Dirac delta function as the limit of a seqquence

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Discussion Overview

The discussion centers around the conditions under which a sequence of functions can be considered to converge to the Dirac delta function, particularly in the context of limits and integrals. Participants explore the necessary criteria for such convergence, including properties of the sequence and the implications of convergence in the sense of distributions.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions whether a sequence that tends to infinity at x=0 and zero elsewhere can be identified with the Dirac delta function.
  • Another participant refines the question by adding that if the integral of the sequence over the entire real line equals one, can it then be identified with the Dirac delta function, or are the initial conditions sufficient?
  • A participant introduces the idea that a sequence of positive functions converging to the Dirac delta may not behave uniformly, citing that multiplying the sequence by 2 does not lead to convergence to the delta function.
  • Another participant points out that the limit of a sequence converging to zero at every point does not necessarily imply convergence to zero in the sense of distributions, suggesting that a sequence could converge to a delta function located at a different point.

Areas of Agreement / Disagreement

Participants express differing views on the necessary conditions for convergence to the Dirac delta function, indicating that multiple competing perspectives remain unresolved.

Contextual Notes

Participants highlight limitations regarding the assumptions required for convergence, the dependence on definitions of convergence in distributions, and the implications of specific sequences on the identification with the Dirac delta function.

krishna mohan
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Dirac delta function as the limit of a sequence

Hi..

If I have a sequence which in some limit tends to infinity for x=0 and goes to zero for x[tex]\neq[/tex]0, then can I call the limit as a dirac delta function?

If not, what are the additional constraints to be satisfied?
 
Last edited:
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Let me refine that..

If the integral of a member of the sequence over the whole real line is one, along with the conditions stated in previous post, can the limit of the sequence be identified with a dirac delta?

Or are the conditions of the first post enough??
 
Let fn be any sequence of positive functions that converges (in the sense of distributions) to the dirac delta.

Then 2fn does not converge to the dirac delta.

Also, I believe [itex]\sqrt{f_n}[/itex] converges to 0.
 
Also,
[tex]\lim_{n \rightarrow +\infty} g_n(x) = 0[/tex]​
does not imply
[tex]\lim_{n \rightarrow +\infty} g_n = 0[/tex]​
(the second limit in the sense of distributions)

In fact, I'm pretty sure you can find a sequence that satisfies the first equation whose limit is [itex]\delta(x-1)[/itex].
 

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