for large , by the definition of . This proves that the th power of approaches the Gaussian function defined in §D.1 for large .
Since the inverse Fourier transform of a Gaussian is another Gaussian (§D.8), we can define a time-domain function as being ``sufficiently regular'' when its Fourier transform approaches in a sufficiently small neighborhood of . That is, the Fourier transform simply needs a ``sufficiently smooth peak'' at that can be expanded into a convergent Taylor series. This obviously holds for the DTFT of any discrete-time window function (the subject of Chapter 3), because the window transform is a finite sum of continuous cosines of the form in the zero-phase case, and complex exponentials in the causal case, each of which is differentiable any number of times in .
The last row of Pascal's triangle (the binomial distribution) approaches a sampled Gaussian function as the number of rows increases.D.3 Since Lagrange interpolation (elementary polynomial interpolation) is equal to binomially windowed sinc interpolation [301,134], it follows that Lagrange interpolation approaches Gaussian-windowed sinc interpolation at high orders.
Gaussian Probability Density Function
Fourier Transform of Complex Gaussian