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MathGroup Archive 2002

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Re: Question about yet another bug in Sum

  • To: mathgroup at smc.vnet.net
  • Subject: [mg35233] Re: [mg35204] Question about yet another bug in Sum
  • From: BobHanlon at aol.com
  • Date: Wed, 3 Jul 2002 05:13:54 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

In a message dated 7/2/02 4:07:18 AM, vvb at mail.strace.net writes:

>Here is another bug in Sum.
>
>            $Version
>
>            "4.2 for Microsoft Windows (February 28, 2002)"
>
>            Sum[Exp[-I Pi n] n /(1 + n^2), {n, 0, Infinity}]
>
>ACTUAL:     Sum::"div": "Sum does not converge."
>
>EXPECTED:   -1/2*HypergeometricPFQ[{2, 1 + I, 1 - I}, {2 - I, 2 + I}, -1]
>
>CHECK-UP:   N[%, 20]//Chop
>
>            -0.26961050270800898180
>
>            NSum[Exp[-I Pi n] n/(1 + n^2), {n, 0, Infinity}, 
WorkingPrecision->20]//Chop
>
>            -0.269610502708010
>
>This bug is present in 4.1, 4.0, 3.0.  Version 2.2 leaves the sum 
unevaluated.
>
>
>Can anyone explain the origin of the bug?

As a general rule it is worth simplifying the argument to Sum when possible.

$Version

4.1 for Mac OS X (November 5, 2001)

Sum[Simplify[Exp[(-I)*Pi*n]*(n/(1 + n^2)), Element[n, Integers]], {n, 0, 
Infinity}]

(1/4)*PolyGamma[0, 1/2 - I/2] +
 
  (1/4)*PolyGamma[0, 1/2 + I/2] -
 
  (1/4)*PolyGamma[0, 1 - I/2] - (1/4)*PolyGamma[0, 1 + I/2]

N[%, 20]//Chop

-0.26961050270800898180

%%//FullSimplify

(1/4)*(HarmonicNumber[-(1/2) - I/2] +
 
   HarmonicNumber[-(1/2) + I/2] - HarmonicNumber[-(I/2)] -
 
   HarmonicNumber[I/2])

N[%, 20]//Chop

-0.26961050270800898180


Bob Hanlon
Chantilly, VA  USA


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