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Relation of Lagrange Interpolation to Windowed Sinc Interpolation

For an infinite number of equally spaced samples, with spacing $ x_{k+1}-x_k = \Delta$, the Lagrange-interpolation basis polynomials converge to shifts of the sinc function, i.e.,

$\displaystyle l_k(x) =$   sinc$\displaystyle \left(x-k\Delta\over\Delta\right), \quad k=\ldots,-2,-1,0,1,2,\ldots
$

Proof: As order $ \to\infty$, the binomial window $ \to$ Gaussian window $ \to$ constant (unity).

Alternate Proof: Every analytic function is determined by its zeros and its value at one nonzero point. Since $ \sin(\pi x)$ is zero on all the integers except 0, and since sinc$ (0)=1$, it therefore coincides with the Lagrangian basis polynomial for $ N=\infty$ and $ k=0$.



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``Bandlimited Interpolation, Fractional Delay Filtering, and Optimal FIR Filter Design'', by Julius O. Smith III, (From Lecture Overheads, Music 420).
Copyright © 2008-02-05 by Julius O. Smith III
Center for Computer Research in Music and Acoustics (CCRMA),   Stanford University
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