% \def\updated{07mar04}%19jun97 \magnification=1200\input carlformat\def\cite#1{[#1]}\parskip10pt % % %Moments and Fourier transform of a B-spline % %
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Moments and Fourier transform of a B-spline %

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Since $$%
\dvd{t_0,\ldots,t_k}f = \int M(\cdot|t_0,\ldots,t_k) D^kf/k!, $$%
with $M$ the B-spline normalized to have integral 1, some integrals of the form $$%
\int M(\cdot|t_0,\ldots,t_k) g $$%
can (and certainly have been) computed from knowledge of divided differences, as long as it is easy to obtain $D^{-k}g$ and compute its divided difference. %

This is certainly the case for the power function, $$%
g=()^n:x\mapsto x^n, $$%
for which $$%
D^{-k}g = ()^{n+k} n!/(n+k)! $$%
while (see pagep101) %basic divided difference formulae $$%
\dvd{t_0,\ldots,t_k}()^{n+k} = \sum (t^\alpha: |\alpha|=n, \alpha \ge 0). $$%
Therefore, the $n$th moment of the B-spline is $$%
\int ()^n M(\cdot|t_0,\ldots,t_k) = \dvd{t_0,\ldots,t_k}()^{n+k}/{\scriptstyle{n+k\choose n}} = \sum (t^\alpha: |\alpha|=n, \alpha \ge 0)/{\scriptstyle{n+k\choose n}}\,. $$%
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In the same way, since $D^k \exp(-\ii(\cdot)\xi) = (-\ii \xi)^k\exp(-\ii(\cdot)\xi)$, the Fourier transform of the B-spline is $$%
\int e^{-\ii(\cdot)\xi}M(\cdot|t_0,\ldots,t_k) = (k!/(-\ii \xi)^k)\dvd{t_0,\ldots,t_k}e^{-\ii(\cdot)\xi}. $$%
An early reference (other than Schoenberg's use of this connection throughout his papers on B-splines, including the very first one, in '46), is %

%Neuman81b % larry %\refJ Neuman, E.; Moments and Fourier transforms of B-splines; J.Comput.Applied Math.; 7; 1981; 51--62; % % \bye