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Journal of Approximation Theory
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Journal of Approximation Theory
Article . 2001
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Orthogonal Expansion of Real Polynomials, Location of Zeros, and an L2 Inequality

Orthogonal expansion of real polynomials, location of zeros, and an \(L^2\) inequality
Authors: Schmeisser, G.;

Orthogonal Expansion of Real Polynomials, Location of Zeros, and an L2 Inequality

Abstract

The following problem is investigated: let \(f\) be a polynomial given by the expansion \(f(z)=a_0 p_0(z)+a_1p_1(z)+\cdots+a_np_n(z)\) in terms of orthogonal polynomials. What can be said about the zeros of \(f\) in terms of the zeros of the orthogonal polynomials \(p_j\) and the Fourier coefficients \(a_j\)? The main result is a condition on the (real) Fourier coefficients \(a_j\) which implies that \(f\) has at least \(k\) distinct real zeros of odd multiplicity. If the condition does not hold, then a region is given outside of which \(f\) has at most \((n-k)/2\) pairs of conjugate zeros. As a corollary, a strip is found where the polynomial has at least \(k\) zeros. Another result, of independent interest, is an upper bound for the ratio \(\|f\|/ \|fg\|\), where \(f\) and \(g\) are polynomials of degree \(n\) and \(k\), respectively, and the norms are in \(L^2(\sigma)\), where \(\sigma\) is the orthogonality measure for the polynomials \(p_k\).

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Keywords

Mathematics(all), Numerical Analysis, weighted L2 inequality, zeros of polynomials, Applied Mathematics, Fourier coefficients, orthogonal expansion, Zeros of polynomials, rational functions, and other analytic functions of one complex variable (e.g., zeros of functions with bounded Dirichlet integral), location of zeros, real polynomials, Orthogonal functions and polynomials, general theory of nontrigonometric harmonic analysis, orthogonal polynomials, Analysis, criterion for real zeros

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
0
Average
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