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anonymous
 one year ago
fourier transform question
first question please
as soon as possible.
anonymous
 one year ago
fourier transform question first question please as soon as possible.

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anonymous
 one year ago
Best ResponseYou've already chosen the best response.0Find the fourier Transform of \[e^{a^{2}x^{2}} ,a>0\]Given \[\int\limits_{\infty}^{\infty} e^{t} dt = \sqrt{\pi}\]

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0I think you meant \(e^{\color{red}{t^2}}\) in that last integral?

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0\[\large\begin{align*}\mathcal{F}\{e^{a^2x^2}\}&=\frac{1}{2\pi}\int_{\infty}^\infty e^{a^2x^2}e^{i\xi x}\,dx\\[2ex] &=\frac{1}{2\pi}\int_{\infty}^\infty e^{a^2\left(x^2+\frac{i\xi}{a^2}x\right)}\,dx \end{align*}\] Complete the square: \[\begin{align*}a^2\left(x^2+\frac{i\xi}{a^2}x\right)&=a^2\left(x^2+\frac{i\xi}{a^2}x+\left(\frac{i\xi}{2a^2}\right)^2\left(\frac{i\xi}{2a^2}\right)^2\right)\\[2ex] &=a^2\left(\left(x+\frac{i\xi}{2a^2}\right)^2+\frac{\xi^2}{4a^4}\right)\\[2ex] &=\left(ax+\frac{i\xi}{2a}\right)^2\frac{\xi^2}{4a^2}\end{align*}\] A substitution will do the rest: \(y=ax+\dfrac{i\xi}{2a}\) with \(dy=a\,dx\).\[\large\begin{align*}\mathcal{F}\{e^{a^2x^2}\}&=\frac{1}{2a\pi}\int_{\infty}^\infty e^{y^2\frac{\xi^2}{4a^2}}\,dy \end{align*}\]

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0yes it was \[e^{t^{2}}\] isn't it \[\frac{ 1 }{ \sqrt{2\pi} }\] instead of \[\frac{ 1 }{ {2\pi} }\] otherwise thank you very much... it had been bugging me for days...

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0I've seen a few variations of the transform's definition, things like \(\dfrac{1}{2\pi}\int e^{i\xi x}\,dx\) and \(\int e^{2\pi i\xi x}\,dx\) (which are identical) as well as \(\dfrac{1}{\sqrt{2\pi}}\) in place of \(\dfrac{1}{2\pi}\). (see here: http://mathworld.wolfram.com/FourierTransform.html ) I'm not sure myself about the details of the advantage to using one definition over another, but I think it has something to do with symmetry. As long as you're consistent, either way is fine.

anonymous
 one year ago
Best ResponseYou've already chosen the best response.0cool..... thanks again
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