Regularizing a divergent integral with a test function or convergence factor

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Discussion Overview

The discussion revolves around the use of test functions or convergence factors to regularize divergent integrals, particularly in the context of quantum field theory. Participants explore the justification for integrating functions that are initially unintegrable and the implications of using these techniques.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the validity of integrating a product of an original unintegrable function and a test function or convergence factor, seeking clarification on why this is acceptable.
  • Another participant suggests that the justification for this practice depends on the context and mentions that there are theorems that may support it, although specifics are not provided.
  • A follow-up inquiry asks for references to the theorems mentioned and seeks clarification on the term "subsidiary integral in the middle of a much larger integration."
  • A participant explains that in quantum field theory, long integrals can be rearranged to isolate simpler integrals, which may involve distributions that do not converge on their own but are manageable within the larger context.
  • The discussion touches on the concept of oscillating functions and how they can be "damped down" by other components of the integral, although the exact nature of this damping is not fully elaborated.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the justification for using test functions and theorems related to this practice. There is no consensus on the specifics of the theorems or the conditions under which the integration is valid.

Contextual Notes

Participants note the lack of references in the paper being discussed, which may limit the ability to verify claims about theorems and their applicability. The discussion also highlights the complexity of integrals in quantum field theory and the potential for divergent behavior in certain functions.

graphicsRat
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I'm trying to understand a paper in which the authors use a number of test functions (are they the same as convergence factors) to make integrate unintegrable functions. Now here is my ignorant question: why is this acceptable? The product of the original function and the test function or convergence factor surely is an entirely new function. Why is the integral of this new function the same as the integral of the old function which I recall was unintegrable? In layperson terms, can we be sure that the integral of the new function is valid and or correct?

Thanks
 
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graphicsRat said:
… why is this acceptable?

In layperson terms, can we be sure that the integral of the new function is valid and or correct?

Hi graphicsRat! :smile:

It depends on the context …

usually this is done to a subsidiary integral in the middle of a much larger integration …

there are theorems that tell you whether it's justified or not …

doesn't the paper give any references?
 
tiny-tim said:
It depends on the context …

usually this is done to a subsidiary integral in the middle of a much larger integration …

there are theorems that tell you whether it's justified or not …

doesn't the paper give any references?

Thanks Tim.

No the paper didn't give any references. Which theorems are you referring to? I'd like to look them up. What do you mean by a "subsidiary integral in the middle of a much larger integration"?

I'm sorry to ask so many annoying questions.
 
graphicsRat said:
Which theorems are you referring to?

dunno … i wasn't really concentrating when they went over that boring stuff :redface:
What do you mean by a "subsidiary integral in the middle of a much larger integration"?

in quantum field theory, there are really long integrals, and you can rearrange the order of the variables so as to get some easy integrals in the middle :smile:

(btw, this rearranging involves combinatorial additions of integrals, and Feynman diagrams are what keep track of the different combinations)

but those easy integrals are of functions (technically called "distributions", such as the Dirac delta "function" … see http://en.wikipedia.org/wiki/Distribution_(mathematics)) which don't actually converge on their own, because they oscillate, but are ok in the larger integral because the oscillations are "damped down" by the rest of the long integral
 

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