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[3] = 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[808]:= Sum[ t[i], {i, 1, Infinity}] > Out[808]= 0 > instead of 1 expected. Any advice? > > 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[3]. 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

**References**:**error with Sum and Infinity***From:*Llewlyn <tommaso.biancalani@gmail.com>