Re: Simplifying with KroneckerDelta

• To: mathgroup at smc.vnet.net
• Subject: [mg101462] Re: [mg101426] Simplifying with KroneckerDelta
• From: Francisco Rojas Fernández <fjrojas at gmail.com>
• Date: Wed, 8 Jul 2009 07:08:43 -0400 (EDT)
• References: <1551538.1246958483039.JavaMail.root@n11>

```Thanks.
That helps, but maybe I should have been more specfic because I actually
have long and more involved expressions.
What would be really useful for me in this case would be a replacement rule
like this:

krule = f[b_] KroneckerDelta[a_, b_] -> f[a] KroneckerDelta[a, b]

where f[ ] is any function.
For example, if I had an expression like: Exp[a-b] KroneckerDelta[a,b] I
would like Mathematica to give me

Exp[0] KroneckerDelta[a,b]

which after simplifying of course it's just KroneckerDelta[a,b]

Is there anyway I can do this? I tried some ways but they didn't work (maybe
writing f[b_] is not the right way?)

-Francsico

On Tue, Jul 7, 2009 at 7:43 AM, David Park <djmpark at comcast.net> wrote:

> Why not use a rule?
>
> krule = b_ KroneckerDelta[a_, b_] -> a KroneckerDelta[a, b]
>
> (m + n) KroneckerDelta[k, m + n] /. Krule
> k KroneckerDelta[k, m + n]
>
> But the real work might be if you have a sum before the KroneckerDelta that
> only includes m + n as sub-terms. You could write a routine that collected
> on the KroneckerDelta forms, then wrapped Hold around the m+n sub-terms
> (both places), then applied the rule and released the Hold.
>
>
> David Park
> djmpark at comcast.net
> http://home.comcast.net/~djmpark/ <http://home.comcast.net/%7Edjmpark/>
>
>
> From: frojasf [mailto:fjrojas at gmail.com]
>
>
> Hello,
>
> Does anybody know how to tell mathematica to use the KroneckerDelta in
> order
> to simplify expressions? For example, it would be great if it could receive
> something like this:
>
> (m+n) KroneckerDelta[k,m+n]  and give
>
> k KroneckerDelta[k,m+n]
>
> Of course this is simple example, but I have an expression which is about
> 200 terms long, so if it could use the delta to simplify things by itself,
> that would be great.
>
> Thanks to all in advance,
> Francisco
> --
> View this message in context:
>
> http://www.nabble.com/Simplifying-with-KroneckerDelta-tp24362541p24362541.ht
> ml<http://www.nabble.com/Simplifying-with-KroneckerDelta-tp24362541p24362541.ht%0Aml>
> Sent from the MathGroup mailing list archive at Nabble.com.
>
>
>
>

```

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