Talk:Parseval's theorem

Proof

Can anybody put a proof of the theorem up?

problem solved in Good Will Hunting?

What was solved by the main character in Good Will Hunting? No problems are mentioned up to this point in the text. Cgibbard 16:40, 26 February 2006 (UTC)

reason for edit

Lest the reader be misled, it should be noted that the article is not a math article. Clearly mathematics is not the context here. the discussion is non-mathematical. there are no real mathematical reference listed. article should be expanded or title should be changed to "...in physics and engineering." Mct mht 14:17, 17 May 2006 (UTC)

sum of inverse squares

There's a very simple proof using this theorem, that

$\sum _{k=1}^{\infty }{1 \over k^{2}}={\pi ^{2} \over 6}$

Anyone know it? It might be cool to add it to the article. Phr (talk) 09:20, 2 August 2006 (UTC)

This actually just showed up on my homework - it consists of finding the Fourier series for the period 2pi extension of the function defined as f(x) = (pi - x)^2 on [0,2pi]. The value of this series for x = 0 gives the summation of that series; Parseval's Theorem gives the sum of 1/k^4. It's a bit computational, though; might be worth mentioning briefly how to do it but without details. Moocowpong1 (talk) 03:57, 11 March 2008 (UTC)

"1/2*pi" in section "Applications"

It looks to me like there is a 1/2*pi missing from the first equation after the heading "Applications," also. Can someone with a bit of knowledge check that, please? —The preceding unsigned comment was added by 4.242.147.1 (talk • contribs) 06:50, 24 October 2006 (UTC)

The integeral variable used in this equation is f, not ω, where

$\omega =2\pi f$

so 1/(2*pi) could be omitted. — Preceding unsigned comment added by Dongliangseu (talk • contribs) 08:00, 23 December 2010 (UTC)

"Discrete time Parseval's Theorem for a Periodic Function"

The discrete time Parseval's Theorem for a periodic function should have the 1/N term in the discrete time domain instead of in the frequency domain. I don't have a textbook to confirm this so I did not edit it in the article.

Here is the original (current) equation:

$\sum _{n=0}^{N-1}|x[n]|^{2}={\frac {1}{N}}\sum _{k=0}^{N-1}|X[k]|^{2}$

Here is what I beleive is correct:

${\frac {1}{N}}\sum _{n=0}^{N-1}|x[n]|^{2}=\sum _{k=0}^{N-1}|X[k]|^{2}$

Any agreements/oppositions? --Gmoose1 (talk) 02:53, 26 November 2007 (UTC)

I believe it simply depends on the definition of the transform that was used to go from x[n] to X[k]. The 1/N can be placed in either the forward or the inverse transform - or often 1/sqrt(N) is placed in both. It may be worth clarifying which transform is used. daviddoria (talk) 18:08, 13 September 2008 (UTC)

I agree with Gmoose1, the 1/N goes to the left side, as the total energy of a stationary random function is its _variance_ wich is defined like

${\frac {1}{N}}\sum _{n=0}^{N-1}|x[n]|^{2}$

See e.g.Uriel Frisch, Turbulence, Cambridge 1995 Page 53 —Preceding unsigned comment added by 130.183.84.206 (talk) 17:52, 18 February 2010 (UTC)

I think the comment of Daviddoria is resonable. If the fourier transform is defined by the following formula, which is the common case,

$X[k]=\sum _{n=0}^{N-1}x[n]exp({\frac {-j2\pi kn}{N}})$

then

$\sum _{n=0}^{N-1}|x[n]|^{2}={\frac {1}{N}}\sum _{k=0}^{N-1}|X[k]|^{2}$

However, if

$X[k]={\frac {1}{N}}\sum _{n=0}^{N-1}x[n]exp({\frac {-j2\pi kn}{N}})$

then,

${\frac {1}{N}}\sum _{n=0}^{N-1}|x[n]|^{2}=\sum _{k=0}^{N-1}|X[k]|^{2}$

There is also another case, that is, if

$X[k]={\frac {1}{\sqrt {N}}}\sum _{n=0}^{N-1}x[n]exp({\frac {-j2\pi kn}{N}})$

then,

$\sum _{n=0}^{N-1}|x[n]|^{2}=\sum _{k=0}^{N-1}|X[k]|^{2}$

— Preceding unsigned comment added by Dongliangseu (talk • contribs) 07:50, 23 December 2010 (UTC)

The current formula is inconsistent with the common definition of DFT on Wikipedia's own DFT page. This should be changed back to the original,

$\sum _{n=0}^{N-1}|x[n]|^{2}={\frac {1}{N}}\sum _{k=0}^{N-1}|X[k]|^{2}$

which is the common form, as Dongliangseu pointed out.

Parseval's Power Theorem is not the same as Rayleigh's Energy Theorem

In the intro paragraph, it says "this is also known as Rayleigh's Energy Theorem". I don't believe this is the case, because Parseval's theorem is for periodic signals (power signals) and Rayleigh's theorem is for energy signals (usually finite duration - have 0 power). Can someone confirm this and then I will fix it? daviddoria (talk) 18:08, 13 September 2008 (UTC)

Parseval's theorem is not just for periodic functions, but it also applies to non-periodic functions. Thenub314 (talk) 19:37, 13 September 2008 (UTC)

I still think there is a difference though.. Isn't Parseval's Power Theorem a sum of the Fourier series coefficients = the power in the signal? And Rayleigh's Energy Theorem that the integral of the signal (energy) = the energy of the spectrum? —Preceding unsigned comment added by Daviddoria (talk • contribs) 16:17, 14 September 2008 (UTC)

I am not sure, other then this article I had not heard of "Rayleigh's Energy Theorem" but when I google the term everything I find seems to simply be Parseval's theorem. Thenub314 (talk) 11:33, 15 September 2008 (UTC)

Link from "Parsevals Theorem

What is the usual procedure on this? If someone searches "Parsevals Theorem" (without an apostrophe) I'd say this page should come up right away, without having to look through the search results to find "Parseval's Theorem". Can I create the article "Parsevals Theorem" and redirect it to here? What is usually done about this? daviddoria (talk) 18:08, 13 September 2008 (UTC)

Yes; it's pretty obvious that this will only help people without confusing any. I've just done exactly this BTW. C xong (talk) 01:39, 10 August 2010 (UTC)

Unnecessary clarification

In the statement of the theorem in the article, it's written

where i is the imaginary unit and horizontal bars indicate complex conjugation.

Aren't both these notations extremely well established, rendering the clarification a bit superfluous? ✎ HannesP · talk 22:30, 8 April 2012 (UTC)

This article is way too obscure

This article makes Parseval's theorem needlessly difficult. It badly needs clarification for people who are not mathematicians.

In almost any book on Fourier series that one looks at, Parseval's theorem is as follows: Suppose $f(x)$ is a square-integrable function over $(-\pi ,\pi )$ (i.e., $f(x)$ and $f^{2}(x)$ are integrable on that interval), with the Fourier series

$f(x)\simeq {\frac {a_{0}}{2}}+\sum _{n=1}^{\infty }(a_{n}\cos(nx)+b_{n}\sin(nx))$.

Then

${\frac {1}{\pi }}\int _{-\pi }^{\pi }f^{2}(x)dx={\frac {a_{0}^{2}}{2}}+\sum _{n=1}^{\infty }(a_{n}^{2}+b_{n}^{2})$.

Is there any objection to adding the above statement to the article?

MathPerson (talk) 20:54, 17 July 2017 (UTC)

Seeing no objection, I added it, with references. MathPerson (talk) 00:22, 29 July 2017 (UTC)