# Finding an integer with matlab fft function

• MATLAB

## Main Question or Discussion Point

Hi, I hope someone could help me, I have been trying to solve this problem with FFT in matlab, why?, because my teacher gave it as homework. The problem is the following.

Obtain the value of the following integer using FFT:

the integer goes from [-infinte, infinite], and the function is x^2*exp(-x^2)*dx

He told us to check the definition of the fourier transform and I checked in books, matlab everywhere and I cant find any info that could help me, I hope someone here can, THANK YOU.

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chroot
Staff Emeritus
Gold Member
I have no idea what you mean by "obtain the value of the following integer."

Can you please post the problem exactly as it was given to you?

- Warren

I have no idea what you mean by "obtain the value of the following integer."

Can you please post the problem exactly as it was given to you?

- Warren
Sure, and thanks for your help

Solve the following integer using the FFT in MATLAB:

integer goes from[-infinite, infinite] and the function is:
x^2*exp(-x^2)dx

TIP: Check the fourier transform definition. (the tip is from my teacher).

By the way I tried to translate the problem the best I can, because even in spanish what my teacher writes is difficult to understand or to find the answer he is looking for by the problem he gave us.

Dr Transport
Gold Member
is he asking for the integral??????

Yes, It should be intergral because of the dx at the end of the fuction.

hi, yes the integral im sorry.

He's asking how you would write a program to find the integral using the Fast Fourier Transform (FFT). I don't know how to do it, but I thought I should clear that up for other people.

chroot
Staff Emeritus
Gold Member
Seems like you should be able to evaluate this integral by using integration by parts. I don't understand what MATLAB has to do with it.

- Warren

Actually this is the solution my teacher gave us. He said that this is a cool way to calculate an integral, I still don't get it, hope this info helps someone, and thank you all foru your help and good intentions.

>> N=1000;
x=linspace(-10,10,N);
dx=x(2)-x(1);
y=x.^2.*exp(-x.^2);
plot(x,y);
sum( y)*dx;
mm=(fft( y));
mm(1)*dx;
>>