothness of a spatially inhomogeneous function from noisy data under the framework of smoothing splines. Most existing studies related to this problem deal with estimation induced by a single smoothing parameter or partially local smoothing parameters, which may not be efficient to characterize various degrees of smoothness of the underlying function when it is spatially varying. In this paper, we propose a new nonparametric method to estimate local smoothness of the function based on a moving local risk minimization coupled with spatially adaptive smoothing splines. The proposed method provides full information of the local smoothness at every location on the entire data domain, so that it is able to understand the degrees of spatial inhomogeneity of the function. A successful estimate of the local smoothness is useful for identifying abrupt changes of smoothness of the data, performing functional clustering and improving the uniformity of coverage of the confidence intervals of smoothing splines. We further consider a nontrivial extension of the local smoothness of inhomogeneous two-dimensional functions or spatial fields. Empirical performance of the proposed method is evaluated through numerical examples, which demonstrates promising results of the proposed method.     

Boundary smoothness properties of Lipα analytic functions

LetU be an open set andb ∈ bdy(U). Let 0 < α< 1. Let A(U) denote the space of Lipα functions that are analytic onU, and a(U) the subspace lipα ∩ A(U). The space a(U ∪b), consisting of the functions that are analytic nearb, is dense in a(U). Letk be a natural number. We say that a(U) admits ak-th order continuous point derivation (cpd) atb if the functionalf → f^{(k) (b)} is continuous on a(U ∪b), with respect to the Lipα norm.     

Pointwise symmetrization inequalities for Sobolev functions and applications

We develop a technique to obtain new symmetrization inequalities that provide a unified framework to study Sobolev inequalities, concentration inequalities and sharp integrability of solutions of elliptic equations.