Re: Why is Mathematica assuming k==l and how do I make it not to?

• To: mathgroup at smc.vnet.net
• Subject: [mg92611] Re: Why is Mathematica assuming k==l and how do I make it not to?
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Wed, 8 Oct 2008 06:25:11 -0400 (EDT)
• Organization: Uni Leipzig
• References: <gcffpl\$f4g\$1@smc.vnet.net>

```Hi,

it doe not do that, it
compute the integral and get
(2*k*Cos[l*Pi]*Sin[k*Pi] - 2*l*Cos[k*Pi]*Sin[l*Pi])/(k^2 - l^2)

and Sin[k*Pi] is 0 for any integer k and Sin[l*Pi] is 0 for any integer
l. So it get 0 with out an assumption about the relation between k and l
and you must treat  the case k==l explicit.

Regards
Jens

Aaron Fude wrote:
> As in
>
> Assuming[Element[{k, l}, Integers] ,
>  Integrate[Cos[k alpha] Cos[l alpha], {alpha, -Pi, Pi}]]
>
> I get 0 whereas the answer is Pi if k=l;
>
> Thanks!
>
> Aaron
>

```

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