       Re: error with Sum and Infinity

• To: mathgroup at smc.vnet.net
• Subject: [mg102250] Re: [mg102245] error with Sum and Infinity
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Mon, 3 Aug 2009 05:44:19 -0400 (EDT)
• References: <200908021001.GAA01730@smc.vnet.net>

```On 2 Aug 2009, at 19:01, Llewlyn wrote:

> Hi experts,
>
> i'm very new to Mathematica and this is my first problem. I define a
> function that is 0 everywhere:
> t[i_] := 0
> expect for point i =3 that is:
> t = 1
> If now i evaluate the finite sum i obtain 1 as expected
> Sum[ t[i], {i, 1, 10}]
> Out = 1
> but if i try do the same with infinity i obtain
> In:= Sum[ t[i], {i, 1, Infinity}]
> Out= 0
>
> tb
>

What do you mean by "advice"? This is exactly the expected behaviour.
When you evaluate Sum[ t[i], {i, 1, 10}] Mathematica simply adds up
the 10 numbers, one of them being 3 and the other all 0, you get zero.
When you evaluate Sum[ t[i], {i, 1, Infinity}] Mathematica does not
try to add up infinitely many numbers because it can't be done 0. It
never, ever sees the value t.
All this shows is that computer programs are not human, which is
hardly surprising.

However, this works as you intended:

Sum[KroneckerDelta[i, 3], {i, 1, Infinity}]

1

Andrzej Kozlowski

```

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