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KroneckerDelta in sum


Hi all,

I guess I have a rather simple problem. If I write
Sum[a[i, j]KroneckerDelta[i - j], {i, -Infinity, Infinity}]

Mathematica answers
a[j, j]

Now, if I use a two-fold sum
Sum[a[i, j]KroneckerDelta[i - j], {i, -Infinity, Infinity}, {j, -Infinity, 
    Infinity}]

the expression is not simplified.
How can I force Mathematica to give results like
Sum[a[j,j],{j,-Infinity, Infinity}]

Thks,
Matthias Reichelt

+--------------------------------------------------+
| Matthias Reichelt     |                          |
| Department of Physics | phone: +49-6421-28-24217 |
| Philipps-University   |                          |
| Renthof 5             | fax:   +49-6421-28-27076 |
| 35032 Marburg         |                          |
| Germany               |                          |
+--------------------------------------------------+



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