# Homework Help: Fourier series: Parseval's identity HELP!

1. Oct 13, 2006

### sarahisme

Hey all,

I am unsure how to do this problem... i find problems where i have to derive things quite difficult! :P

http://img143.imageshack.us/img143/744/picture2ao8.png [Broken]

this is the Full Fourier series i think and so the Fourier coeffiecients would be given by:

http://img144.imageshack.us/img144/8200/picture5yv4.png [Broken]

ok so first i need to take the inner product, so i did this:

http://img99.imageshack.us/img99/7193/picture4ol9.png [Broken]

but then i am stuck... anyone got an idea of how to proceed from here?

Cheers! :)

Sarah

Last edited by a moderator: May 2, 2017
2. Oct 13, 2006

### quasar987

$$\int_{-L}^{+L}|f(x)|^2dx = \int_{-L}^{+L} f(x) \left( \frac{A_0}{2} + \sum_{n\geq 1} A_n\cos (\frac{n\pi x}{L})+B_n\sin (\frac{n\pi x}{L}) \right)dx$$

does this inspire you more? (It should)

3. Oct 13, 2006

### sarahisme

it does actually, hold on.... i'll post my answer in a sec

4. Oct 13, 2006

### sarahisme

http://img136.imageshack.us/img136/8739/picture6tt5.png [Broken]

however i do have one question, are we allowed to substitute the integral inside the summation? (i am have never heard of any rules telling me whether this is or is not allowed....)

Last edited by a moderator: May 2, 2017
5. Oct 13, 2006

### quasar987

Haven't you covered series of functions (or at least sequences of functions) in an earlier class?

Last edited: Oct 13, 2006
6. Oct 13, 2006

### sarahisme

not for a couple of years i don't think...

thanks for the help by the way :)

7. Oct 13, 2006

### quasar987

Strange, since Fourier series are precisely series of functions. In occurence, sine and cosine. They are usually covered in a second analysis class together with the theory of Riemann integration.

8. Oct 13, 2006

### sarahisme

yeah, we did Riemann integration about a year and half ago i think, in a first analysis class i'm pretty sure.

9. Dec 2, 2006

### Swapnil

So when is it OK to move the integral inside the summation?

10. Dec 2, 2006

### quasar987

Thm: If the series of function $\sum_{n=1}^{\infty} f_n(x)$ converges uniformly towards S(x) on [a,b] and if $f_n$ is integrable on [a,b] $\forall n\in \mathbb{N}$, then the function S is integrable on [a,b] and

$$\sum_{n=1}^{\infty}\int_a^b f_n(x)dx=\int_a^b \sum_{n=1}^{\infty} f_n(x)dx$$

11. Mar 17, 2008

### Higgs_Boson

In this case it is surely ok to include the integral in the summation. Don't know the rule exactly but I remember my professor claiming that it's ok to do the above one.

I will try to find supporting evidence in due course!

Last edited by a moderator: May 3, 2017
12. May 6, 2011

### maxverywell

In this case, how do we check if the Fourier series converges uniformly to the f(x)?
Actually we don't know if the function f is continuous and C^1 in [-L,L] so I think that we can't "move the integral inside the summation". If it was saying, for example, that the function f is periodic with L=pi and f is square integrable than we could do this.

Last edited: May 6, 2011