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 >