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Abstract  

We consider the classical extremal problem of estimating norms of higher order derivatives of algebraic polynomials when their norms are given. The corresponding extremal problem for general polynomials in uniform norm was solved by A. A. Markov, while Bernstein found the exact constant in the Markov inequality for monotone polynomials. In this note we give Markov-type inequalities for higher order derivatives in the general class of k-monotone polynomials. In particular, in case of first derivative we find the exact solution of this extremal problem in both uniform and L 1-norms. This exact solution is given in terms of the largest zeros of certain Jacobi polynomials.

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Abstract  

Let P n denote the linear space of polynomials p(z:=Σk=0 n a k(p)z k of degree ≦ n with complex coefficients and let |p|[−1,1]: = maxx∈[−1,1]|p(x)| be the uniform norm of a polynomial p over the unit interval [−1, 1]. Let t nP n be the n th Chebyshev polynomial. The inequality

\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{bbm} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\frac{{\left| p \right|_{\left[ { - 1,1} \right]} }} {{\left| {a_n (p)} \right|}} \geqq \frac{{\left| {t_n } \right|_{\left[ { - 1,1} \right]} }} {{\left| {a_n (t_n )} \right|}},p \in P_n$$ \end{document}
due to P. L. Chebyshev can be considered as an extremal property of the Chebyshev polynomial t n in P n. The present note contains various extensions and improvements of the above inequality obtained by using complex analysis methods.

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Abstract  

We consider biorthogonal systems of functions associated to derivatives of orthogonal polynomials in the case of general weights. For Freud polynomials, it is proved that the derivatives of any orders of them constitute Hilbertian bases in the space of weighted square integrable functions.

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Abstract

The uniform weighted approximation errors of Baskakov-type operators are characterized for weights of the form for γ 0,γ ∊[−1,0]. Direct and strong converse theorems are proved in terms of the weighted K-functional.

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Abstract

Relations between ω r(f,t)B and ω r+1(f,t)B of the sharp Marchaud and sharp lower estimate-type are shown to be satisfied for some Banach spaces of functions that are not rearrangement invariant. Corresponding results relating the rate of best approximation with ω r(f,t)B for those spaces are also given.

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Abstract

For a Banach space B of functions which satisfies for some m>0
∗
a significant improvement for lower estimates of the moduli of smoothness ω r(f,t)B is achieved. As a result of these estimates, sharp Jackson inequalities which are superior to the classical Jackson type inequality are derived. Our investigation covers Banach spaces of functions on ℝd or for which translations are isometries or on S d−1 for which rotations are isometries. Results for C 0 semigroups of contractions are derived. As applications of the technique used in this paper, many new theorems are deduced. An L p space with 1<p<∞ satisfies () where s=max  (p,2), and many Orlicz spaces are shown to satisfy () with appropriate s.
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Abstract  

Let f be an entire function of exponential type satisfying the condition
\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{bbm} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$f(z) \equiv e^{i\gamma } e^{i\tau z} \overline {f(\bar z)}$$ \end{document}
for some real γ. Lower and upper estimates for ∫−∞ |f′(x)|p dx in terms of ∫−∞ |f(x)|p dx, for such a function f belonging to L p(R), have been known in the case where p ∊ [1, ∞) and γ = 0. In this paper, these estimates are shown to hold for any p ∊ (0, ∞) and any real γ.
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Abstract  

Let f be a real continuous 2π-periodic function changing its sign in the fixed distinct points y iY:= {y i}i∈ℤ such that for x ∈ [y i, y i−1], f(x) ≧ 0 if i is odd and f(x) ≦ 0 if i is even. Then for each nN(Y) we construct a trigonometric polynomial P n of order ≦ n, changing its sign at the same points y iY as f, and

\documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{upgreek} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage{bbm} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} $$\left\| {f - P_n } \right\| \leqq c(s)\omega _3 \left( {f,\frac{\pi } {n}} \right),$$ \end{document}
where N(Y) is a constant depending only on Y, c(s) is a constant depending only on s, ω 3(f, t) is the third modulus of smoothness of f and ∥ · ∥ is the max-norm.

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Abstract

Letting 0<p≦∞, we prove Remez-, Bernstein–Markoff-, Schur- and Nikolskii-type inequalities for algebraic polynomials with exponential weights on (−1,1) multiplied by another weight function, which will satisfy the doubling or the A condition at different occurrences. Moreover, we state embedding theorems between some function spaces related with exponential weights.

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The norm estimation problem for Fourier operators acting from L w p (
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) to L υ q (
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) where 1 < pq < ∞ was investigated. These results has been generalized to the two-dimensional case and applied to obtain generalizations of the Bernstein inequality for trigonometric polynomials of one and two variables. Also, the rates of convergence of Cesaro and Abel-Poisson means of functions fL w p (
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) has been estimated in the case p = q and υw . The generalized Bernstein inequality applied to estimate the order of best trigonometric approximation of the derivative of functions fL w p (
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) in the space L υ q (
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).
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