Does the Convolution Theorem Hold for Fourier Transforms Using Omega?

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Discussion Overview

The discussion centers around the validity of the convolution theorem for Fourier transforms when using different definitions, specifically comparing the use of ordinary frequency and angular frequency. Participants explore whether the theorem holds under these varying definitions, with references to sources that support differing views.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions if the convolution theorem holds when using the definition of the Fourier transform that incorporates angular frequency (omega) instead of ordinary frequency (xi).
  • Another participant asserts that the convolution theorem is valid for all variants of the Fourier transform and emphasizes that the inclusion of factors like 2π and normalization constants is arbitrary as long as consistency is maintained.
  • A later reply references Wikipedia, indicating that while the convolution theorem holds for angular frequency, a specific normalization adjustment (multiplying by √2π) is necessary for it to apply correctly.

Areas of Agreement / Disagreement

Participants express differing views on the applicability of the convolution theorem based on the definition used. Some assert it holds universally, while others highlight specific conditions under which it may not apply.

Contextual Notes

There are unresolved aspects regarding the normalization constants and their impact on the validity of the convolution theorem across different definitions of the Fourier transform.

dimension10
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I have a question on the Convolution theorem for Fourier Transforms. The convolution theorem states that

[tex]\mathscr{F}\{f(t) g(t)\}=\mathcal{F}\{f(t)g(t)\}=\mathcal{F}\{f(t)\}\ast \mathcal{F}\{g(t)\}-\mathscr{F}\{f(t)\} \ast \mathscr{F}\{g(t)\}[/tex]

[tex]\mathscr{F}\{f(t) \ast g(t)\}=\mathcal{F}\{f(t)\ast g(t)\}=\mathcal{F}\{f(t)\}\mathcal{F}\{g(t)\}= \mathscr{F}\{f(t)\}\mathscr{F}\{g(t)\}[/tex]

and the same for the Laplace Transform.

However, my sources use the definition of the Fourier Transform as

[tex]\mathscr{F}\{f(t)\}=\mathcal{F}\{f(t)\}=\int_{-\infty}^{\infty}{e}^{-2i\pi\xi t}f(t) \mbox{d}t[/tex]

But my question is that if I were to use the definition,[tex]\mathscr{F}\{f(t)\} = \mathcal{F}\{f(t)\}=\frac{1}{ \sqrt{2\pi} }\int_{-\infty}^{\infty}{e}^{-i\omega t}f(t) \mbox{d}t[/tex]

then will the convolution theorem for Fourier transforms still hold? There is one article that states that it does hold. However, when I checked Wikipedia, they say that the convolution theorem for Fourier transforms does not hold for the definition which uses angular frequency instead of ordinary frequency.

So, does the Convolution theorem for Fourier Transforms still hold for the definition involving omega instead of xi.
 
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Hi dimension10! :smile:

Yes, the convolution theorem holds for all variants of the Fourier transform and the Laplace transform.

It is an arbitrary choice whether you include 2pi in the exponent or not.
It is also an arbitrary choice where you put the normalization constant, which can be in the Fourier transform, the Inverse Fourier transform, or in both.
As long as you're consistent of course.
 
I like Serena said:
Hi dimension10! :smile:

Yes, the convolution theorem holds for all variants of the Fourier transform and the Laplace transform.

It is an arbitrary choice whether you include 2pi in the exponent or not.
It is also an arbitrary choice where you put the normalization constant, which can be in the Fourier transform, the Inverse Fourier transform, or in both.
As long as you're consistent of course.

Ok thanks a lot.
 
I was curious where you may have found that the convolution theory would not hold, so I tried to look it up in wikipedia.

What I found is this:
http://en.wikipedia.org/wiki/Fourier_transform#Functional_relationships
In the table, you can see that the convolution theorem holds, but in the case of angular frequency with a normalization constant of [itex]1 \over\sqrt{2\pi}[/itex] in the Fourier transform, you also need to multiply by [itex]\sqrt{2\pi}[/itex] for the convolution theorem to hold.
 
I like Serena said:
I was curious where you may have found that the convolution theory would not hold, so I tried to look it up in wikipedia.

What I found is this:
http://en.wikipedia.org/wiki/Fourier_transform#Functional_relationships
In the table, you can see that the convolution theorem holds, but in the case of angular frequency with a normalization constant of [itex]1 \over\sqrt{2\pi}[/itex] in the Fourier transform, you also need to multiply by [itex]\sqrt{2\pi}[/itex] for the convolution theorem to hold.

Thanks.
 

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