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Re: FFT in Mathematica

• To: mathgroup at smc.vnet.net
• Subject: [mg93670] Re: FFT in Mathematica
• From: "Nasser Abbasi" <nma at 12000.org>
• Date: Fri, 21 Nov 2008 05:33:17 -0500 (EST)
• References: <gg3c81$k40$1@smc.vnet.net>
• Reply-to: "Nasser Abbasi" <nma at 12000.org>

"Oliver" <sch_oliver2000 at yahoo.de> wrote in message 
news:gg3c81$k40$1 at smc.vnet.net...
> Hallo,
>
> i wanted to plot the following function:
>
> s(t) = (1/(4*(Pi*1*t)^(3/2))) * Exp[-(x^2 + y^2 + z^2)/(4*1*t)]
> when x=1, y=1, z=1
> so i wrote:
>
> eq = (1/(4*(Pi*1*t)^(3/2))) * Exp[-(x^2 + y^2 + z^2)/(4*1*t)]
>
> Plot[Evaluate[eq /. {x -> 1, y -> 1, z -> 1}], {t, 0, 3}]
>
> My Question is, how can i plot s(f), which is the function depending on 
> the frequency. In other word, how to find the FFT of s(t) in Mathematica?
> thanks.
>

Your function s(t) is continues. FFT takes sequence of numbers (samples). In 
Mathematica use Fourier (which is DFT (Discrete Fourier Transform). Note FFT 
is the name of an implementation (algorithm) of DFT.

You do not plot the spectrum 's(f)', you plot its magnitude and phase. (it 
is complex numbers).

Fyi, in version 7, WRI has added many more functions to support spectrum 
analysis. Check
http://www.wolfram.com/products/mathematica/newin7/content/EnhancedFourierAnalysis/

Nasser