How to Prove \(\lim_{y\rightarrow0}\frac{y}{x^2+y^2}=\pi\delta(x)\)?

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Discussion Overview

The discussion revolves around the mathematical proof of the limit \(\lim_{y\rightarrow0}\frac{y}{x^2+y^2}=\pi\delta(x)\), exploring methods to demonstrate this relationship and the implications of the Dirac Delta function in this context.

Discussion Character

  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant suggests integrating \(\frac{y}{x^2+y^2}\) with respect to \(x\) over an interval containing 0, then taking the limit as \(y\) approaches 0 to see if the result approaches \(\pi\).
  • Another participant questions the validity of this approach, suggesting that it may be too informal and proposes a more systematic method for proving related expressions, such as \(\frac{1}{x+i\eta}=P\frac{1}{x}-i\pi\delta(x)\).
  • A different participant defends the original method, asserting that the limit behavior is essential to justify the use of Dirac Delta formalism in this context.

Areas of Agreement / Disagreement

Participants express differing views on the adequacy of the proposed proof methods, with some advocating for a more rigorous approach while others defend the current reasoning as sufficient. The discussion remains unresolved regarding the best method to prove the limit.

Contextual Notes

There are concerns about the rigor of the proposed methods and the assumptions underlying the use of Dirac Delta functions, which may not be fully addressed in the discussion.

daudaudaudau
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What is the way to show that

[tex] \lim_{y\rightarrow0}\frac{y}{x^2+y^2}=\pi\delta(x)[/tex]
?
 
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Integrate y/(x2 + y2) with respect to x over an interval containing 0. You will have a function of y. Let y ->0, and see if you get π. Further integrate over an arbitrary interval not including 0, then the limit should be 0.
 
But isn't that a bit hand-waving? What if you have something a little harder, like showing that
[tex] \frac{1}{x+i\eta}=P\frac{1}{x}-i\pi\delta(x)[/tex]
? Don't you need a more systematic way of doing it?
 
No, it is no hand-waving involved.

It is precisely the limit behaviour mathman points to that you need to prove is present, in order to legitimize the introduction of the Dirac Delta-formalism.
 

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