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Re: plot dirac delta function? [message #49518 is a reply to message #49366]

Sun, 30 July 2006 07:18

James Kuyper
Messages: 425
Registered: March 2000

Senior Member

swingnut@gmail.com wrote:
> Ah, I see we're about to head off into the realm of defining functions,
> ever a popular discussion (not the least of which is because I relish
> the opportunity to learn more about it, not being a mathematician). Why
> do you say this? In plasma kinetic theory a while back, the prof give
> us the pop quiz about the value of particle distributions made from
> sums of delta functions. As with the "How do you put an elephant into
> the refrigerator?" test, everyone jumped to the wrong answer since the
> integral of the Dirac delta is 1. The function wasn't a sum of
> integrals, its a sum of deltas, so the value of the distribution at a
> particle's parameters in the phase space we were working in is
> infinity.

That's just a test question about a intrinsically meaningless
intermediate step in a calculation. The only thing you can ever
meaningfully do with such a particle distribution function is integrate
over it, one way or another. The value of such a function at a given
point is meaningless, except in those places where it's zero, in which
case it's meaningful only as a simplified approximation to reality.
It's value at any point where it's non-zero is not a measurable
quantity. Can you show me any actual calculations where the value of a
dirac delta function at 0 is actually used to calculate something
meaningful, without being integrated over?

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[Message index]

		plot dirac delta function? By: Nic on Wed, 19 July 2006 03:30
		Re: plot dirac delta function? By: James Kuyper on Sun, 23 July 2006 19:07
		Re: plot dirac delta function? By: swingnut on Sat, 29 July 2006 22:27
		Re: plot dirac delta function? By: swingnut on Sat, 29 July 2006 22:17
		Re: plot dirac delta function? By: James Kuyper on Sun, 30 July 2006 07:32
		Re: plot dirac delta function? By: Carsten Lechte on Fri, 11 August 2006 14:18
		Re: plot dirac delta function? By: James Kuyper on Sun, 30 July 2006 07:18
		Re: plot dirac delta function? By: James Kuyper on Fri, 11 August 2006 15:35
		Re: plot dirac delta function? By: priorbee on Fri, 25 November 2016 02:13

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