       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.

~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:= FullSimplify[x DiracDelta[x]]
>
>     Out= 0
>
>
>     In:= FullSimplify[(x-y) DiracDelta[x-y]]
>
>     Out= (x - y) DiracDelta[x - y]
>
> whereas:
>
>     In:= FullSimplify[(x-y) DiracDelta[x-y]/.x-y->z]
>
>     Out= 0
>
> Is it simply that Mathematica is a little weak on Out, or does it have a
> good reason for not simplifying, i.e., am I missing something at the
> mathematics level?
>