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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>
  • Reply-to: kuska at informatik.uni-leipzig.de

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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