- #1
rayman123
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Homework Statement
Compute the integral
[tex] \int_{-\infty}^{\infty}\frac{\sin(a\xi)}{(1+i\xi)^3\xi}d \xi[/tex]
by using Parseval's formula for Fourier transform
[tex] <\overbrace{f}^{\wedge},\overbrace{g}^{\wedge}>=2 \pi<f,g>[/tex]
where [tex] \wedge[/tex] means the Fourier transform of a function
The Attempt at a Solution
Using Parseval's formula we can rewrite the integral as
[tex] \int_{-\infty}^{\infty}\frac{\sin(a\xi)}{(1+i\xi)^3\xi}d \xi = \Bigl<\overbrace{\frac{\sin(a\xi)}{\xi}}^{\mathcal{F}},\overbrace{\frac{1}{(1+i\xi)^3}}^{\mathcal{F}}\Bigr>[/tex]
with their inversions as follows
[tex] \Bigl(\frac{\sin(a \xi)}{\xi}\Bigr)^{\wedge \wedge }=\frac{1}{2} \chi_{[-a,a]}(x)[/tex] by the table 2 in Folland p.223
and the other one
[tex] \Bigl(\frac{2}{(i \xi+1)^3}\Bigr)^{\wedge \wedge}=\frac{1}{2}x^2e^{-x}[/tex]
which I calculated myslef as follows
we let [tex] g(x)= e^{-x}[/tex] and then the Fourier transform of g(x) is [tex]\mathcal{F}[g(x)]= \frac{1}{1+i \xi}[/tex]
then using formula in Folland again table 2 [tex] \mathcal{F}[xf(x)]=i(\overbrace{f}^{\wedge})^{'})(\xi)[/tex]
and applying it to our function [tex] x^2e^{-x}[/tex]
we get
[tex] \mathcal{F}[x^2e^{-x}]=i(\overbrace{f}^{\wedge})^{''})(\xi)=\frac{2}{(1+i \xi)^3}[/tex]
Homework Statement
going back to the integral and plugging in everything in the Parseval's formula gives me smth like this
[tex] \int_{-\infty}^{\infty}\frac{\sin(a\xi)}{(1+i\xi)^3\xi}d \xi = \Bigl<\overbrace{\frac{\sin(a\xi)}{\xi}}^{\mathcal{F}},\overbrace{\frac{1}{(1+i\xi)^3}}^{\mathcal{F}}\Bigr>=2 \pi \Bigl<\frac{1}{2} \chi_{[-a,a]}, \frac{1}{2}x^2e^{-x}\Bigr>[/tex]
in the solution given by our teacher we find
[tex] \frac{1}{4}<f^{\wedge}, g^{\wedge}>= \frac{\pi}{2}<f,g>[/tex]
how did he get this [tex]\frac{1}{4}[/tex] on the left hand side and then [tex] \frac{\pi}{2}[/tex] on the right hand side??