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In this paper, we study Bernstein, Markov and Nikol’skii type inequalities for arbitrary algebraic polynomials with respect to a weighted Lebesgue space, where the contour and weight functions have some singularities on a given contour.
Keyword: In this paper, we study Bernstein, Markov and Nikol’skii type inequalities for arbitrary algebraic polynomials with respect to a weighted Lebesgue space, where the contour and weight functions have some singularities on a given contour
Let φ={φk}k=−∞∞ denote the extended Takenaka–Malmquist system on unit circle T and let σn,φ(f), f∈L1(T), be the Fejér-type operator based on φ, introduced by V. N. Rusak. We give the convergence criteria for σn,φ(f) in Banach space C(T). Also we prove the Voronovskaya-type theorem for σn,φ(f) on class of holomorphic functions representable by Cauchy-type integrals with bounded densities.
Keywords: Holomorphic functions; Takenaka–Malmquist system; Fejér type operator; Blaschke product; Frostman condition
Let Tn be the linear Hadamard convolution operator acting over Hardy space Hq, 1≤q≤∞. We call Tn a best approximation-preserving operator (BAP operator) if Tn(en)=en, where en(z):=zn, and if ∥Tn(f)∥q≤En(f)q for all f∈Hq, where En(f)q is the best approximation by algebraic polynomials of degree a most n−1 in Hq space. We give necessary and sufficient conditions for Tn to be a BAP operator over H∞. We apply this result to establish an exact lower bound for the best approximation of bounded holomorphic functions. In particular, we show that the Landau-type inequality ∣∣fˆn∣∣+c∣∣fˆN∣∣≤En(f)∞, wher ...More