About <Emphasis Type="Italic">I</Emphasis>(<Emphasis Type="Italic">J</Emphasis>)-approximately continuous functions

A generalization of density point with respect to category and I(J)-density topology has been presented in [5]. This papers deals with some basic properties of the corresponding I(J)-approximately continuous functions and gives answer when these functions are of the first Baire class and when they are Darboux functions.     

Some remarks concerning the Fourier transform in the space <Emphasis Type="Italic">L</Emphasis><Subscript>2</Subscript> (ℝ)

Two useful estimates are proved for the Fourier transform in the space of square integrable functions on certain classes of functions characterized by the generalized continuity modulus.     

Approximation of periodic functions by Steklov averages in <Emphasis Type="Italic">L</Emphasis><Subscript>2</Subscript>

In the space L₂ of periodic functions, we establish exact (in the sense of constants) estimates from below for the deviation of the Steklov functions of the first and second order in terms of the modulus of continuity of the second order. Similar results are also established for even continuous periodic functions with nonnegative Fourier coefficients in the space C. Bibliography: 5 titles. __________ Translated from Problemy Matematicheskogo Analiza, No. 35, 2007, pp. 79–90     

The use of anti-A priori estimates in the theory of bases in the space <Emphasis Type="Italic">L</Emphasis><Subscript>2</Subscript>

We consider a one-dimensional Schrödinger operator defined on a finite interval. We show that anti-a priori estimates without a positive power on the right-hand side on the inequality are necessary and sufficient conditions for the unconditional basis property in the space of square integrable functions for an arbitrary choice of associated functions. We suggest a new definition of associated functions of linear operators in a Hilbert space.     

Problem of uniqueness of an element of the best nonsymmetric <Emphasis Type="Italic">L</Emphasis><Subscript>1</Subscript>-approximation of continuous functions with values in <Emphasis Type="Italic">KB</Emphasis>-spaces

We consider the problem of characterization of subspaces of uniqueness of an element of the best nonsymmetric L ₁-approximation of functions that are continuous on a metric compact set of functions with values in a KB-space. We find classes of test functions that characterize the uniqueness of an element of the best nonsymmetric approximation. Translated from Ukrains'kyi Matematychnyi Zhurnal, Vol. 60, No. 7, pp. 867–878, July, 2008.     

On Geodesic <Emphasis Type="Italic">E</Emphasis>-Convex Sets,Geodesic <Emphasis Type="Italic">E</Emphasis>-Convex Functions and <Emphasis Type="Italic">E</Emphasis>-Epigraphs

In this paper, we introduce a new class of sets and a new class of functions called geodesic E-convex sets and geodesic E-convex functions on a Riemannian manifold. The concept of E-quasiconvex functions on R ⁿ is extended to geodesic E-quasiconvex functions on Riemannian manifold and some of its properties are investigated. Afterwards, we generalize the notion of epigraph called E-epigraph and discuss a characterization of geodesic E-convex functions in terms of its E-epigraph. Some properties of geodesic E-convex sets are also studied.     

Affine equivalence for rotation symmetric Boolean functions with 2<Superscript><Emphasis Type="Italic">k</Emphasis></Superscript> variables

Rotation symmetric Boolean functions have been extensively studied in the last 10 years or so because of their importance in cryptography and coding theory. Until recently, very little was known about the basic question of when two such functions are affine equivalent. Even the case of quadratic functions is nontrivial, and this was only completely settled in a 2009 paper of Kim, Park and Hahn. The much more complicated case of cubic functions was solved for permutations using a new concept of patterns in a 2010 paper of Cusick, and it is conjectured that, as in the quadratic case, this solution actually applies for all affine transformations. The patterns method enables a detailed analysis of the affine equivalence classes for various special classes of cubic rotation symmetric functions in n variables. Here the case of functions with 2 ^k variables (this number is especially relevant in computer applications) and generated by a single monomial is examined in detail, and in particular a formula for the number of classes is proved.     

<Emphasis Type="Italic">E′</Emphasis> and its relation with vector-valued functions on<Emphasis Type="Italic">E</Emphasis>

We study the relation between different spaces of vector-valued polynomials and analytic functions over dual-isomorphic Banach spaces. Under conditions of regularity onE andF, we show that the spaces ofX-valuedn-homogeneous polynomials and analytic functions of bounded type onE andF are isomorphic wheneverX is a dual space. Also, we prove that many of the usual subspaces of polynomials and analytic functions onE andF are isomorphic without conditions on the involved spaces.     

On a <Emphasis Type="Italic">PT</Emphasis>-symmetric waveguide with a pair of small holes

Positive definite piecewise linear functions are applied to the study of the properties of nonnegative trigonometric polynomials and to the analysis of some extremal problems on sets of positive definite functions.     

Jensen inequalities for <Emphasis Type="Italic">P</Emphasis>-class functions

In this paper, we first give some characterizations for P-class functions. Then giving a Hermite–Hadamard type inequality for P-class functions, we prove equivalency of some significant metrics in normed linear spaces. We also obtain an operator version of the Jensen inequality for P-class functions. Introducing operator (mid) P-class functions, we present some characterizations for such functions.     

Boundary Harnack Principle for <Emphasis Type="Italic">p</Emphasis>-harmonic Functions in Smooth Euclidean Domains

We establish a scale-invariant version of the boundary Harnack principle for p-harmonic functions in Euclidean C ^1,1-domains and obtain estimates for the decay rates of positive p-harmonic functions vanishing on a segment of the boundary in terms of the distance to the boundary. We use these estimates to study the behavior of conformal Martin kernel functions and positive p-superharmonic functions near the boundary of the domain. H. A. was partially supported by Grant-in-Aid for (B) (2) (No. 15340046) Japan Society for the Promotion of Science. N. S. was partially supported by NSF grant DMS-0355027. X. Z. was partially supported by the Taft foundation.     

