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More properties of optimal polynomial approximants in Hardy spaces. (English) Zbl 1535.30119

Summary: We study optimal polynomial approximants (OPAs) in the classical Hardy spaces on the unit disk, \(H^p\) (\(1 < p < \infty \)). For fixed \(f\in H^p\) and \(n\in\mathbb{N} \), the OPA of degree \(n\) associated to \(f\) is the polynomial which minimizes the quantity \(\|qf-1\|_p\) over all complex polynomials \(q\) of degree less than or equal to \(n\). We begin with some examples which illustrate, when \(p\neq2\), how the Banach space geometry makes the above minimization problem interesting. We then weave through various results concerning limits and roots of these polynomials, including results which show that OPAs can be witnessed as solutions of certain fixed-point problems. Finally, using duality arguments, we provide several bounds concerning the error incurred in the OPA approximation.

MSC:

30H10 Hardy spaces
30E10 Approximation in the complex plane
30C10 Polynomials and rational functions of one complex variable
30C15 Zeros of polynomials, rational functions, and other analytic functions of one complex variable (e.g., zeros of functions with bounded Dirichlet integral)

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