Mathematica 9 is now available
Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2005
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2005

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Re: (x-y) DiracDelta[x-y] does not simplify to 0

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56361] Re: [mg56297] Re: (x-y) DiracDelta[x-y] does not simplify to 0
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Fri, 22 Apr 2005 06:24:56 -0400 (EDT)
  • References: <d42kg5$39t$1@smc.vnet.net> <d45agf$ieu$1@smc.vnet.net> <200504210936.FAA05048@smc.vnet.net> <0e5459acc0a6eae9a16bda863b79434c@mimuw.edu.pl> <16999.49105.639076.187149@localhost.localdomain> <913e334e36e5b09dce4f89131131357d@mimuw.edu.pl> <17000.4781.157008.559505@localhost.localdomain>
  • Sender: owner-wri-mathgroup at wolfram.com

On 22 Apr 2005, at 05:53, Alain Cochard wrote:

> *This message was transferred with a trial version of CommuniGate(tm) 
> Pro*
> Andrzej Kozlowski writes:
>
>> Since [use inside Integrate] is the only context in which [x
>> DiracDelta[x] == 0] is useful and makes sense, I can't see any
>> justification for your application of Simplify.
>
> OK, I think I now understand what you say and I disagree with your
> conclusion.  I indeed do find practical justifications for the use of
> that identity and had so far to perform the simplifications manually.
>
>

Of course I did not mean to doubt that this identity or this particular 
way of treating the DiracDelta can't be useful for you or others. There 
is no end to unexpected and ingenious uses that people make of various 
built in functions in Mathematica. However, unless you actually 
integrate your expressions containing the DiracDelta I do not think you 
are really making use of the mathematical notion of a distribution. For 
example, I could define a function MyDiscreteDelta by

MyDiscreteDelta[x_] := DiracDelta[x]/DiracDelta[0]


and then I would have


FullSimplify[x*MyDiscreteDelta[x]]

0

(Note that, as i pointed out earlier, the 
FullSimplify[x*DiscreteDelta[x]] does not return 0). So one could argue 
that I "used" the DiracDelta to get a "superior" version of the the 
DiscreteDelta, and this may be further useful for something else. Even 
if this were the case it would still be true that the mathematical 
notion of a distribution is not being used here at all because that 
notion is only meaningful when used in th context of integration.

Andrzej



  • Prev by Date: Re: a conflicting StringReplace
  • Next by Date: Re: removing sublist . Again and Different
  • Previous by thread: Re: Re: (x-y) DiracDelta[x-y] does not simplify to 0
  • Next by thread: Re: Re: Re: (x-y) DiracDelta[x-y] does not simplify to 0