Inequalities for Cyclic Functions

The nth cyclic function is defined by

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We prove that if k is an integer with 1kn−1, then

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holds for all positive real numbers x with the best possible constantsα=1 and β= 2n-k over n.     

Orthogonal Rational Functions and Nested Disks

In Akhiezer's book [“The Classical Moment Problem and Some Related Questions in Analysis,” Oliver & Boyd, Edinburghasol;London, 1965] the uniqueness of the solution of the Hamburger moment problem, if a solution exists, is related to a theory of nested disks in the complex plane. The purpose of the present paper is to develop a similar nested disk theory for a moment problem that arises in the study of certain orthogonal rational functions. Let {α_n}^∞_n=0be a sequence in the open unit disk in the complex plane, let

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( /|α_k|=−1 whenα_k=0), and let

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We consider the following “moment” problem: Given a positive-definite Hermitian inner product ·, · on × , find a non-decreasing functionμon [−π, π] (or a positive Borel measureμon [−π,π)) such that

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In particular we give necessary and sufficient conditions for the uniqueness of the solution in the case that If this series diverges the solution is always unique.     

On algebraic K-theory of real algebraic varieties with circle action

Assume that X is a compact connected orientable nonsingular real algebraic variety with an algebraic free S¹-action so that the quotient Y=X/S¹ is also a real algebraic variety. If π : X→Y is the quotient map then the induced map between reduced algebraic K-groups, tensored with ,

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is onto, where , denoting the ring of entire rational (regular) functions on the real algebraic variety X, extending partially the Bochnak–Kucharz result that

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for any real algebraic variety X. As an application we will show that for a compact connected Lie group G .     

Boundedness by 3 of the Whitney Interpolation Constant

Let the function fC[0,1] satisfy f( . We prove the estimate

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.     

Oblique Projections and Abstract Splines

Given a closed subspace of a Hilbert space and a bounded linear operator AL( ) which is positive, consider the set of all A-self-adjoint projections onto :

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In addition, if ₁ is another Hilbert space, T: → ₁ is a bounded linear operator such that T^*T=A and ξ , consider the set of (T, ) spline interpolants to ξ:

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A strong relationship exists between (A, ) and sp(T, ,ξ). In fact, (A, ) is not empty if and only if s p(T, ,ξ) is not empty for every ξ . In this case, for any ξ it holds

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and for any ξ , the unique vector of s p(T, ,ξ) with minimal norm is (1−P_A, )ξ, where P_A, is a distinguished element of (A, ). These results offer a generalization to arbitrary operators of several theorems by de Boor, Atteia, Sard and others, which hold for closed range operators.     

Weierstrass'' Theorem in Weighted Sobolev Spaces

We characterize the set of functions which can be approximated by polynomials with the following norm

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for a big class of weights w₀, w₁, …, w_k     

Functions of class Lip(α, p) and their Taylor mean

The object of this paper is to study the rapidity of convergence of the Taylor mean of the Fourier series of ƒ(x) when ƒ(x) belongs to the class Lip(α, p). We show that it is of Jackson order provided that a suitable integrability condition is imposed upon the function

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.     

An Improvement of the Constants in a Reverse Integral Inequality

We present an upper bound for the ratio [formula], where f is a positive decreasing function satisfying

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for all t (0, a]. Our result sharpens an inequality of L. Nania.     

Partially dynamic maintenance of minimum weight hyperpaths

We address the problem of dynamically maintaining minimum weight hyperpaths in a directed hypergraph in a decremental setting. For such a problem, we provide a new efficient algorithm that works for a wide class of hyperpath weight measures. This algorithm explicitly updates minimum weight hyperpaths in

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worst case time under a sequence of L hyperarc weight increments and hyperarc deletions, where C is the maximum weight of minimum hyperpaths in and is the size of the representation of the hypergraph. Hyperpath weight measures are only required to belong to the class of strict weakly superior functions.     

The Bernstein Constant and Polynomial Interpolation at the Chebyshev Nodes

In this paper, we establish new asymptotic relations for the errors of approximation in L_p[−1,1], 0<p∞, of x^λ, λ>0, by the Lagrange interpolation polynomials at the Chebyshev nodes of the first and second kind. As a corollary, we show that the Bernstein constant

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is finite for λ>0 and .     

Periodic solutions of a nonlinear second order differential equation with delay

It is proved that the autonomous difference-differential equation

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(0) has, for a broad class of functions f, a nonconstant periodic solution whenever the associated characteristic equation has a root with positive real part.     

