How can I solve the problem with Fourier transforms in critical phenomena?

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Discussion Overview

The discussion revolves around the application of Fourier transforms in the context of critical phenomena, specifically addressing the transition between equations 7.37 and 7.38. Participants are exploring the mathematical intricacies involved in this transformation and the implications of convolution in Fourier space.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant expresses difficulty in understanding how to transition from equation 7.37 to 7.38 using Fourier transforms.
  • Another participant suggests multiplying equation 7.37 by the denominator of the right-hand side (RHS) and applying the convolution theorem, indicating that a product in Fourier space becomes a convolution in direct space.
  • A participant questions whether the Fourier transform of C (with hat) is simply C without the hat, expressing confusion about the arguments of the functions in equation 7.38.
  • One participant clarifies that the difference in variables (r1-r2) is valid in a homogeneous system, where translation invariant quantities depend on their difference rather than individual positions.
  • Another participant emphasizes the need to understand that the integral represents a convolution.
  • A participant mentions that despite using the convolution theorem, their results differ from the author's, particularly in the arguments of the functions, which adds to their confusion.
  • A suggestion is made to rename variables to clarify the relationships in the equations.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the application of the convolution theorem and the interpretation of function arguments. There is no consensus on the resolution of the confusion surrounding the differences in arguments of functions under the integral versus those outside of it.

Contextual Notes

Participants reference earlier discussions in the text regarding the properties of Fourier transforms and convolution, but the specific assumptions and definitions that lead to their confusion remain unresolved.

enrikofermi
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Hi all.
I'm learning something about critical phenomena and I have one problem.

I'm bad with Fourier transforms so I don't know how from 7.37 we have 7.38.
I have tryed everything I knew, but fruitless. I have attached picture of my problem.

Does anybody has any idea how I can solve this?
 

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Multiply 7.37 with the denominator of the RHS and then Fourier transform using that a product in Fourier space becomes a convolution in direct space (and vice versa), i.e. the integral on the RHS of eq. 7.38.

https://en.wikipedia.org/wiki/Convolution_theorem
 
Thanks!
Is the Fourier transform of C (with hat) just C without hat, taking into account deffinition of C?
What make me confused are the arguments of functions in 7.38. Why r-r', and so on...?
 
Anybody? To help with this manybody? :)
 
That C and ##\hat{C}## are Fourier transforms of each other is clearly stated in the article.
r1-r2 is as valid a variable as e.g. x or y. In a homogeneous system, translation invariant quantities cannot depend on both r1 and r2 but via their difference r1-r2. I suppose this has been discussed in your text previously
 
Yes, I know, it was discussed earlier. But that what confused me was arguments of those functions under integral. Why are they different? And why they differ from those arguments of function outside of integral?
 
I think you first have to convince yourself that the integral represents a convolution.
 
I have used convolution theorem, but I didn't get exactly same solution as author. Look how I calculated that. Arguments of my functions are considerably different. And that is what confuses me...
 

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Rename your variables:
r->r-r'
And
r''->r''-r'
 
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Thank you very much Doc!
 

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