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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: [mg56262] Re: (x-y) DiracDelta[x-y] does not simplify to 0
  • From: "Dr. Wolfgang Hintze \(SBS\)" <wolfgang.hintze at siemens.com>
  • Date: Wed, 20 Apr 2005 05:32:06 -0400 (EDT)
  • References: <d42kg5$39t$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

I was even more surprised by this:

In[3]:= Simplify[(x - y)*DiracDelta[x - y], {x, y}]
Out[3]=0

As I don't know what {x,y} means here.
I found it by playing around with the more meaningful {x>y},{x<y},{x==y}
Wolfgang

"Alain Cochard" <alain at geophysik.uni-muenchen.de> wrote in message
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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