Optimal Domains for <Emphasis Type="Italic">L</Emphasis><Superscript>0</Superscript>-valued Operators Via Stochastic Measures

We study extension of operators T: E→ L⁰([0, 1]), where E is an F–function space and L⁰([0, 1]) the space of measurable functions with the topology of convergence in measure, to domains larger than E, and we study the properties of such domains. The main tool is the integration of scalar functions with respect to stochastic measures and the corresponding spaces of integrable functions. Partially supported by D.G.I. #MTM2006-13000-C03-01 (Spain).     

Exact order of relative widths of classes <Emphasis Type="Italic">W</Emphasis><Stack><Subscript>1</Subscript><Superscript>r</Superscript></Stack> in the space <Emphasis Type="Italic">L</Emphasis><Subscript>1</Subscript>

We determine the exact order of relative widths of classes W ₁ ^r of periodic functions in the space L ₁ as n → ∞ under restrictions on higher derivatives of approximating functions. __________ Translated from Ukrains’kyi Matematychnyi Zhurnal, Vol. 57, No. 10, pp. 1409–1417, October, 2005.     

Characterizations of Hardy-Orlicz spaces on strictly pseudoconvex domains of C<Superscript>n</Superscript>

In this paper, we characterize Hardy-Orlicz spaces on strictly pseudoconvex domains of Cⁿ using real variable methods. Concretely, we prove the basic properties for a boundary behavior of functions in Hardy-Orlicz spaces, an approximation of functions in Hardy-Orlicz spaces by some functions holomorphic up to a neighborhood of the boundary, and a boundedness of the Szegö projection.     

On the outcome of false images of linear <Emphasis Type="Italic">k</Emphasis>-valued functions for composite numbers <Emphasis Type="Italic">k</Emphasis> when the number of variables increases

The problem of constructing discrete functions such that parts of their value sets determine (generate) arbitrary linear functions is considered. A case in which k is a prime number was considered earlier by the author. It is proved that the existence of such partial functions wshen the number of independent variables is no less then two implies they exists for any arbitrary greater number of independent variables. Upper estimates linear with respect to the number of independent variables are proved for the size of the domain of universal functions. The existence of two-variable universal functions is proved for sufficiently large k.     

Close‐to‐convexity of normalized Dini functions

In this paper necessary and sufficient conditions are deduced for the close‐to‐convexity of some special combinations of Bessel functions of the first kind and their derivatives by using a result of Shah and Trimble about transcendental entire functions with univalent derivatives and some newly discovered Mittag–Leffler expansions for Bessel functions of the first kind.     

Pointwise behaviour of <Emphasis Type="Italic">M</Emphasis><Superscript>1,1</Superscript> Sobolev functions

Our main objective is to study Haj?asz type Sobolev functions with the exponent one on metric measure spaces equipped with a doubling measure. We show that a discrete maximal function is bounded in the Haj?asz space with the exponent one. This implies that every such function has Lebesgue points outside a set of capacity zero. We also show that every Haj?asz function coincides with a Hölder continuous Haj?asz function outside a set of small Hausdorff content. Our proofs are based on Sobolev space estimates for maximal functions.     

On the Relation between Piecewise Polynomial and Rational Approximation in <Emphasis Type="Italic">L</Emphasis><Subscript><Emphasis Type="Italic">p</Emphasis></Subscript>(<Emphasis Type="Bold">R</Emphasis><Superscript>2</Superscript>)

In the univariate case there are certain equivalences between the nonlinear approximation methods that use piecewise polynomials and those that use rational functions. It is known that for certain parameters the respective approximation spaces are identical and may be described as Besov spaces. The characterization of the approximation spaces of the multivariate nonlinear approximation by piecewise polynomials and by rational functions is not known. In this work we compare between the two methods in the bivariate case. We show some relations between the approximation spaces of piecewise polynomials defined on n triangles and those of bivariate rational functions of total degree n which are described by n parameters. Thus we compare two classes of approximants with the same number Cn of parameters. We consider this the proper comparison between the two methods.     

On the <Emphasis Type="Italic">p</Emphasis>-ary (cubic) bent and plateaued (vectorial) functions

Plateaued functions play a significant role in cryptography, sequences for communications, and the related combinatorics and designs. Comparing to their importance, those functions have not been studied in detail in a general framework. Our motivation is to bring further results on the characterizations of bent and plateaued functions, and to introduce new tools which allow us firstly a better understanding of their structure and secondly to get methods for handling and designing such functions. We first characterize bent functions in terms of all even moments of the Walsh transform, and then plateaued (vectorial) functions in terms of the value distribution of the second-order derivatives. Moreover, we devote to cubic functions the characterization of plateaued functions in terms of the value distribution of the second-order derivatives, and hence this reveals non-existence of homogeneous cubic bent (and also (homogeneous) cubic plateaued for some cases) functions in odd characteristic. We use a rank notion which generalizes the rank notion of quadratic functions. This rank notion reveals new results about (homogeneous) cubic plateaued functions. Furthermore, we observe non-existence of a function whose absolute Walsh transform takes exactly 3 distinct values (one being zero). We finally provide a new class of functions whose absolute Walsh transform takes exactly 4 distinct values (one being zero).