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Re: Question: DiracDelta simplifies/integrates incorrectly?
*To*: mathgroup at smc.vnet.net
*Subject*: [mg65152] Re: [mg65132] Question: DiracDelta simplifies/integrates incorrectly?
*From*: Daniel Lichtblau <danl at wolfram.com>
*Date*: Wed, 15 Mar 2006 23:59:26 -0500 (EST)
*References*: <200603151129.GAA23260@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
John Harker wrote:
> Hello,
>
> I have a question regarding the behavior of the DiracDelta function in
> Mathematica 5.2.
>
> The following two inputs produce the following results:
>
> In[61]:=
> Simplify[Pi*DiracDelta[Pi*x]]
>
> Out[61]=
> DiracDelta[x]
>
> In[60]:=
> Simplify[Pi*DiracDelta[Pi*(x-3)]]
>
> Out[60]=
> \[Pi] DiracDelta[\[Pi] (-3+x)]
>
> As you can see, although the Pi is correctly simplified out in the first
> case, it is not simplified in the second case. This is a problem because
> of the following result:
>
> In[68]:=
> Clear[f];
>
> In[69]:=
> Integrate[f[x]*Pi*DiracDelta[Pi*x],{x,-Infinity,Infinity}]
>
> Out[69]=
> f[0]
>
> In[70]:=
> Integrate[f[x]*Pi*DiracDelta[Pi*(x-3)],{x,-Infinity,Infinity}]
>
> Out[70]=
> 0
>
> As you can see, the output [69] is correct, but the output [70] should
> correctly be f[3], and instead it returns 0.
>
> Is there a flaw in my understanding of the DiracDelta function, or is this
> a bug?
>
> All of the above poses a problem because Mathematica will return results
> such as the following:
>
> In[78]:=
> Simplify[
> FourierTransform[Exp[I*2*Pi*3*x],x,f,
> FourierParameters\[Rule]{0,-2*\[Pi]}]
> ]
>
> Out[78]=
> \[Pi] DiracDelta[(-3+f) \[Pi]]
>
> So you see that just by asking for a simple Fourier transform, I can get
> an output result which Mathematica cannot integrate correctly.
>
> Does anyone have any ideas about a better way to perform this math
> in order to get around the problem? Or something illuminating about how
> the DiracDelta function works?
>
> Many thanks!
>
> John
Your second integral example
Integrate[f[x]*Pi*DiracDelta[Pi*(x-3)],{x,-Infinity,Infinity}]
should certainly return f[3], and the failure to do so is a bug. This
will be fixed in a future release (the next one).
Daniel Lichtblau
Wolfram Research
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