# Homework Help: Hard fourier transform

1. Oct 18, 2011

### saxen

1. The problem statement, all variables and given/known data
Using that the Fourier transform of e$^{|x|}$ is $\frac{2}{\xi^2+1}$. Compute the Fourier transform of $\frac{x}{(x^2+1)^2}$

2. Relevant equations

3. The attempt at a solution

My first thought was to try and rewrite the problem in a form I recognized, tried a couple of things but what I though was best was to write:

$\frac{d}{dx}$ $\frac{-1}{x^2+1}$

And transform that to e$^{i*\xi}$*f($\xi$). This was wrong. Very wrong actually.

Anyone have any hints for me?

thanks!

edit: Missed some things but should be right this time.

Last edited: Oct 18, 2011
2. Oct 18, 2011

### susskind_leon

http://en.wikipedia.org/wiki/Fourier_transform#Functional_relationships Is has nothing to do with convolution! Fourier transform turns DERIVATIVE into MULTIPLICATION with i times argument, so $i \xi$ in this case, so the result should be something like $i \xi e^{|x|}$ (i assume you are supposed to perform the inverse FT, right?

3. Oct 18, 2011

### saxen

Hello sussking_leon,

My bad, the * means multiplication and not convolution. How do you get the $i \xi$ infront of the exponent?

4. Oct 18, 2011

### susskind_leon

Check the wiki page, forumla 106 and 107 do the trick. So if FT{e^|x|} = f(v), then FT{x e^|x|} = i d/dv f(v). (Fourier transform non-unitary, angular frequency)

5. Oct 18, 2011

### saxen

Ah, I was thinking right at least. Its so stupid, we don't get any of these rules on our exam so without wiki, I would have never solved this exercise. Thanks for the help.