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27 April 2024
 
  » arxiv » 1610.6748

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On Hahn polynomial expansion of a continuous function of bounded variation
René Goertz ; Philipp Öffner ;
Date 21 Oct 2016
AbstractWe consider the well-known method of least squares on an equidistant grid with $N+1$ nodes on the interval $[-1,1]$. We investigate the following problem: For which ratio $N/n$ and which functions, do we have pointwise convergence of the least square operator ${LS}_n^N:mathcal{C}left[-1,1 ight] ightarrowmathcal{P}_n$? To solve this problem we investigate the relation between the Jacobi polynomials $P_k^{alpha,eta}$ and the Hahn polynomials $Q_kleft(cdot;alpha,eta,N ight)$. Thereby we describe the least square operator ${LS}_n^N$ by the expansion of a function by Hahn polynomials. In particular we present the following result: The series expansion $sum_{k=0}^n{hat{f} Q_k}$ of a function $f$ by Hahn polynomials $Q_k$ converges pointwise, if the series expansion $sum_{k=0}^n{hat{f} P_k}$ of the function $f$ by Jacobi polynomials $P_k$ converges pointwise and if ${n^4}/N ightarrow 0$ for $n,N ightarrowinfty$. Furthermore we obtain the following result: Let $finleft{ginmathcal{C}^1left[-1,1 ight]:g^primeinmathcal{BV}left[-1,1 ight] ight}$ and let $(N_n)_{n}$ be a sequence of natural numbers with ${n^4}/{N_n} ightarrow 0$. Then the least square method ${LS}_n^{N_n}[f]$ converges for each $xin[-1,1]$.
Source arXiv, 1610.6748
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