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MathGroup Archive 2005

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

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56299] Re: (x-y) DiracDelta[x-y] does not simplify to 0
  • From: "Scout" <user at domain.com>
  • Date: Thu, 21 Apr 2005 05:36:17 -0400 (EDT)
  • References: <d42kg5$39t$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Alain,
you can try this:

    FullSimplify[(x - y) DiracDelta[x - y], ExcludedForms -> {x - y}]

I think that the unvaluated answer is due to the use of the math. 
associative
law of the expression.
In fact, (a+b)*c would become a*c+b*c and so...
the ExcludedForms option forces Math. to don't use the associative law, in
this particular context.
I hope can help you.

~Scout~

"Alain Cochard" <alain at geophysik.uni-muenchen.de> ha scritto nel messaggio 
news:d42kg5$39t$1 at smc.vnet.net...
> Mathematica 4.0 for Linux
> Copyright 1988-1999 Wolfram Research, Inc.
> -- Motif graphics initialized -- 
>
> Considering that
>
>     In[1]:= FullSimplify[x DiracDelta[x]]
>
>     Out[1]= 0
>
> I was surprised about this one:
>
>     In[2]:= FullSimplify[(x-y) DiracDelta[x-y]]
>
>     Out[2]= (x - y) DiracDelta[x - y]
>
> whereas:
>
>     In[3]:= FullSimplify[(x-y) DiracDelta[x-y]/.x-y->z]
>
>     Out[3]= 0
>
> Is it simply that Mathematica is a little weak on Out[2], or does it have a
> good reason for not simplifying, i.e., am I missing something at the
> mathematics level?
>
> Thanks in advance,
> Alain
> 


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