       Re: Simplifying with KroneckerDelta

• To: mathgroup at smc.vnet.net
• Subject: [mg101447] Re: [mg101426] Simplifying with KroneckerDelta
• From: "David Park" <djmpark at comcast.net>
• Date: Tue, 7 Jul 2009 07:50:35 -0400 (EDT)
• References: <1551538.1246958483039.JavaMail.root@n11>

```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/

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.

Francisco
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