Precise asymptotics in some strong limit theorems for multidimensionally indexed random variables

Consider Z₊^d (d2)—the positive d-dimensional lattice points with partial ordering , let {X_k,kZ₊^d} be i.i.d. random variables with mean 0, and set S_n=∑_knX_k, nZ₊^d. We establish precise asymptotics for ∑_n|n|^r/p−2P(|S_n||n|^1/p), and for

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, (0δ1) as 0, and for

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as .     

Asymptotics for Lp extremal polynomials on the unit circle

Let p > 1, and dμ a positive finite Borel measure on the unit circle Γ: = {z ε C: ¦z¦ = 1}. Define the monic polynomial φ_{n, p}(z)=zⁿ+…εP_n >(the set of polynomials of degree at most n) satisfying

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. Under certain conditions on dμ, the asymptotics of φ_{n, p}(z) for z outside, on, or inside Γ are obtained (cf. Theorems 2.2 and 2.4). Zero distributions of φ_{n, p} are also discussed (cf. Theorems 3.1 and 3.2).     

Restricted T-Universal Functions

We prove the existence of a function which is holomorphic exactly in the unit disk and has universal translates with respect to a prescribed closed set E∂ and satisfies C^∞(∂ \E). If Q is a subsequence of ₀ with upper density d(Q)=1 then the function can be constructed such that in addition

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Prophet inequalities for parallel processes

Generalizations of prophet inequalities for single sequences are obtained for optimal stopping of several parallel sequences of independent random variables. For example, if {X_{i, j}, 1 ≤ i ≤ n, 1 ≤ j < ∞} are independent non-negative random variables, then

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and this bound is best possible. Applications are made to comparisons of the optimal expected returns of various alternative methods of stopping of parallel processes.     

Target pattern and spiral solutions to reaction-diffusion equations with more than one space dimension

We prove the existence of homogeneous target pattern and spiral solutions to equations of the form

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; the spatial dimension is greater than one. As in the one-dimensional case, such solutions exist for discrete values of the asymptotic wave number (or equivalently, the frequency of oscillation of the entire solution). For target patterns, we construct solutions for a sequence of frequencies. For spirals, we construct only the “lowest mode” solution.     

Inequalities for Rational Functions

A new hyperbolic area estimate for the level sets of finite Blaschke products is presented. The following inversion of the usual Sobolev embedding theorem is proved:

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Hereris a rational function of degreenwith poles outside . This estimate implies a new inverse theorem for rational approximation of analytic functions with respect to areaL^pnorm.     

Best Approximation and Cyclic Variation Diminishing Kernels

We study best uniform approximation of periodic functions from

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where the kernelK(x, y) is strictly cyclic variation diminishing, and related problems including periodic generalized perfect splines. For various approximation problems of this type, we show the uniqueness of the best approximation and characterize the best approximation by extremal properties of the error function. The results are proved by using a characterization of best approximants from quasi-Chebyshev spaces and certain perturbation results.     

Markov-Type Inequalities for Products of Müntz Polynomials

Let Λ(λ_j)^∞_j=0 be a sequence of distinct real numbers. The span of {x^λ₀, x^λ₁, …, x^λ_n} over is denoted by M_n(Λ)span{x^λ₀, x^λ₁, …, x^λ_n}. Elements of M_n(Λ) are called Müntz polynomials. The principal result of this paper is the following Markov-type inequality for products of Müntz polynomials. T 2.1. LetΛ(λ_j)^∞_j=0andΓ(γ_j)^∞_j=0be increasing sequences of nonnegative real numbers. Let

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Then

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18(n+m+1)(λ_n+γ_m).In particular ,

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Under some necessary extra assumptions, an analog of the above Markov-type inequality is extended to the cases when the factor x is dropped, and when the interval [0, 1] is replaced by [a, b](0, ∞).     

Biholomorphically invariant families amongst Carleson class

Given α (0, 1), let T_α be the Carleson class of all meromorphic maps ƒ from the unit disk to the extended complex plane with

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where ƒ^# and dm mean the spherical derivative of ƒ and Lebesgue area measure on separately. And, let BIT_α and BIT_α,0 be the biholomorphically invariant families (amongst the Carleson class) consisting of those ƒ T_α with sup and lim_{|w| → 1} ||ƒ ο φ_w||_Tα = 0 respectively, where

. The main purpose of this article is to study BIT_α and BIT_α,0 via the Ahlfors-Shimizu characteristic, canonical factorization and bounded holomorphic maps.